Hellcat (yet another 38mm MD)

[Clusterphiliac]

So, 2015 has been kind of a hit or miss year for me rocketry-wise. There have been a couple crashes and a bunch of cancelled launches, but I finally got my level 1. Now, my plans for this Spring are fairly simple: make a new drop-in avbay for my Level 1 and most future high-power rockets I build, finishing fixing up for MPR cluster model I crashed last spring and add electronic deployment so I can actually fly it as a cluster, and hopefully build a 24mm or 29mm MD.

But after that, in the 2016-2017 Academic year, I don't really have a major project on the to-do list. So... how about going as high as possible on I impulse?

So, here's my initial design.

Currently, specs are as follows:

• Empty mass should be under 16 oz.
• Length is 32"
• My goal is to reach 2.5 miles (13,200 ft), but sims with RASAERO II suggest that it might be possible to break 15,000 feet.
• The airframe will be 38mm thin-wall G12 from Rocketry Warehouse. I'm guessing this will hold up fairly well at the speeds expected of this rocket. My club does make custom fiberglass tubes, but with the coarse cloth available I'm not confident that I could make a tube that's strong, light, and smooth.
• The nose cone will be a 38mm pinnacle.
• The fins will be 1/16" G10 with about a 1/4" bevel on the leading and trailing edges.
• Recovery will be dual-deploy with a Raven 3 firing a BP charge at apogee and a Cable Cutter around 700 feet, and a 24" chute. I'm looking at a Top Flight thin mill as an option.
• The avionics bay will be mounted in the nose cone, and aside from the Raven will use either an Eggfinder or, if I get my HAM license and have money an Adept tracker for location of the rocket.
• For motor retention I was looking at the AeroPack MD retainer. However, 1.5 oz is a little heavy when the rocket is under a pound. I think I can make something lighter but still with plenty of strength using a bit of fiberglass coupler, a weld nut, some allthread, and scrap G10 or plywood for a bulkhead and "ribs." The shock cord will be attached with a kevlar loop.

Questions and concerns I have include:

• The fin profile is a bit swept-back. Should the 1/16" G10 hold up below Mach 2, and will fillets be strong enough or will T2T be required?
• Rocksim 9 indicates about 2 calibers of stability at low speeds, while RASAERO II claims around 1.8. I've seen 2 calibers recommended as a rule of thumb for high-speed flights, but will slightly less still work?
• Will the plastic Pinnacle nosecone survive top speeds of Mach 1.5-2.0, or will it be damaged by aerodynamic heating?
• Launching from NCR, which is at about 5,000 feet, a 15,000 foot flight results in the apogee being about 20 kft ASL. At this point will I need to use extra confinement for the apogee charge?
• I'm thinking of screwing the retainer into the airframe instead of epoxying it for easier maintenance and future compatibility with possible EX cases. However, screw heads create extra drag. Is it practical to countersink the hole so that the screws can be flush with the outside of the airframe?

View attachment Hellcat (38mm MD).rkt

 

[AndersLT]

Very cool subbed!

 

[T34zac]

So, 2015 has been kind of a hit or miss year for me rocketry-wise. There have been a couple crashes and a bunch of cancelled launches, but I finally got my level 1. Now, my plans for this Spring are fairly simple: make a new drop-in avbay for my Level 1 and most future high-power rockets I build, finishing fixing up for MPR cluster model I crashed last spring and add electronic deployment so I can actually fly it as a cluster, and hopefully build a 24mm or 29mm MD.

But after that, in the 2016-2017 Academic year, I don't really have a major project on the to-do list. So... how about going as high as possible on I impulse?

So, here's my initial design.

Currently, specs are as follows:

• Empty mass should be under 16 oz.
• Length is 32"
• My goal is to reach 2.5 miles (13,200 ft), but sims with RASAERO II suggest that it might be possible to break 15,000 feet.
• The airframe will be 38mm thin-wall G12 from Rocketry Warehouse. I'm guessing this will hold up fairly well at the speeds expected of this rocket. My club does make custom fiberglass tubes, but with the coarse cloth available I'm not confident that I could make a tube that's strong, light, and smooth.
• The nose cone will be a 38mm pinnacle.
• The fins will be 1/16" G10 with about a 1/4" bevel on the leading and trailing edges.
• Recovery will be dual-deploy with a Raven 3 firing a BP charge at apogee and a Cable Cutter around 700 feet, and a 24" chute. I'm looking at a Top Flight thin mill as an option.
• The avionics bay will be mounted in the nose cone, and aside from the Raven will use either an Eggfinder or, if I get my HAM license and have money an Adept tracker for location of the rocket.
• For motor retention I was looking at the AeroPack MD retainer. However, 1.5 oz is a little heavy when the rocket is under a pound. I think I can make something lighter but still with plenty of strength using a bit of fiberglass coupler, a weld nut, some allthread, and scrap G10 or plywood for a bulkhead and "ribs." The shock cord will be attached with a kevlar loop.

Questions and concerns I have include:

• The fin profile is a bit swept-back. Should the 1/16" G10 hold up below Mach 2, and will fillets be strong enough or will T2T be required?
• Rocksim 9 indicates about 2 calibers of stability at low speeds, while RASAERO II claims around 1.8. I've seen 2 calibers recommended as a rule of thumb for high-speed flights, but will slightly less still work?
• Will the plastic Pinnacle nosecone survive top speeds of Mach 1.5-2.0, or will it be damaged by aerodynamic heating?
• Launching from NCR, which is at about 5,000 feet, a 15,000 foot flight results in the apogee being about 20 kft ASL. At this point will I need to use extra confinement for the apogee charge?
• I'm thinking of screwing the retainer into the airframe instead of epoxying it for easier maintenance and future compatibility with possible EX cases. However, screw heads create extra drag. Is it practical to countersink the hole so that the screws can be flush with the outside of the airframe?

To answer a couple of your questions:

-The G10 fins should be fine, although T2T never hurts at that speed.

-The pinnacle nose cone is iffy. I use a polycarbonate nose cone on my 38mm MD.

-Using countersunk screws flush with the airframe is a good option. It eliminates the drag as much as possible. I did this with my 54mm MD to mount the av-bay.

-At 20,000' I don't think you need anything extra for your apogee charge, but hopefully someone a bit more knowledgeable on the subject chimes in.

-I wouldn't trust Rocksim with transonic and supersonic flights, it hasn't been known to handle it well. However, if Rasaero says its stable, then you can probably bet it's stable. I use OR for all my sims since I run a Mac and Rasaero is unavailable to me.

Hope this helps.

 

[Aksrockets]

[*] The fin profile is a bit swept-back. Should the 1/16" G10 hold up below Mach 2, and will fillets be strong enough or will T2T be required?
[*] Rocksim 9 indicates about 2 calibers of stability at low speeds, while RASAERO II claims around 1.8. I've seen 2 calibers recommended as a rule of thumb for high-speed flights, but will slightly less still work?
[*] Will the plastic Pinnacle nosecone survive top speeds of Mach 1.5-2.0, or will it be damaged by aerodynamic heating?
[*] Launching from NCR, which is at about 5,000 feet, a 15,000 foot flight results in the apogee being about 20 kft ASL. At this point will I need to use extra confinement for the apogee charge?
[*] I'm thinking of screwing the retainer into the airframe instead of epoxying it for easier maintenance and future compatibility with possible EX cases. However, screw heads create extra drag. Is it practical to countersink the hole so that the screws can be flush with the outside of the airframe?
[/LIST]

1. Im just going to go ahead and say that T2T is basically almost never necessary on anything smaller then 54. Most 54s don't even need T2T. Focus on good strong attachments and ample fillets and you'll be just fine.
2. You're good. RAS 2.0 actually looks great for high speed stability.
3. Yes. It might get a little toasted, but it's fine. I flew an estes big daddy NC to 1.7ish without any problems. That was a thin walled 3in. You're flying a thick walled 1.5in.
4. Probably not, but it couldnt hurt.
5. Masking/electrical tape makes a nice motor retainer.

 

[AndersLT]

To answer a couple of your questions:

-The G10 fins should be fine, although T2T never hurts at that speed.

-The pinnacle nose cone is iffy. I use a polycarbonate nose cone on my 38mm MD.

-Using countersunk screws flush with the airframe is a good option. It eliminates the drag as much as possible. I did this with my 54mm MD to mount the av-bay.

-At 20,000' I don't think you need anything extra for your apogee charge, but hopefully someone a bit more knowledgeable on the subject chimes in.

-I wouldn't trust Rocksim with transonic and supersonic flights, it hasn't been known to handle it well. However, if Rasaero says its stable, then you can probably bet it's stable. I use OR for all my sims since I run a Mac and Rasaero is unavailable to me.

Hope this helps.

+ 1 to T2T and screwing the retainer into the airframe

 

[T34zac]

+ 1 to T2T and screwing the retainer into the airframe

Here's a hint though, if you want the best MD possible, ditch the retainers. Use friction fit. Less weight, shorter airframe, higher altitude.

 

[smapdiage9]

Friction fit works super well and saves a couple inches of internal space versus a md forward retainer. Less slick, but you can make the rocket shorter which makes it more aerodynamically slick.

 

[Clusterphiliac]

To answer a couple of your questions:

-The G10 fins should be fine, although T2T never hurts at that speed.

-The pinnacle nose cone is iffy. I use a polycarbonate nose cone on my 38mm MD.

-Using countersunk screws flush with the airframe is a good option. It eliminates the drag as much as possible. I did this with my 54mm MD to mount the av-bay.

-At 20,000' I don't think you need anything extra for your apogee charge, but hopefully someone a bit more knowledgeable on the subject chimes in.

-I wouldn't trust Rocksim with transonic and supersonic flights, it hasn't been known to handle it well. However, if Rasaero says its stable, then you can probably bet it's stable. I use OR for all my sims since I run a Mac and Rasaero is unavailable to me.

Hope this helps.

1. Im just going to go ahead and say that T2T is basically almost never necessary on anything smaller then 54. Most 54s don't even need T2T. Focus on good strong attachments and ample fillets and you'll be just fine.
2. You're good. RAS 2.0 actually looks great for high speed stability.
3. Yes. It might get a little toasted, but it's fine. I flew an estes big daddy NC to 1.7ish without any problems. That was a thin walled 3in. You're flying a thick walled 1.5in.
4. Probably not, but it couldnt hurt.
5. Masking/electrical tape makes a nice motor retainer.

1. Regarding fillets: in other threads, I've seen people suggest that a radius of 4-8% of the root edge is optimal for drag purposes. For this rocket, this translates to about a 1/2" to 3/4" diameter. Should this be strong enough, or is it better to go up to a 1" diameter (I prefer diameter because that's how big a washer I need to smooth the fillets out)? I'd most likely use Aeropoxy: would glass microbubbles or powdered silica be good thickeners, or should I use chopped fiberglass to strengthen the epoxy a little? (I'd prefer to use the other thickeners if possible since chopped fiberglass is a PITA to get smooth both before and after curing). Are there any techniques for strengthening a surface-mount attachment besides "sand the tube and fin"?

3. Hmm... is the polycarb nose cone this one? I might actually go with that since it's only slightly more expensive than the Pinnacle, it's a Von Karman, and it looks easier to put an avbay into.

Also, is it worth covering the surface of a plastic nose cone with some kind of heat-resistant material to reduce possible erosion? E.G. putting a coat or two of BBQ paint on over the primer and under the colored coat?

Here's a hint though, if you want the best MD possible, ditch the retainers. Use friction fit. Less weight, shorter airframe, higher altitude.

Hmm... if I used friction fit, would the shock cord be attached to the end of the motor case?

Also, regarding friction fitting:

According to RASAero, the drag at burnout is about 16 lbs on an I216, and assuming no venting of the parachute compartment there is an additional 4 lbs of force from internal pressure. Adding a 50% safety margin, it should take about 30 lbf to separate the nose cone.

So...

1. For purposes of consistency and angular alignment of altimeter vent holes in the airframe and nose cone shoulder, I would like to attach the nose with shear pins. However, 2-56 nylon screws take 20-25 lbs to shear, so two of them would require a force of 40-50 lbs. More force to separate the nose cone = more force that the motor friction fit must withstand. I've heard that using a single shear pin is a bad idea, so are there things which shear at somewhere in the 10-15 lbf range that I could use?

2. If the motor case is a "tight" fit before launch (defined as I can semi-vigorously shake the rocket up and down without the motor falling out, but can still remove the motor by hand with a little twisting), will thermal expansion of the case when the motor burns make it tight enough to withstand the pressure from the ejection charge?

 

[smapdiage9]

You can use an internal bulkhead/cr or other supporting device to retain the shock cord while still occupying a bit less space than the hardware require for forward retention. A small amount of coupler with an anchor like a metal rod running across its inner diameter could be made very short and strong and even still allow motor ejection gasses to pass forward.

When I friction fit my 29mm motors I shim with masking tape to the point that it requires a fair effort to twist the motor in all the way. Not enough to make me want to get the sledgehammer out, but enough that it would be quite difficult to remove by hand. They rarely move at all from the ejection charge, and since I run the tape in a spiral along the length of the case I've never had one push out by more than a few mm.

But the real way to prevent it from popping out at all is to use aluminum tape wrapped over the joint of thrust ring and body tube.

 

[T34zac]

1. Regarding fillets: in other threads, I've seen people suggest that a radius of 4-8% of the root edge is optimal for drag purposes. For this rocket, this translates to about a 1/2" to 3/4" diameter. Should this be strong enough, or is it better to go up to a 1" diameter (I prefer diameter because that's how big a washer I need to smooth the fillets out)?

a 1/2" diameter fillet should be fine. It's what I used on my MD. I also go by the 4-8%.

I'd most likely use Aeropoxy: would glass microbubbles or powdered silica be good thickeners, or should I use chopped fiberglass to strengthen the epoxy a little? (I'd prefer to use the other thickeners if possible since chopped fiberglass is a PITA to get smooth both before and after curing). Are there any techniques for strengthening a surface-mount attachment besides "sand the tube and fin"?

I don't know really that much about Aeropoxy, so I'll let someone else chime in on that. I used Proline 4500, and my next will likely use Rocketpoxy G5000.

Sanding tube and fin is what I did. I don't think there would be much of an issue. But if your worried, T2T is a bulletproof option.

3. Hmm... is the polycarb nose cone this one? I might actually go with that since it's only slightly more expensive than the Pinnacle, it's a Von Karman, and it looks easier to put an avbay into.

Yes that's the nose cone I used. The 54mm version (made the same way) has gone to over Mach 2 before without any added treatment other than regular spray paint.

Also there is a neat little av-bay sled that you can get for those of your interested in that includes a charge cup. https://www.ape-rc.com/#!/Head-End-...Molded-Nose-Cone/p/49409110/category=11909535

Also, is it worth covering the surface of a plastic nose cone with some kind of heat-resistant material to reduce possible erosion? E.G. putting a coat or two of BBQ paint on over the primer and under the colored coat?

I just answered above.

Hmm... if I used friction fit, would the shock cord be attached to the end of the motor case?

You can do that. My motor is just friction fit in. The av-bay sits on top of it and the shock cord is attached to that.

Also, regarding friction fitting:

According to RASAero, the drag at burnout is about 16 lbs on an I216, and assuming no venting of the parachute compartment there is an additional 4 lbs of force from internal pressure. Adding a 50% safety margin, it should take about 30 lbf to separate the nose cone.

So...

1. For purposes of consistency and angular alignment of altimeter vent holes in the airframe and nose cone shoulder, I would like to attach the nose with shear pins. However, 2-56 nylon screws take 20-25 lbs to shear, so two of them would require a force of 40-50 lbs. More force to separate the nose cone = more force that the motor friction fit must withstand. I've heard that using a single shear pin is a bad idea, so are there things which shear at somewhere in the 10-15 lbf range that I could use?

2. If the motor case is a "tight" fit before launch (defined as I can semi-vigorously shake the rocket up and down without the motor falling out, but can still remove the motor by hand with a little twisting), will thermal expansion of the case when the motor burns make it tight enough to withstand the pressure from the ejection charge?

I don't use shear pins on my MD, but I also don't put the electronics there. Therefore I can't really help you. I'd say ground test with the motor being unused, until your satisfied it won't come out.