Rush-E6: Taking a stab at some E-class altitude records

[MouserWilliams]

I recently joined a new rocketry club, ARS, and I noticed on their website that the club altitude records for some of the smaller motor classes weren’t particularly high, and I thought it might be fun to try to beat one. In particular, their E motor altitude record was just over 300m and I thought it was very likely I could go higher than that on a long burn motor like the Aerotech E6-RCT.

While looking up motor performance data on thrustcurve.org, I came across the Apogee Medalist E6 motor which, while the second longest burn time E motor available at 5.8s, is noticeably shorter in burn time than the Aerotech E6-RCT at 7.1s. However, the Apogee motor weighs only 48g while the Aerotech motor (with case) weighs 93g [EDIT: as pointed out below, I was mistaken here, the Aerotech E6-RCT is 52g fully loaded including the case]. I wanted to see how the trade-off between motor mass and motor burn time played out for these two motors that otherwise have almost identical total impulse.

I whipped up a couple quick simulations in OpenRocket to compare the two and found that, all other things being equal, the Apogee E6 tended to outperform the Aerotech E6-RCT in terms of apogee achieved for a given airframe. Then I noticed that the simulated apogee I was achieving in OpenRocket of >1,800m not only greatly exceeded the club’s E motor record of 300m, but it also exceeded the NAR record for E motor altitude of 1,639m.

So of course now I have to build it and see how closely my building skills could match my optimized theoretical design and how well the OpenRocket simulations match reality. This thread is my build log for this effort.

I am leaning pretty heavily on the OpenRocket simulation engine to guide design decisions, particularly fin geometry, nose weight, boat tail geometry, etc. I've spent a lot of time min-maxing minor changes in the design to optimize for simulated apogee according to OR. Here is the OR screen shot of the initial design:

Most of it is pretty typical altitude-optimized stuff:

• Minimum diameter - 24.8 mm
• Minimum weight - shooting for <100 g total including motor
• Minimum length - shooting for <25 cm
• Boat tail
• 3 very thin carbon fiber fins
• Single deployment

Unfortunately, I can’t take advantage of the motor’s built-in ejection charge – the longest available delay in an Apogee E6 is eight seconds, which is far too short (the simulation says 11.9 seconds would be optimal). So I’ll have to remove the ejection charge from the motor and add electronic deployment. I will, however, be able to reclaim the volume typically taken up by the motor’s ejection charge for my own ejection charge plus some of the shock cord.

Since this thing is quite small and is going to go very high and not have dual-deployment, I expect it to drift a long way before landing. Having some sort of GPS tracking or RFDF option onboard would be nice, though weight and volume are real concerns. Ultimately I opted to go with the Altus Metrum TeleMini which will control the apogee deployment but also provide RFDF and buzzer capabilities for finding the rocket (which must be done if a record is to count…). The total mass of the TeleMini including the antenna and battery is only 8.7g which is very reasonable considering the functionality I get out of it.

The simulated maximum speed of this rocket is Mach 0.69, so an elliptical nose cone would be optimal for drag reduction. However, the existence of the wire antenna on the TeleMini means that I’d have to make room with an extended body tube when using the elliptical nose cone. The alternative is to go with a shorter body tube and the more typical pointy nose cone. I didn’t see any meaningful difference in simulated apogee between these two options in OR, so I just went with an Apogee 24mm ogive plastic nose cone because I already had some on-hand. I could have 3D printed a Haack-series nose cone, but the surface finish would have been much worse and the drag savings at low speeds don’t appear to be worth the effort.

For body tubing, I’m going to go with Apogee’s thin-walled kraft paper tube, backed up by their kraft-paper couplers for both the nose cone and the ejection piston which, when fully assembled, will give me full double-walled structural support for the body tube all the way from the forward end of the motor to the nose cone. This should be adequate to withstand the estimated 20 g’s of acceleration at takeoff. With the thin paper tube’s mass of 0.23g per centimeter of length, it is lighter per unit length than any commercially available carbon fiber or fiberglass tubing and I don’t have the skills or experience to build ultra-thin CF tubing myself.

I’m going to go with a piston ejection system because it uses less axial length than wadding or a chute protector, and the mass penalty vs wadding washes out when compared with the drag savings on shorter tubing. I’ll place the piston bulkhead as far aft as possible, leaving just enough room below it for my ejection charge and shock cord (and most of this will fit in the ejection charge well of the motor casing). This leaves the remainder of the piston’s internal volume available for the recovery system.

Similarly, I’ll put a bulkhead in the nose cone coupler as far forward as I can while still leaving room for the TeleMini, battery, antenna, some shock cord, some padding, and possible nose weight in the nose cone. The recovery system thus lives between the two coupler bulkheads. When fully assembled, the two couplers are in contact within the body tube, offering maximum structural advantage.

Getting the TeleMini as far forward as possible in the nose cone will require some folding of the wire antenna, which will affect RF performance. I’ll be doing some testing on this to see how bad it gets. Also, I’ll be flying a 120mAh battery for the TeleMini, and the Altus Metrum documentation doesn’t give any hints as to expected power draw or lifetimes vs. battery capacity–I’ll be testing this too.

Recovery will be a 12” aluminized polyester film parachute (the “Hang Time” competition chute from Apogee) with a total mass, including shroud lines and swivel, of 1.4g. I’m reluctant to use a streamer only because I don’t know how resilient the ultra-thin fins and paper body tubing will be to a harder landing. I may consider cutting a central hole in the parachute to save weight and volume at the cost of a modest increase in descent rate. I recognize that this chute is going to lead to some serious hikes to retrieve the rocket.

All of my shock cord is 350# kevlar line, which is probably more strength than is absolutely necessary, but it’s wider than the 100# kevlar strings and should help reduce the likelihood of zippers in the weak body tubing. The aft-most section of shock cord is the one most likely to cause zippers, but it is also the one that is directly in contact with the ejection charge so swapping this out for elastic is off the table. I could swap out the upper shock cord (between the parachute and the nose cone) with thin elastic cord to reduce the likelihood of zippers, or with smaller kevlar to reduce weight, but I think the difference in both cases is not worth the effort.

The motor is friction-fit into the body tube. The nose cone coupler is friction-fit into the nose cone. The lower end of the shock cord is epoxied to the inner wall of the motor casing’s ejection charge well, the upper end is epoxied into the tip of the nose cone. The boat tail will be epoxied to the aft end of the motor with JB Weld.

I’m going to build everything except the fins and nose weight, then re-simulate with the actual sizes and weights as-built to optimize the fin geometry and nose weight for maximum apogee while still retaining at least 1.0 caliber of stability.

Enough about the design; in the next post I’ll start covering my build process (which, at the time of this writing, is just getting started). Comments/advice/criticism welcome; I’m still early in this process and don’t necessarily know what I’m doing…

 

[MouserWilliams]

Nose Cone Subassembly

I’m using an Apogee 24mm nose cone which comes in two parts, the nose cone itself and a separate shoulder piece which is meant to be glued in. I’m not using the shoulder piece, opting instead for a longer piece of paper coupler to accommodate all of the components I need to fit within the nose cone. Unfortunately the Apogee 24mm kraft paper coupler tubes don’t fit inside as a replacement for the shoulder piece. The nose cone ID is 22.66 mm while the coupler OD is 23.83 mm.

I used a Dremel sanding drum to remove half a millimeter from the inside surface of the nose cone. I also lightly sanded one end of a coupler tube and the result was a 0.5g reduction in weight on the nose cone and a coupler tube that could fit inside the nose cone snugly. In flight this joint will just be a friction fit, so snugness was a priority here.

It wasn’t clear how little coupler length I needed sticking out aft of the nose cone in order to have a stable joint between the nose cone and body tube. I can fit everything I need inside the nose cone without any need for the coupler volume itself, and the parachute chamber only needs 11.3 mm or so of the nose cone coupler’s length to fit what needs to be below the nose cone bulkhead. However, I don’t want the coupler to be too short or it will invite the nose cone to fold over during boost. The rule of thumb in high power rocketry is generally two calibers of coupler, and for lower power rockets I see the one caliber number thrown around a lot, which for me would be 24mm.

I measured the length of the stock Apogee nose cone shoulder piece, which sticks aft of the nose cone by 19mm. Presumably this is sufficient in most scenarios, and the low-thrust motor I’m using should keep the lateral forces on the body reasonably sane. Assuming all the internals fit inside (and my math suggests they should), I’ll cut the nose cone coupler down to 19 mm of extension beyond the nose cone.

I stuck the coupler as far into the thinned nose cone as I could without damaging it, marked that distance, added 19 mm, then cut the tube using a razor blade in a jig that kept the cut perpendicular to the tube and at the correct length. I’m using the cut end inside the nose cone so that the aft end, which should butt up against the piston coupler when fully assembled, will have a perfect factory edge.

Here’s the final-length nose cone coupler temporarily installed in the nose cone, and the 24mm fiberglass coupler bulkhead from Mach 1 Rocketry that will get epoxied into the coupler after a few modifications for wires and shock cord:

I drilled out a hole in the center of the bulkhead large enough for my 350# kevlar shock cord to pass through twice, then filed a notch in one side of the bulkhead allowing me to pass the shock cord through the bulkhead in a loop but without needing a knot.

Then I filed another notch opposite the first large enough for the JST connector wires to pass through. Once these modifications to the bulkhead were complete, I epoxied it into place in the nose cone coupler and let it cure overnight.

I cut a ~50cm section of the 350# kevlar line and ensured that both ends had a dab of thin cyanoacrylate in them to ease in passing them through through the bulkhead without fraying. I passed the line through all four mounting holes of the TeleMini and left enough line at the top end to run to the top of the nose cone while still allowing the TeleMini to be fully extracted from the nose cone once the line gets epoxied into the tip.

I then passed the lower end of the line through the nose cone bulkhead’s central hole until there was enough slack above the bulkhead that the TeleMini could just barely be extracted from the coupler while attached to the line. The aft end of the line was then threaded up through one of the notches in the bulkhead edge, and back down through the center hole.

I also added a thin foam disc to the forward face of the bulkhead to act as a cushion for the electronics. I’d also like to add a thin foam sheet around the sides and top of the electronics just for protection in possibly rough landings, but I don’t have the material for that yet.

The current weight of the nose subassembly is 19.09 grams, and this includes:

• Nose cone
• Coupler
• Bulkhead w/ epoxy
• Foam disc
• TeleMini w/ antenna wire and switch jumper
• 120mAh battery
• Shock cord (350# kevlar, 50cm)

What is still missing from the nose cone subassembly is:

• Forward ejection charge wires
• Foam sleeve for electronics
• Epoxy attachment between shock cord and nose cone tip
• Nose weight (if needed)

The wiring will get done later since there are remaining questions about the type of connector I want to use between the upper and lower half of the ejection charge wiring. The foam sleeve will weigh nothing and can get done at any time later with minimal effect on performance. I'm not going to epoxy the shock cord into the front until I know if I need to add a few lead shot spheres or not. Stay tuned for that. For now, I'll move on to another subassembly of the rocket.

 

[MouserWilliams]

Motor Subassembly

The starting point for the Rush-E6 motor is the Apogee Components E6 Medalist.

They don’t seem to sell a plugged version currently, so I got the E6-8 version and removed the ejection charge. The charge is held in with a small paper seal which can be easily removed with tweezers.

Once the seal is removed, the bulk of the black powder can be dumped out (and saved!) and what remains can be wiped out with a damp cloth.

To keep hot gasses from the delay grain from interfering with the shock cord and ejection charge that I would be storing in this space, I put a small piece of aluminum tape over the hole.

The cleaned-out ejection charge well is over 7mm deep and 20mm wide, which is big enough to hold 70 cm of 350# kevlar thread tightly packed.

My intention is to epoxy one end of the kevlar to the wall of the motor’s ejection charge well and pack my ejection charge canister and most of the kevlar that sits between the motor and the ejection piston into this volume. Taking advantage of this volume will allow me to position the piston bulkhead as low as possible and minimize body tube length. Also, if I can fit the ejection canister into the well sideways (spoiler alert–I can), putting it in the thick-walled plastic ejection well of the motor casing may protect the thin cardboard body tube and coupler from getting damaged (as much?) by hot, fast ejection particles and blast pressure.

Nothing much left to do with the motor other than epoxy on the shock cord and boat tail, but both of those tasks will come later and be part of separate posts.

The total mass of the modified motor with the ejection charge removed (and a tiny piece of tape added) is 48.1g.

 

[bad_idea]

Love this build!

If you've not used it before, you might want to ground test that aluminum tape on a small composite motor (presumably one cheaper than the E6). I'd be concerned about it blowing out when the delay grain burns through.

 

[burkefj]

I recently joined a new rocketry club, ARS, and I noticed on their website that the club altitude records for some of the smaller motor classes weren’t particularly high, and I thought it might be fun to try to beat one. In particular, their E motor altitude record was just over 300m and I thought it was very likely I could go higher than that on a long burn motor like the Aerotech E6-RCT.

While looking up motor performance data on thrustcurve.org, I came across the Apogee Medalist E6 motor which, while the second longest burn time E motor available at 5.8s, is noticeably shorter in burn time than the Aerotech E6-RCT at 7.1s. However, the Apogee motor weighs only 48g while the Aerotech motor (with case) weighs 93g. I wanted to see how the trade-off between motor mass and motor burn time played out for these two motors that otherwise have almost identical total impulse.

I whipped up a couple quick simulations in OpenRocket to compare the two and found that, all other things being equal, the Apogee E6 tended to outperform the Aerotech E6-RCT in terms of apogee achieved for a given airframe. Then I noticed that the simulated apogee I was achieving in OpenRocket of >1,800m not only greatly exceeded the club’s E motor record of 300m, but it also exceeded the NAR record for E motor altitude of 1,639m.

So of course now I have to build it and see how closely my building skills could match my optimized theoretical design and how well the OpenRocket simulations match reality. This thread is my build log for this effort.

I am leaning pretty heavily on the OpenRocket simulation engine to guide design decisions, particularly fin geometry, nose weight, boat tail geometry, etc. I've spent a lot of time min-maxing minor changes in the design to optimize for simulated apogee according to OR. Here is the OR screen shot of the initial design:

Most of it is pretty typical altitude-optimized stuff:

• Minimum diameter - 24.8 mm
• Minimum weight - shooting for <100 g total including motor
• Minimum length - shooting for <25 cm
• Boat tail
• 3 very thin carbon fiber fins
• Single deployment

Unfortunately, I can’t take advantage of the motor’s built-in ejection charge – the longest available delay in an Apogee E6 is eight seconds, which is far too short (the simulation says 11.9 seconds would be optimal). So I’ll have to remove the ejection charge from the motor and add electronic deployment. I will, however, be able to reclaim the volume typically taken up by the motor’s ejection charge for my own ejection charge plus some of the shock cord.

Since this thing is quite small and is going to go very high and not have dual-deployment, I expect it to drift a long way before landing. Having some sort of GPS tracking or RFDF option onboard would be nice, though weight and volume are real concerns. Ultimately I opted to go with the Altus Metrum TeleMini which will control the apogee deployment but also provide RFDF and buzzer capabilities for finding the rocket (which must be done if a record is to count…). The total mass of the TeleMini including the antenna and battery is only 8.7g which is very reasonable considering the functionality I get out of it.

The simulated maximum speed of this rocket is Mach 0.69, so an elliptical nose cone would be optimal for drag reduction. However, the existence of the wire antenna on the TeleMini means that I’d have to make room with an extended body tube when using the elliptical nose cone. The alternative is to go with a shorter body tube and the more typical pointy nose cone. I didn’t see any meaningful difference in simulated apogee between these two options in OR, so I just went with an Apogee 24mm ogive plastic nose cone because I already had some on-hand. I could have 3D printed a Haack-series nose cone, but the surface finish would have been much worse and the drag savings at low speeds don’t appear to be worth the effort.

For body tubing, I’m going to go with Apogee’s thin-walled kraft paper tube, backed up by their kraft-paper couplers for both the nose cone and the ejection piston which, when fully assembled, will give me full double-walled structural support for the body tube all the way from the forward end of the motor to the nose cone. This should be adequate to withstand the estimated 20 g’s of acceleration at takeoff. With the thin paper tube’s mass of 0.23g per centimeter of length, it is lighter per unit length than any commercially available carbon fiber or fiberglass tubing and I don’t have the skills or experience to build ultra-thin CF tubing myself.

I’m going to go with a piston ejection system because it uses less axial length than wadding or a chute protector, and the mass penalty vs wadding washes out when compared with the drag savings on shorter tubing. I’ll place the piston bulkhead as far aft as possible, leaving just enough room below it for my ejection charge and shock cord (and most of this will fit in the ejection charge well of the motor casing). This leaves the remainder of the piston’s internal volume available for the recovery system.

Similarly, I’ll put a bulkhead in the nose cone coupler as far forward as I can while still leaving room for the TeleMini, battery, antenna, some shock cord, some padding, and possible nose weight in the nose cone. The recovery system thus lives between the two coupler bulkheads. When fully assembled, the two couplers are in contact within the body tube, offering maximum structural advantage.

Getting the TeleMini as far forward as possible in the nose cone will require some folding of the wire antenna, which will affect RF performance. I’ll be doing some testing on this to see how bad it gets. Also, I’ll be flying a 120mAh battery for the TeleMini, and the Altus Metrum documentation doesn’t give any hints as to expected power draw or lifetimes vs. battery capacity–I’ll be testing this too.

Recovery will be a 12” aluminized polyester film parachute (the “Hang Time” competition chute from Apogee) with a total mass, including shroud lines and swivel, of 1.4g. I’m reluctant to use a streamer only because I don’t know how resilient the ultra-thin fins and paper body tubing will be to a harder landing. I may consider cutting a central hole in the parachute to save weight and volume at the cost of a modest increase in descent rate. I recognize that this chute is going to lead to some serious hikes to retrieve the rocket.

All of my shock cord is 350# kevlar line, which is probably more strength than is absolutely necessary, but it’s wider than the 100# kevlar strings and should help reduce the likelihood of zippers in the weak body tubing. The aft-most section of shock cord is the one most likely to cause zippers, but it is also the one that is directly in contact with the ejection charge so swapping this out for elastic is off the table. I could swap out the upper shock cord (between the parachute and the nose cone) with thin elastic cord to reduce the likelihood of zippers, or with smaller kevlar to reduce weight, but I think the difference in both cases is not worth the effort.

The motor is friction-fit into the body tube. The nose cone coupler is friction-fit into the nose cone. The lower end of the shock cord is epoxied to the inner wall of the motor casing’s ejection charge well, the upper end is epoxied into the tip of the nose cone. The boat tail will be epoxied to the aft end of the motor with JB Weld.

I’m going to build everything except the fins and nose weight, then re-simulate with the actual sizes and weights as-built to optimize the fin geometry and nose weight for maximum apogee while still retaining at least 1.0 caliber of stability.

Enough about the design; in the next post I’ll start covering my build process (which, at the time of this writing, is just getting started). Comments/advice/criticism welcome; I’m still early in this process and don’t necessarily know what I’m doing…

The E-6 in reloadable case is 53 grams not 90+
I have carried single use E-6 motors that aerotech made for FAI folks that weigh 46 grams rtf. Those would be my choice rather than the medalist since you don't have the weight of the delay grain. You don't need a delay.

Frank

 

[MouserWilliams]

The E-6 in reloadable case is 53 grams not 90+
I have carried single use E-6 motors that aerotech made for FAI folks that weigh 46 grams rtf. Those would be my choice rather than the medalist since you don't have the weight of the delay grain. You don't need a delay.

Frank

Thanks for pointing this out.

I suspect I may have been led astray by inaccurate specifications and/or I may not be looking at the right place for the information I need and/or I was confused about the numbers I saw in my initial cursory search. The Aerotech E6-RCT motor needs an RMS-RC 24/20-40 case, which Apogee indicate weighs 41g empty. The reload itself is listed as having a total mass of 52.0g and a propellant mass of 21.5g. Now that I'm digging a bit deeper, I see that these numbers (52g and 21.5g) match what is on Aerotech's website for the loaded weight and propellant weight. So I suspect the problem here is that Apogee's indication of a 41g mass for the empty case just can't be right (and the 52.0g mass is the total mass including the case, which Apogee didn't make clear. Any chance someone has an empty 24/20-40 case they could weigh and just verify that it is more in the realm of <30g?

I'm looking at my OpenRocket simulations to see what it was using for motor mass internally:

• Aerotech E6-RCT: 52.7g
• Apogee E6: 46.3g

OK, so at least my simulations were using something approaching sane numbers. Having said that, the Apogee motor is still six grams lighter and the OR simulation does still show a (slightly) higher apogee with the Apogee motor, even though I'm lugging along a useless delay grain.

Interestingly, the 46.3g mass given in the Apogee E6 file I downloaded from thrustcurve.org is ~2g less than the measured mass of my E6 after having removed the ejection charge and charge seal (not that these weight much... but still). 5% discrepancy seems like a lot for it to just be manufacturing variance...

Manufacturers' and vendors' absolute commitment to flawless documentation and consistency is the stuff of legend.

-Mouser

 

[MouserWilliams]

Love this build!

If you've not used it before, you might want to ground test that aluminum tape on a small composite motor (presumably one cheaper than the E6). I'd be concerned about it blowing out when the delay grain burns through.

Thanks!

I was assuming that after an eight-second delay, the internal pressure in the motor would be ~ambient and that the delay grain burn rate would be more at the smolder end of the smolder<->detonation spectrum. But this is entirely supposition on my part; do I need to worry about the gas on the back side of my tape seal being either hot enough to melt aluminum or high pressure enough to tear it (or both)?

Contrast this with the fact that the kevlar line that will be on top of the tape will be directly exposed to my ejection charge and presumably will be able to handle that. So as long as whatever comes through that aluminum tape seal isn't more destructive than a BP charge at zero range, I should be OK, right?

If the answer is that I'm an idiot and I need to plug this thing in some other manner, could someone recommend a path forward?

Cheers,
-Mouser

 

[burkefj]

An empty 24/40 rc case is what you need, it is around 20 grams empty, the grains are too short to seal with the standard casing. I can verify a loaded motor is 52-53 grams, I would seal the hole with a little dab of epoxy, I dont think the flame will damage the kevlar but there will be some little pressure you might not want.

 

[MouserWilliams]

An empty 24/40 rc case is what you need, it is around 20 grams empty, the grains are too short to seal with the standard casing. I can verify a loaded motor is 52-53 grams, I would seal the hole with a little dab of epoxy, I dont think the flame will damage the kevlar but there will be some little pressure you might not want.

OK, I will do that. Thanks for the tip!

 

[bad_idea]

I was assuming that after an eight-second delay, the internal pressure in the motor would be ~ambient and that the delay grain burn rate would be more at the smolder end of the smolder<->detonation spectrum. But this is entirely supposition on my part; do I need to worry about the gas on the back side of my tape seal being either hot enough to melt aluminum or high pressure enough to tear it (or both)?

Brainfade on my part, sorry. I was forgetting the E6 was an end burner. With an end burner, maybe tape alone will work; I don't have the experience to say, myself, so I'd probably sacrifice a motor to a ground test if I were doing this.

 

[MouserWilliams]

At the advice of burkefj, I replaced my bit of aluminum tape with a little dab of epoxy. In this case, a tiny bit of Hysol 9462:

While that cures for the next 24 hours, I worked on my ejection canister. This is something I’ve never done, so I’m anticipating errors. Please, do tell me when/how I’m screwing this up.

I started at the ejection charge calculator on Rocketry Calculator, which suggests that for this rocket I only need 0.01g of black powder. After a bit of playing around with the calculator, I found that this is the minimum quantity the calculator will output. As I increased the value for tube length, the suggested quantity value stays the same until the tube length is more than double what it’ll actually be. So I suspect that I really need something more like 0.005g. This is, however, a minute quantity of black powder. Here’s 0.01g on my scale:

I tried dumping this 0.01g of BP into the red cap that came with the e-matches that I have and it was so little that it fell to the back of the cap and didn’t really hang out near the actual pyrogen. I’m worried it’ll fail to ignite reliably if at least some of it isn’t in intimate contact with the e-match pyrogen bulb.
I tried plugging the aft half of the cap with hot glue, but this was a disaster and just ended up coating an entire e-match in hot glue which is probably even worse in terms of keeping the BP in contact with the bulb… So I gave up on the red caps and went with a 10mm long section of ⅛” launch lug.

I removed the red cap, then put a healthy blob of hot glue just below the pyrogen on the e-match. Then I quickly slid down my section of launch lug so that it covered the pyrogen but only just barely. The bottom end of it was plugged by hot glue and hopefully the hot glue plug will keep the black powder from sliding down too far away from the pyrogen.

I then poured the measly quantity of BP into the launch lug and plugged the end with another drip of hot glue. When everything had cooled, I trimmed off the excess hot glue.

I haven’t test-fired this yet, and I’m only mildly confident that it’ll work. Please tell me if there is a fool-proof way to make a very small BP charge around an e-match. Also, is RocketryCalculator leading me astray for such a small ejection volume? I feel like I should have more BP in here but I don't want to pop my rocket at apogee...

I did test that the charge fits in the ejection well of the Apogee E6 motor:

It fits great! Plenty of room in there for the kevlar attachment point and lots of shock cord storage as well.

I also trimmed the e-match wires down to 5cm, which is plenty for my application, and weighed the entire piece at 0.74g.

 

[BEC]

This is an interesting project. Good luck!

I will just add for calibration's sake that the last time E Altitude was flown at a NARAM (at NARAM-61 in Muncie Indiana in 2019) we pretty much all used that Apogee single-use E6-8 with motor ejection. My model, which placed third in C Division, was a little bigger than an Alpha and used an Alpha VI nose cone and carried a PerfectFlite Firefly for altitude reading (and a bit of ballast) in that nose cone. It recovered by shiny streamer and went to 1546 meters or 5072 feet. I used thin carbon fins not unlike those in your OR screen shot above.

The winner went to 1641 meters (5384 feet). His model was a study in drag reduction via a smooth finish, clean joints, etc. It was beautiful. Most all the models flown in the event were 24mm minimum diameter designs.

We pretty much depended on finding each others' models after the flights. One contestant at least had a tiny tracker in his model (Walston?). It didn't really help. And yes, an 8-second delay is too short to allow the model to coast all the way to apogee. It will be interesting to see what sort of results you get with your very different approach. Your OR projection beats Mr. Rains' actual results by over 200m.

 

[aerostadt]

There is a controversy about plugging motors.
https://www.rocketryforum.com/threa...ed-rocket-engines-please.183750/#post-2524324

 

[SolarYellow]

It would depend on what Apogee or whoever actually makes the motor for them (I believe that's AeroTech) says is an allowed modification for that motor.

My recollection is that AT has approved adjusting or removing the ejection charge from motors and replacing the plastic cap. I'm not aware of them having publicly approved "dog barf and tape" as an alternative closure to the plug, nor epoxy plugging of the housing.

TVM still has a video on his YT channel where he shows how to plug a -0 BP motor with epoxy, so I would guess he'd be OK with this, but I'd want to ask him, myself.

 

[MouserWilliams]

Perhaps I'm a bit far into this to be looking carefully at the NAR contest rules for the first time... but here we are. I notice the following section:

4.4 Alterations
A model rocket motor must not be altered in any manner that changes its dimensions and/or its performance characteristics. No material may be permanently affixed to the motor.

I'm not going to be making any modifications to my motor that change its dimensions and/or performance, but it is my intention to epoxy the boat tail to the aft face of the motor and to attach the aft end of the shock cord to the inner wall of the ejection charge well. I've read threads here of other people doing this for TRA altitude records; is this something that is allowed by TRA but not by NAR? Would either of these actions actually disqualify me from an altitude record? I'm happy to go to my second E6-8 motor that I have and go back to the aluminum tape solution if the epoxy plug is an issue...

 

[SolarYellow]

I've read threads here of other people doing this for TRA altitude records; is this something that is allowed by TRA but not by NAR? Would either of these actions actually disqualify me from an altitude record?

Yes. TRA allows "flying case" rockets with fins glued to the motor case. NAR does not allow anything to be glued to the motor.

 

[MouserWilliams]

<stops print on boat tail that has a horizontal mounting flange for attaching to the motor...>

Any chance TRA will start keeping track of records for sub-F motors? I'm not sure why they track F and G records but not E...

Anyway, I'm going to spend some time thinking about how I'm going to attach a shock cord to the inside of the body tube just above the motor and have a boat tail but also be able to insert a motor. Stay tuned. Engineering wouldn't be fun if it was straightforward...

 

[SolarYellow]

<stops print on boat tail that has a horizontal mounting flange for attaching to the motor...>

I only meant for the NAR competition rule book, which includes any record setting.

I'm not sure specifically about gluing to motors as far as the safety code. It might need a judgement call whether the particular instance counted as a "modification" of the motor or its function with respect to the safety code language. Read through the thread linked above if you want to go through the details at length.

 

[BEC]

I'm usually hard and fast about following the Safety Codes, but in this case I'd be tempted to discuss it with the ARS safety officer (if they have one) or whoever is the RSO on the day of the launch.

You could easily beat that 998 foot E record by just putting an unmodified E6-8 in something like an Estes Eliminator or some other BT-55/56 model. But of course that's not what you're going for...

Having once been out to ARS' Rio Rancho site once about 10 years ago, good luck on your recovery! A 12 inch 'chute at over a mile in the air is going to require some really really good eyes to see and then some interesting times navigating that landscape. It'll be rather more challenging than blacktop roads (and runways) and close cropped grass flying fields as we were dealing with at the AMA's Muncie site at NARAM-61.

Good luck!!

 

[SolarYellow]

You can beat 998 feet with an A motor.

 

[MouserWilliams]

Yeah the 998' record is not going to be particularly difficult to top, but as long as I'm in the ballpark I'd like to at least try for the NAR record. I'm not confident I'll be able to beat it, because I suspect engineering details and my inexperience with doing really smooth/light airframes are going to get in the way of my rocket matching my simulation, but I like a good challenge.

So I'm working on a couple of design details that will allow me to proceed with the same external geometry without significant increases in mass and without gluing anything to the motor.

1) The aft end of the shock cord will be attached to the inside of the body tube immediately above the motor. This will require me to put a notch in the side of the aft part of the piston and push the piston bulkhead forward by a few millimeters to accommodate this epoxy joint. This will result in a few millimeters less length in the parachute storage area unless I lengthen the rocket. I need to actually fabricate the parachute and see how well it packs down to know whether I can get away with this. I'm also reconsidering my parachute vs. streamer decision here... more on that later.

2) I'm going to shorten the body tube by ~5mm or so such that the motor and fins protrude out the back of the rocket slightly, then re-print the boat tail to have a short cylindrical section at the forward end which will cover the protruding motor and slip under the fins, sitting flush with the aft end of the body tube. Finally, I'd like to apply a dab of glue/epoxy behind each of the three fins which will serve two purposes -- it'll firmly hold the boat tail in place and it will complete the fin root fillet at the back of the rocket for aerodynamics. The major downside to this approach that I see is that it reduces the fin attachment area to the body tube, possibly making the fins less resilient to damage on hard landings.

Question for the forum: How do we feel this solution sits with the NAR contest rules? I'm not permanently attaching anything to the motor, but the motor is inside the rocket in a way that I'll have to tear the boat tail off (irreversibly) to extract it. Kosher? Dirty pool?

 

[Art Upton]

Nothing in the rules says you can't tear off your "Engine Retainer system", to remove the motor, or reglue a new one in place to fly the rocket again with a new motor.

Contest lawyers may not like it, but written rules are written rules; no legislation from the lawyers.
Anything they don't like must now pass an RCP rule making to change it.

* Back in the day they were called Pink Book Lawyers, as the rule book originally was printed on 'free' pink paper, and thru the years the cover of the book was done in Pink because of that.

 

[NTP2]

This is really cool, I love the tail cone design!

 

[kjhambrick]

GREAT design and mission, @MouserWilliams !

You made me look at the E6 at ThrustCurve.org > E6


As you can see in the linked page, there are three E6 motors listed: the two you 'found' in OpenRocket plus an Aerotech Single Use Motor ( maybe the same as Apogee's E6 ? )

However, when I click on the Aerotech Single Use motor, the metadata describes a reload for the RMS 24/20-40 R/C case:


When I look at the linked NAR cert docs that @JohnCoker, links from the motor pages, they were definitely different motors ...

It would be nice to score an AT Single use E6, but all I can find is the AEROTECH E6-RCT RMS-R/C-24/20-40 RELOAD KIT when I look at Aerotech > Search > E6.

Too bad about that ... the AT Single Use E6 has just a wee bit more total impulse than the Apogee E6 ...

But then again, I am guessing that the Apogee E6 is a rebranded AT E6 so maybe not

Please keep your thread going and do let us know how you fare -- GREAT engineering and documentation and VERY interesting techniques !

Thanks again !!

-- kjh

p.s. Too bad Tripoli doesn't have an E-class record category. I know there are TRA members out there who roll their own 18mm E-motors

 

[burkefj]

This is the single use E-6 data from nar, I had aerotech do a couple of runs in the last few years as well as the fai folks.

 

[MouserWilliams]

I was planning on doing a bunch of tasks on this rocket this evening, but I got caught up in family stuff and didn't get anything done, so I'll just leave you with this list of remaining tasks and component status. I'll update this as I get more done on the rocket.

Tasks with no remaining prerequisites

1. Assemble parachute
2. Cut body tube to length
3. Cut piston coupler to length
4. Notch and drill piston bulkhead
5. Cut aft kevlar to length
6. Choose electrical connector for ejection wiring
7. Test fire ejection charge canister(s) by themselves for reliability
8. Build suitable launch tower (maybe this should be dependent on a final weight and balance for simulating speed clearing the tower?)

Tasks awaiting prerequisite(s)

1. Prep body tube for shock cord attachment
2. Aft kevlar attachment to body tube
3. Notch piston coupler tube
4. Attach piston coupler bulkhead
5. Determine space for kevlar aft of piston coupler
6. Thread shock cord around piston bulkhead loop
7. Attach fwd electrical connector to nose cone coupler aft volume
8. Solder aft electrical connector to ejection charge wires, heat shrink junctions
9. Optimize weight & balance simulation for apogee, selecting fin geometry and nose weight
10. Order fins from sendcutsend.com
11. Bevel fins
12. Attach fins
13. Epoxy fwd shock cord and nose weight (if necessary) into nose cone tip
14. Ground test ejection
15. FLY EET!

Component	Status	Notes
Nose Cone	100%
Nose Weight	0%	Waiting for final weight and balance
Fwd. Shock Cord	95%	Just needs knot at swivel
Flight Computer	90%	Needs cushion sleeve
Nose Cone Coupler	100%
Nose Cone Bulkhead	100%
Body Tube	0%	Have parts, not started
Fwd. Ejection Charge Wiring / Connector	0%	Didn’t like first connector choice, acquiring second option
Swivel	100%
Parachute	0%	Have parts, need to assemble
Aft Shock Cord	0%	Have parts, not started
Aft Ejection Charge Wiring / Connector	0%	Didn’t like first connector choice, acquiring second option
Piston Coupler	0%	Have parts, not started
Piston Bulkhead	0%	Have parts, not started
Ejection Charge	50%	First candidate produced, need to test it
Motor	100%
Boat Tail	80%	1st prototype fabricated, need to figure out how to attach it without gluing to the motor
Fins	0%	Waiting on final weight & balance
Launch Tower	0%	Not started, have identified candidate design

 

[MouserWilliams]

The Boat Tail
My OpenRocket simulations show an improvement in apogee of over 100m just by adding a boat tail to the aft end of the rocket, so naturally I'm going to do that.

I didn't have a good sense of how long to make it, or how large to make the aft opening of the boat tail, so I just iterated in design space with OR and let it do the work. I'm trusting in the simulation engine to be close to reality because I don't have any better way to come to a final design. I did notice that in certain areas of design space if I made a miniscule change to one parameter it would result in a weird discontinuous jump in apogee (up or down), which makes me concerned that there are some non-physical artifacts in the results, but it's the best tool I have access to currently.

Ultimately, OR led me to a boat tail that is 1.45cm long, 2.48cm in outer diameter at the forward end, and 2.00cm in outer diameter at the aft end. The curvature is a Haack series curve, but it isn't clear to me if the benefits of Haack are valid when looking at reductions at the aft end of a tube rather than a nose cone. But whatever, it simulated better than anything else in OR.

Initially, when I had planned to epoxy the boat tail to the motor (in violation of NAR contest rules), I had designed the part to have a small 4mm flange at the forward end to support the epoxy joint. When it became clear that I had failed to read the manual, I considered redesigning the part to have a sleeve to fit around the very aft end of the motor and then make epoxy joints between the fins and boat tail that didn't touch the motor.

But then I realized that I could probably do that same thing without redesigning anything and just use the part that I already have... so I'm going to start there and see how it goes. This route also keeps my fin/body tube joints as strong as possible which was the big downside of shortening the body tube at the aft end without moving the fins forward.

To minimize the weight of the part, I designed it to be printable in “vase mode” on the 3D printer, resulting in walls that are just a single line of material thick, about 0.4mm. The material has to stand up to intense radiative heat loads for ~7 seconds and a fair amount of turbulent pressure effects, so I went with a reasonably strong carbon-reinforced nylon and we'll see how it holds up.

I didn't have a good way to take the description of the shape from OR and translate that into something the 3D printer could understand, so I zoomed way in to the image of the rocket in OR, took a screen shot of that, and then gave the result to a friend of mine who is mechanically inclined. He was able to use the image of the curve to make a shape in SolidWorks, then rotate that into a solid, and send that to his slicer.

This is the screenshot from OR that I gave to my mechanical engineer friend to help transform it into a physical object.

​

OpenRocket’s curve transferred into SolidWorks and rotated into a 3D shape, then moved into SuperSlicer and ready to print. Printing this part took seven minutes total, and used approximately $0.03 worth of filament.

Zero mass boat tail in carbon-reinforced nylon! Still working on the negative mass version that helps with stability…

Better scale, half a gram. Not bad for a ~120m improvement in apogee.

And here it is resting on top of the aft end of the body tube. The blue is just tape over the motor nozzle. I should be able to put little mini fillets of epoxy between the aft edge of the fins and the outer edge of the boat tail to keep it in place. Filing these fillets off will be part of the motor retrieval process, and the boat tail will be destroyed in the process... but this isn't a rocket I intend to fly many times and the boat tail was really cheap and quick to print, so a bit of extra hassle is probably OK.

Looking at that picture... I'm going to have to fill and sand all those spiral grooves, aren't I. Another thing I've never done before, and one that doesn't sound like fun.

 

[lr64]

The following may perhaps best be left until the second version of this rocket. Better to have something that's flying with 80 or 90 percent of the performance than just a bunch of ideas.
--------
If you're going to fill and sand the grooves, but you're impatient, do a better job on the grooves closest to the nose, where the boundary layer will be thinner. I admit I haven't estimated just what that thickness will be.
----------
I'm curious what Openrocket thinks is the optimum weight for your rocket. Lighter than your build will be? Heavier? I think lighter is probably possible if that's what's best. For instance, maybe a glassed foam, thin shell cone with some lead right up at the tip.
----------
If you're only getting to Mach 0.69, you don't need really thin fins. You could probably have a bit less drag with a 6 or even 9 percent thick airfoil. I'm guessing that it's fast enough that a NACA 60 series would have less drag than an 006 or 009, but, so far, it's only a guess. Or maybe the sweep takes care of that. With an airfoil, you could make, say, balsa fins with a touch of reinforcement that would weigh less than the carbon ones. I doubt if they'd have to be terribly sophisticated to be better than a thin plate, even if bevelled.

It may be that using an unswept fin might give you a bit steeper lift curve and therefore you could have less "wetted area", possibly giving less drag with the same initial stability.

----------
My guess is that, even for a rocket like this, more impulse ends up devoted to fighting air drag than to fighting gravity. It's easy to get Openrocket to give you a plot or table with drag in it. If not, I doubt that boat tail would get you 100 meters. A substantial part of the drag is probably from the fins.

 

[MouserWilliams]

Thanks for the response! I have a feeling you know a lot more about the details of rocket aerodynamics than I do. I recognize the value of having something flying; I'm trying not to get too paralyzed with optimization, but I do appreciate this glimpse into the art of the possible and it gives me some things to read up on.

I'm curious what Openrocket thinks is the optimum weight for your rocket. Lighter than your build will be? Heavier? I think lighter is probably possible if that's what's best. For instance, maybe a glassed foam, thin shell cone with some lead right up at the tip.

I'm not certain how to get OpenRocket to tell me this. I tried running the "Rocket Optimization" tool, letting it play with the nose ballast and fin geometry, and it spit out this gem of a design right here:


It says this will do 2029m apogee, so 140m of additional altitude... but I'm pretty confident I can't make those fins survive a real world flight.

If I leave the geometry alone and just have OR optimize the nose ballast, the results vary with the shape of fins I have selected, but the answer is never zero, is that equivalent to saying that the optimum weight is heavier than just the stock components combined?

If you're only getting to Mach 0.69, you don't need really thin fins. You could probably have a bit less drag with a 6 or even 9 percent thick airfoil. I'm guessing that it's fast enough that a NACA 60 series would have less drag than an 006 or 009, but, so far, it's only a guess. Or maybe the sweep takes care of that. With an airfoil, you could make, say, balsa fins with a touch of reinforcement that would weigh less than the carbon ones. I doubt if they'd have to be terribly sophisticated to be better than a thin plate, even if bevelled.

I think for the purposes of my initial attempts with this, I'm probably not going to go down the rabbit hole of trying to match specific airfoil cross sections in my fins. I'm currently targeting the 1mm thick CF plate stock offered by SendCutSend, which is extraordinarily light. I have zero experience in this, but I'm surprised to hear that a reinforced balsa core fin would be lighter by area than very thin CF. How thin can you make a fiberglass reinforcement layer, assuming good vacuum bagging, etc?

 

[Al_delaIglesia]

Yeah the 998' record is not going to be particularly difficult to top, but as long as I'm in the ballpark I'd like to at least try for the NAR record. I'm not confident I'll be able to beat it, because I suspect engineering details and my inexperience with doing really smooth/light airframes are going to get in the way of my rocket matching my simulation, but I like a good challenge.

So I'm working on a couple of design details that will allow me to proceed with the same external geometry without significant increases in mass and without gluing anything to the motor.

1) The aft end of the shock cord will be attached to the inside of the body tube immediately above the motor. This will require me to put a notch in the side of the aft part of the piston and push the piston bulkhead forward by a few millimeters to accommodate this epoxy joint. This will result in a few millimeters less length in the parachute storage area unless I lengthen the rocket. I need to actually fabricate the parachute and see how well it packs down to know whether I can get away with this. I'm also reconsidering my parachute vs. streamer decision here... more on that later.

2) I'm going to shorten the body tube by ~5mm or so such that the motor and fins protrude out the back of the rocket slightly, then re-print the boat tail to have a short cylindrical section at the forward end which will cover the protruding motor and slip under the fins, sitting flush with the aft end of the body tube. Finally, I'd like to apply a dab of glue/epoxy behind each of the three fins which will serve two purposes -- it'll firmly hold the boat tail in place and it will complete the fin root fillet at the back of the rocket for aerodynamics. The major downside to this approach that I see is that it reduces the fin attachment area to the body tube, possibly making the fins less resilient to damage on hard landings.

Question for the forum: How do we feel this solution sits with the NAR contest rules? I'm not permanently attaching anything to the motor, but the motor is inside the rocket in a way that I'll have to tear the boat tail off (irreversibly) to extract it. Kosher? Dirty pool?

Since your question points to NAR contest rules, according to the USMRSC...
2. Motors. I will use only certified, commercially made model rocket motors, and will not tamper with these motors or use them for any purposes except those recommended by the manufacturer.
3.5 Reusability A model rocket must be so constructed as to be capable of more than a single flight;
4.4 Alterations A model rocket motor must not be altered in any manner that changes its dimensions and/or its performance characteristics. No material may be permanently affixed to the motor.
Also, if you want a NAR record, then you have to use a NAR approved altimeter and fly at a NAR sanctioned event.