An Obsession

Yes, this stuff is an obsession.

Now here is a look at THE Obsession. She is 9 feet 4 inches long. She has a 3" waistline. She can be flown on 54mm or 38mm Propulsion. The Obsession weighed 7 pounds no motor in the form shown in this pic, and she is currently being glassed and will pic up 1 pound after that is completed. She has a 1700-6000 foot flight envelope on Contrail 54mm Hybrids and this envelope can be expanded on APCP motors.

The flight shown here was her maiden voyage at Freedom Launch 2007 and she went 3,884 feet ona J416 Contrail Sparky II Hybrid Motor.

The Obsession was designed and scratch built by me. It uses all Giant Leap Components. It has a 5:1 Ogive Pinnacle NC, 48" Flex Phenolic Booster BT, 48" Flex Phenolic Payload slit into two 24" sections where the Avbay sits, she has a 8" zipperless removeable coupler, 36" MMT, 3 Birch Ply CR's, .093 G10 Fins, and 3" to 54mm Slimline Conical Boatail Retainer.

The Obsession Carries a Perfectflite HA45K (Primary), and a Ozark ARTS1 Backup. She uses 20 feet of 1/4" Payload TK, 25 feet of Booster TK, Fireball Anti-Zipper Device (Payload Area), 18" Drogue, and she had a 60" Spherachute Plus on her first flight. The main will be replaced with a larger chute for future flights.

I'd like to thank Contrail Rockets and Both Tom's for all their help, and the folks on the Hybrid Rockets Yahoo Group for all their help. Doug Pratt at Pratt Hobbies, man that guy has the best GSE equipment, and was a big help getting it ordered and going. I'd also like to thank the many fine folks who have helped me at the launches, and helped me here at this forum in my absence. To the guys at The Freedom Launch, man, you guys are the greatest! This would've never happened without the many fine people who freely gave their time to help me.

Here's a liftoff Shot Of The Obsession.

Welcome back and awesome rocket!!!

What was the top speed?

She got up to 475.53 ft/s on that flight. The ARTS said 11G which would be on liftoff.

Rock_It said:

She got up to 475.53 ft/s on that flight. The ARTS said 11G which would be on liftoff.

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Is that kind of accuracy really valid within the limits of the altimeter?

Regardless - nice flight
I'm surprised that with that weight and size though that the max altitude would only be 6k on hybrids. I would expect more like 8-10 (or more) on a decent K motor.

On the Hybrids the motor weight is about 4 pounds or there abouts. The all up weight before tanking was 12 pounds.

On the ARTS accuracy, it's pretty dang accurate. That was read off the Data Analyzer program. The ARTS has a 200 Hz sampling frequency.

That velocity number I posted was taken from the "Flight Data Summary Page" in the software.

I simmed it on a K1275R just for kicks and got .98 mach. I think it'll push through it though. We may have to see one day, but not before a bunch more flights. It was really designed for the Hybrids. It has room for the 48" motors. They will stick up above the MMT 12" though. That's why I added the zipperless coupler. It gives me the room to go full length. Contrail makes a K555 that sims at 6000 with the extra pound I'm gonna pick up with the glass. I'm using a 10.8 oz Soller sleeve down to the fins, and a 8.4 oz cloth for the fin area. I know that's not the way it's done, but I thought I'd try the rocket without the glass. Had a small zipper and some minor zipperage of the zipperless coupler bolt holes so that wasn't a good idea. The 6000 foot number is figured after glassing. That would be a 13 pound rocket with full length motor.

Rock_It said:

On the Hybrids the motor weight is about 4 pounds or there abouts. The all up weight before tanking was 12 pounds.

On the ARTS accuracy, it's pretty dang accurate. That was read off the Data Analyzer program. The ARTS has a 200 Hz sampling frequency.

That velocity number I posted was taken from the "Flight Data Summary Page" in the software.

I simmed it on a K1275R just for kicks and got .98 mach. I think it'll push through it though. We may have to see one day, but not before a bunch more flights. It was really designed for the Hybrids. It has room for the 48" motors. They will stick up above the MMT 12" though. That's why I added the zipperless coupler. It gives me the room to go full length. Contrail makes a K555 that sims at 6000 with the extra pound I'm gonna pick up with the glass. I'm using a 10.8 oz Soller sleeve down to the fins, and a 8.4 oz cloth for the fin area. I know that's not the way it's done, but I thought I'd try the rocket without the glass. Had a small zipper and some minor zipperage of the zipperless coupler bolt holes so that wasn't a good idea. The 6000 foot number is figured after glassing. That would be a 13 pound rocket with full length motor.

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Yes, it records to that accuracy, however the way that it computes it is by summing the acceleration values. This sums the errors as well, though it's probably good to +/- 5fps or so without much difficulty. I have 2 altimeters that record at 200Hz myself - sometime I'll need to fly both next to each other and compare all the fancy data (one is a Parrot, the other an R-DAS tiny).

I'm still surprised about those numbers BTW - I have flown a rocket that was 16lbs at liftoff and 4" diameter to 5120 on a small to mid K motor (K550). I would have expected quite a bit more from this with both the smaller diameter and the lighter weight (though the contrail specific impulse numbers for several motors are hardly impressive - that could easily be the cause of the numbers below my expectations).

Is it definitely limited to the 54mm stuff though? The Ratt K240 is amazing, though it is an odd size (64mm by 35").

cjl,
watch your software. There are lots of tricks they use in these software programs to fix little innacuracies. That might make some things not seem right if you test two units against each other. I know the Data Analyzer the ARTS uses does some things. It's hard to get into here as it would get into a looooong discussion on math. Just watch for that.

Hybrids are much different than APCP motors. They can vary by a great deal. One thing I've learned already is there is an art to them. I've been very careful to keep pressures within a narrow range, and stuff like that, but there is still much to learn with them. Just to give you an idea. My Rocksim for that flight was 3,500 almost right on the button. It went 3,884! I really didn't want it to go that high, but it did. heh In comparison, my Talon 3 with a J570 in it went 4,441 feet. That motor is awesome BTW!

The Obsession has a 54mm MMT so yes that is as large as it will hold with the booster it has now. RATT's I'd have to be careful with. Those things seem to be designed for serious altitude! I saw what they got at LDRS and that is CRAZY altitude in that kind of heat with a Hybrid. That Tribrid M is insane! :lol: I designd this rocket to havew a wide flight envelope so it could be flown out of the smaller fields too. The fins are slightly larger than needed. It has about 6-6.5 calibers of stability. It needs the extra stability too. It has over a 30:1 L : D ratio so I had to watch the dynamic stability with it. It doesn't change much in flight but I wasn't gonna take any chances being new to this real high power stuff. I'm getting into areas now where if something goes wrong it's gonna be a real bad day so I'm being a little conservative.

Rock_It said:

cjl,
watch your software. There are lots of tricks they use in these software programs to fix little innacuracies. That might make some things not seem right if you test two units against each other. I know the Data Analyzer the ARTS uses does some things. It's hard to get into here as it would get into a looooong discussion on math. Just watch for that.

Click to expand...

The software can't do a thing about slight sensor variation, and because of this, there will always be some error. Not enough to matter, but enough to make readouts to the hundredth of a fps completely pointless. I would definitely say that they are on to +/- 5fps or so though without trouble. If I knew the sensor, as well as its accuracy and resolution specs, I could do more analysis to give the exact number.

Rock_It said:

Hybrids are much different than APCP motors. They can vary by a great deal. One thing I've learned already is there is an art to them. I've been very careful to keep pressures within a narrow range, and stuff like that, but there is still much to learn with them. Just to give you an idea. My Rocksim for that flight was 3,500 almost right on the button. It went 3,884! I really didn't want it to go that high, but it did. heh In comparison, my Talon 3 with a J570 in it went 4,441 feet. That motor is awesome BTW!

Click to expand...

Yes, they are different, but the contrails in particular have terrible Isp's in many cases, even compared against other hybrids (such as Hypertek). As for the rocksim, have you looked at the measured thrust curve and coefficient of drag data from the ARTS? How long does that record at 200Hz - I can't remember the specs on it. My 2 alts will both record for several minutes - the Parrot for 8mins @200Hz (followed by a low rate measurement at a couple of hz for a lot longer, in case the flight outlasts the 8 mins), and the R-DAS has ~30mins of memory @200Hz. As long as the ARTS has enough to get good clean data at least through apogee though, you should be able to get a decent Cd and thrust curve plot, which can help determine the location of the errors in the sim. Also, check the finish on the parts in rocksim - my Cirrus simmed to 10500, and got 11950, until I realized that the surface finish in rocksim should have been set to polished. After that change, the sim was around 0.1% off of the measured flight data.

As for your Talon, that thing must have been a brick (relatively speaking) - I've seen a clubmember's Talon 4 go roughly that same altitude on a J350. That is certainly an awesome motor though

Rock_It said:

The Obsession has a 54mm MMT so yes that is as large as it will hold with the booster it has now. RATT's I'd have to be careful with. Those things seem to be designed for serious altitude! I saw what they got at LDRS and that is CRAZY altitude in that kind of heat with a Hybrid. That Tribrid M is insane! :lol: I designd this rocket to havew a wide flight envelope so it could be flown out of the smaller fields too. The fins are slightly larger than needed. It has about 6-6. calibers of stability. It needs the extra stability too. It has over a 30:1 L ratio so I had to watch the dynamic stability with it. It doesn't change much in flight but I wasn't gonna take any chances being new to this real high power stuff. I'm getting into areas now where if something goes wrong it's gonna be a real bad day so I'm being a little conservative.

Click to expand...

Just be careful with that and hybrids - in many cases (as I'm sure you have noticed), the hybrids have relatively low liftoff thrust. This combined with a long skinny overstable rocket could cause significant weathercocking in any kind of wind - definitely use a nice long rail. I have several designs with similar calibers of stability though, and they definitely work fine. I simply don't tend to fly them on the windier days.

A K550 would be great in that thing IMO, if you do want to fly solid sometime. Wonderful motor. K700's are nice too (well, both the AT and the Animal, but I'm specifically referring to the Aerotech), but might be a bit much.

Ahhh, that's why I went with the Contrails. Liftoff thrust. There are some long burn motors that I'd use a 12' rail with, but Contrail makes butt kicking load lifters too. That J416 had a good balance of liftoff thrust and altitude. A freind of mine flew a K555 down there and it's impressive. As soon as I get a couple of flights on this thing, that K555 will be flown in it at Orangeburg.

I avoid the wind like the plague! We had wind down there at the Freedom Launch, but I just walked around, helped some guys, looked at stuff, prepped a little, and waited it out. I normally fly in the afternoon. I like it when it's calm. There was a little more wind when I flew my Talon the next day and it landed in the woods back behind the pads down there. Took a 1/2 hour to find it. It was laid out nicely accross a trail back in there. Both went straight as an arrow. The down part is why I avoid the wind.

That is an advantage of the contrails, but also a disadvantage. That high thrust, short burning characteristic biases it towards acting like a monopropellant in many ways, and decreases energy and efficiency for a given motor size (which is why they need a 48" long motor in the first place). If you care more about thrust than efficiency though, they are a great choice. Of course, you could fly an Aerotech J1999 and have thrust and efficiency

You may want to look at the definitions of precision and accuracy. Precision looks like the correct term for the ARTS II. Accuracy I doubt it that good.

Doug

Nice write up on the Obsession Rock_it...welcome bacl too!

That first pic of the "O" makes her look like a 54mm MD rocket...

Johnnie

dknight said:

You may want to look at the definitions of precision and accuracy. Precision looks like the correct term for the ARTS II. Accuracy I doubt it that good.

Doug

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It's probably decent in accuracy, just not to the full reported level.

dknight said:

You may want to look at the definitions of precision and accuracy. Precision looks like the correct term for the ARTS II. Accuracy I doubt it that good.

Doug

Click to expand...

Doug is correct. What we were discussing was decimal precision which the unit reports to 2 decimal places of precision...not accuracy. Accuracy to two decimal places? Probably not. I just report what the unit reports, because it's as good as I can get without going to alot of trouble.

cjl said:

That is an advantage of the contrails, but also a disadvantage. That high thrust, short burning characteristic biases it towards acting like a monopropellant in many ways, and decreases energy and efficiency for a given motor size (which is why they need a 48" long motor in the first place). If you care more about thrust than efficiency though, they are a great choice. Of course, you could fly an Aerotech J1999 and have thrust and efficiency

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At the mass fractions that are used in our hobby rockets ISP is not very significant. Sim it for yourself, in a typical 10-15# rocket the difference in altitude between an ISP of 120 and 200 is only about 10% in the J-K impulse range.

jderimig said:

At the mass fractions that are used in our hobby rockets ISP is not very significant. Sim it for yourself, in a typical 10-15# rocket the difference in altitude between an ISP of 120 and 200 is only about 10% in the J-K impulse range.

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A motor with a ISP of 120 will be about one impulse range below a motor with a ISP of 200. So a 120 ISP motor that is J impulse while a motor of the same size and weight with a ISP of 200 would be a K.

Hi John,
I tend to agree with that statement. I've found through messing with the numbers that Total Impulse and Average Impulse seems to be a better measure of performance for what we are doing.

Through research and study I've found that there are theoretical cases where Isp's of 285 are possible. Solids perform pretty well, but their Isp numbers are considerably lower than Liquid propellents. In the case of the space shuttle it uses both to perform it's mission of high thrust boost at liftoff, and then stage to liquid only propulsion after the solids are consumed. Isp's are useful in some cases, but I don't think they are everything when it comes to these motors we fly. Everything is relative. It's whatever does the job.

All of these motors have their pluses and minuses. It is a tradeoff with every change that is made to them. If you want long burn, you're gonna give up peak thrust. If you want peak thrust, you're gonna give up burn time. I'm talking hobby propellants here.

When I pick a motor, I pick a motor based on the performance I'm looking for. On the first flight of the Obsession, I wanted a 2,500 flight, and good liftoff acceleration. I chose the J416 because it gave the liftoff performance I needed. It did sim out to 3,500 though and I settled for that. Going 3,884 feet was a bonus. I really did not want it to go that high but it sure was nice getting that ALT when it got back.

I am also aware that Contrail is currently waiting on their Platinum Performace fuels to be certified. There are lots of choices with that system, and it's why I chose it.

Rock_It said:

Hi John,
I tend to agree with that statement. I've found through messing with the numbers that Total Impulse and Average Impulse seems to be a better measure of performance for what we are doing.

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Assuming of course that the motor desired physically fits in the rocket...

That's my biggest problem with Contrail's Isp - I don't really have anything that fits a 48" long 54mm case, but I do have rockets that would take that much impulse and more.

Rock_It said:

Through research and study I've found that there are theoretical cases where Isp's of 285 are possible. Solids perform pretty well, but their Isp numbers are considerably lower than Liquid propellents. In the case of the space shuttle it uses both to perform it's mission of high thrust boost at liftoff, and then stage to liquid only propulsion after the solids are consumed. Isp's are useful in some cases, but I don't think they are everything when it comes to these motors we fly. Everything is relative. It's whatever does the job.

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What types of propellants are you talking about?

For solid fuel, 260 or so is about the practical limit, and most hobby AP motors are in the 180-220 range. IIRC, W9 is the highest of all commercial propellants and it is around 230-240. For liquids, 400 is relatively easy with LH2/LOX, and 450 is attainable. Mid 500's is doable, but only with nasty propellants such as liquid lithium, fluorine, and a little hydrogen thrown in.

Rock_It said:

All of these motors have their pluses and minuses. It is a tradeoff with every change that is made to them. If you want long burn, you're gonna give up peak thrust. If you want peak thrust, you're gonna give up burn time. I'm talking hobby propellants here.

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Well, not always, but in most cases that is true. For a counterexample, look at the Apogee motors such as the F10. Plenty of kick of the pad with a decent spike, followed by a long tail. A similar case also occurs with the Loki K350 (amazing motor).

Rock_It said:

When I pick a motor, I pick a motor based on the performance I'm looking for. On the first flight of the Obsession, I wanted a 2,500 flight, and good liftoff acceleration. I chose the J416 because it gave the liftoff performance I needed. It did sim out to 3,500 though and I settled for that. Going 3,884 feet was a bonus. I really did not want it to go that high but it sure was nice getting that ALT when it got back.

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As do most people. As for not wanting to go that high, why? It isn't that high for dual deployment, and it is if anything safer than a lower altitude flight, as it gives it more time to assume a stable configuration under drogue. I don't think I've ever seen a dual deploy flight under 4k lost before.

Rock_It said:

I am also aware that Contrail is currently waiting on their Platinum Performace fuels to be certified. There are lots of choices with that system, and it's why I chose it.

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Yes, there are lots of choices. The main reason I don't fly them is the aforementioned problem with physical size in the impulse ranges I design my rockets for.

As for platinum performance, what exactly will this fuel be? High Isp? High thrust? Something else? I haven't heard anything about it before...

Bullpup said:

A motor with a ISP of 120 will be about one impulse range below a motor with a ISP of 200. So a 120 ISP motor that is J impulse while a motor of the same size and weight with a ISP of 200 would be a K.

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You are equalizing the wrong quantities.

Take a 10# rocket with 1# of propellant. Lets say the ISP is 200. That would put it in about a mid J range. So fly that 11# rocket on that J.

Now fly it with it with a propellant with an ISP of 100 or half the previous. To get the same impulse you will need an extra 1# of propellant. Now the rocket with the same impulse motor will weigh 12# or 1/11th more. So the rocket will fly about 1/11th less altitude. Actually the difference will be less than this because the flight will be slower and less drag will be effected.

The effect of ISP is almost negligible. It's within the variation of motor impulse and the altimeter.

cjl said:

Assuming of course that the motor desired physically fits in the rocket...

That's my biggest problem with Contrail's Isp - I don't really have anything that fits a 48" long 54mm case, but I do have rockets that would take that much impulse and more.

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That's the general issue with hybrids. You usually build the rocket to fit the motor.

cjl, the reason on the ALT is because I work up in progressive steps. Another reason is because of the area I fly in. This is not the desert here. There are trees everywhere. Even at Orangeburg which is an exceptional field there are trees. It is a completely different kind of flying, flying at or near sea level east of the Mississippi then it is out west. Wide open out there a rocket could land miles away and not be seen on decent and not be lost. Here in the east, a rocket can be seen, come down, and never be found again. I've seen rockets lost that were LPR at some fields where they only went 800 feet went down in the woods and were never found or were found months later.

This is why I fly lower, and in less wind, and get these rockets going off the pad so they go straight at the angle I launch them at. You've got to be conservative around here. Guys that do these High ALT flights around here are really good at getting them to go precisely where they want them, but they've worked up to that. A mile in the desert or some of the large fields out west is nothing to recover a rocket. Here, a mile means lost unless you are lucky and can still get a signal.

The platinum performance is a new fuel developed by Contrail. I don't know speciffics, but there are some examples shown on their website. We probably won't know details until they are certed. It looks like it has pretty good average thrust. I'm sure it has good peak thrust too as that is what Contrails are known for.

jderimig said:

You are equalizing the wrong quantities.

Take a 10# rocket with 1# of propellant. Lets say the ISP is 200. That would put it in about a mid J range. So fly that 11# rocket on that J.

Now fly it with it with a propellant with an ISP of 100 or half the previous. To get the same impulse you will need an extra 1# of propellant. Now the rocket with the same impulse motor will weigh 12# or 1/11th more. So the rocket will fly about 1/11th less altitude. Actually the difference will be less than this because the flight will be slower and less drag will be effected.

The effect of ISP is almost negligible. It's within the variation of motor impulse and the altimeter.

Click to expand...

Well that is one way to look at it. For me, I have X amount of space in a rocket for a motor. I fill that void with motor. I don't have unlimited space to try make sugar motors look on par with APCP. Apples and oranges to me.