Altitude control problem. Help!

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I'm assuming this is for the ESRA IREC/Spaceport America Cup competition. What school are your from?

Under-filling an M hybrid to get from 25K to 10K will not work under the stability margin requirements and thrust-to-weight minimum. It might also disqualify you from the "COTS" motor category. If this is for the student-built motor category, starting with a Contrail motor will probably disqualify you. But, I bet nobody will check any of that.

No test flight is required for IREC. No experience needed. An experienced mentor isn't required. These (among other reasons) are why I didn't go last year and won't be there this year even though I live nearby.

I understand the COTs thing, but underfilling a hybrid should not affect the thrust at all (or minimally), it will just affect the burn time I think. If you reduce the tank volume the liquid pressure is unchanged.
 
The startup thrust for an underfilled hybrid does not decrease. The burn time decreases. The mass is decreased. The thrust curve is altered a bit. The thrust to weight is increased. The mass of unburned fuel at the end of the burn is also increased. But yes the CG will be quite different!

Gerald
 
Just put something draggy on the outside of the rocket, maybe use tube fins. MikeC has a good idea too... maybe just a big round disk at the end of the fins... you're gonna need really good glue joints, though.
 
Grid fins can give you the ability to tune the drag to your needs. You just block more or less of the grid squares to up or decrease the drag.
 
I'm assuming this is for the ESRA IREC/Spaceport America Cup competition. What school are your from?

<snip>

No test flight is required for IREC. No experience needed. An experienced mentor isn't required. These (among other reasons) are why I didn't go last year and won't be there this year even though I live nearby.

This is the major concern I have with IREC also…
 
+1 on Cris and Mike C with drag components at the bottom of the rocket. Flared cone transition, tube fins, or even side pods made of tubes with nose cones. One of our members is working on his L3 with a Delta 4 heavy. It has a central 4&#8221; tube, two side 4&#8221; tubes and tube fins all around. When the motor burns out it decelerates dramatically like it&#8217;s slamming on the brakes. If it was a cartoon you&#8217;d hear squealing tires.
 
This is the major concern I have with IREC also&#8230;

I'm not familiar with IREC- is there a low amount of oversight at their launches?


To the OP- the answer here is clear- you need a different motor based on your design parameters. Otherwise you're going to end up with a rocket that will have a hard time passing an RSO without a whole lot of data backing up your design decisions.
 
Another idea, cant the fins and spin the rocket like crazy. Divert motor energy into some angular energy instead of linear energy.
 
The ESRA IREC/Spaceport America Cup competition is not a well run affair in my opinion for many of the reasons noted......but that's a subject for another thread.
 
... To the OP- the answer here is clear- you need a different motor based on your design parameters. Otherwise you're going to end up with a rocket that will have a hard time passing an RSO without a whole lot of data backing up your design decisions.

I don't think an RSO would have any trouble with a drag plate at the aft end as long as the glue joints looked good. It would be nice to see a successful flight first or see first flight on a smaller motor than an M though. The OP would need to do that anyway most likely to calibrate the amount of drag. You could get close with calcs, but getting dead on might be tough.

Experienced mentors stifle innovation.

I don't agree with that at all. An experienced mentor may not have the flexibility that you need for innovation, but I don't think that's true of all experienced mentors. An experienced and flexible mentor is how you get to innovation that works, IMHO.
 
I'm not familiar with IREC- is there a low amount of oversight at their launches?


To the OP- the answer here is clear- you need a different motor based on your design parameters. Otherwise you're going to end up with a rocket that will have a hard time passing an RSO without a whole lot of data backing up your design decisions.

When I had to do research on HPR rockets during design phase before hands on build time, we had to research other reports, and also try to read basic materials related to HPR design and anything practical we could get our hands on. We read some IREC reports of teams flying stuff so Overstable it didn't clear the tree tops. I read some that went really high Mach and they were thinking it was stable and the cp shifted enough to bend and destroy aluminum airframes.
For SEDS competitions their oversight was limited to a few skype calls maybe under four hours. The rest was whatever students could learn without practical hands on experience through academia report bs... We were trying to get a prof L-1 certified before we were launching a project rocket, and it was our first multistage. We had mentors bail. Thank God I got stability down. None of our designs were unstable. We ended up buying kits and trying to put those together before the build of an HPR airframe to teach ourselves crap. Had to teach ourselves, open rocket, HPR concepts, and AeroFinSim. Motor selection practically screwed up first flight only looked at alt and didn't know more burn time was better for lower G's. And these student teams usually got a budget to do one flight. It legit sucks. My team chipped in and did a second flight because the first flight was for us to gain experience. We got kinks worked out as we gained experience. By trying new designs we had more problems than usual which requires more iteration of bugs. They expect the world when you don't know squat. And what you don't know could hurt you so they say sign a death waiver. They want all the credit when the university launch goes well but they don't really care if it goes bad. You are basically assigned something outside of your skill level and accept the fact your professor is liable if you screw up design and get someone hurt.

The new teams that launch stuff can't even understand how the rocket behavior is in a crosswind. They just don't "get" it. We had to explain what we learned in the field for reasons why some theory crap doesn't work practically. And the timeframe the college wants projects done on top of other courses is just hard. Your lucky to have an airworthy rocket launch actually happen. We designed stuff but honestly practically we learned more by launching them in the field. Everyone have feedback. It would be a simple thing we didn't have experience on that would basically screw up a flight.
Now imagine teams starting with M motor and they don't have an Estes build or launch experience or a mentor, it's an accident waiting to happen. Had a team wire a freakin altimeter this semester as motor eject both stages. I literally screamed. They did it anyways. This next team is actually going to use all channels on Raven like I did and other did last year.
 
I don't agree with that at all. An experienced mentor may not have the flexibility that you need for innovation, but I don't think that's true of all experienced mentors. An experienced and flexible mentor is how you get to innovation that works, IMHO.

I forgot to use the sarcasm font.
 
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My group started at supersonic MD and multistage. It was nuts but we figured it out. It's bad when the mentor says I don't have enough experience, I'm out. We refused a free NDA hybrid... That hybrid scared us more than commercial solids with data. No I feel bad for those teams that don't even finish it in the same academic calendar year and some team ends up launching what they originally didn't design do to the complexity.
 
I understand the COTs thing, but underfilling a hybrid should not affect the thrust at all (or minimally), it will just affect the burn time I think. If you reduce the tank volume the liquid pressure is unchanged.

Underfilling to the extent that a 25K flight goes to 10K will give a large gaseous volume of nitrous oxide in the tank. That is an unpredictable condition for thrust curve prediction. Also, the temperatures at the Spaceport America Cup launch in June will be over 100F. It wouldn't be the first tank to blow with a difficult partial fill. (I've been too close to two of them).
 
The nitrous injector could be starved by a partially filled tank and all that spinning. :-O
That Jennings SEDS judge guy told us how a team put forward canards spin stabilized on a liquid rocket and it exploded and disintegrated at 8k ft. He recommended do not spin stabilize if low experience. Dr. Newman asked if we wanted spin stabilize code, we said no. Loooooooollll... They got a liquid rocket engine to work and the fin spin didn't. Spinning+student launch=Bad. It won't be the spin rate or fin angles. It'll be all that structural analysis garbage that just got tenfold more scary.
 
Underfilling to the extent that a 25K flight goes to 10K will give a large gaseous volume of nitrous oxide in the tank. That is an unpredictable condition for thrust curve prediction. Also, the temperatures at the Spaceport America Cup launch in June will be over 100F. It wouldn't be the first tank to blow with a difficult partial fill. (I've been too close to two of them).

I was suggesting decreasing the tank volume so that it essentially converts the M motor to a "normal" well behaved "L" motor. The tank volume can be decreased by putting objects in tank that displace the volume. Like an N2O inert cylinder (aluminum tube).

Upon further thinking a 100F full fill of that motor might by itself decrease the impulse enough to hit 10K feet.

Also, all hybrid motors have a "large gaseous volume of nitrous" in the tank for over 50% of their burn. They don't blow up.
 
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If you think you are capable of handling and launching a min diameter hybrid M, you should already understand the physics well enough to figure this out on your own. If you do not have mentors with this ability and knowledge, you need to find them locally and not begin by posting here.

I cannot agree with this more. 100^100 upvotes. The competition is less than 80 days away and the team has not made enough progress to figure this out? And skin friction is their best idea?

This year I'm working with three teams and all are in the final fabrication of their vehicles and final subscale flight testing. Some are still testing final full scale motors.

It will be HOT AS BALLS(TM) at the Spaceport. Cooling the supply nitrous tanks is highly encouraged to get reasonable nitrous density.

Edward
 
I forgot to use the sarcasm font.

But, your statement is the goal of ESRA's competition. They do not want to limit the event to NFPA or even TRA Research rules. They have stated that TRA Research rules would not allow students to experiment and innovate. The team's professor with no direct rocket flying experience is good enough. Very easy decision when there is no insurance covering the flights other than the college's bank account or the individuals who help.

There is at least one final check for craziness and safety... a couple of TRA guys dumb enough to be involved (directly or indirectly) with the Spaceport America Cup. :wink:
 
But, your statement is the goal of ESRA's competition. They do not want to limit the event to NFPA or even TRA Research rules. They have stated that TRA Research rules would not allow students to experiment and innovate. The team's professor with no direct rocket flying experience is good enough. Very easy decision when there is no insurance covering the flights other than the college's bank account or the individuals who help.

There is at least one final check for craziness and safety... a couple of TRA guys dumb enough to be involved (directly or indirectly) with the Spaceport America Cup. :wink:

I support several ESRA teams. They are good customers, some teams go through alot of altimeters......

Not all ESRA teams are qualified mentor free, I think the competitive teams have some. Its just not encouraged, curiously.
 
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Just put something draggy on the outside of the rocket, maybe use tube fins. MikeC has a good idea too... maybe just a big round disk at the end of the fins... you're gonna need really good glue joints, though.

Cover it in shag carpet
 
Seriously some of you guys are acting like complete jerks. Calm down and maybe we can get something more productive out of this other than making me quit this hobby because the people act like assholes. If you would like a little context for the dilemma my class group started out designing a rocket for the FAR 1030 competition with the intended goal of 30k. We submitted our proposal, got it accepted, budgeted out the whole damn thing and made the material order. A few days later, the professor "informs" us that we are required to go for 10k instead which incited a bit of a panic as we literally already blew the entire budget and can't just spit out an additional $120 for 5.5" blue tube or whatever would make it go closer to target. At that point I started to look for ways to bleed kinetic energy quickly post burn and mathematically, boosting the skin friction can work, providing it doesn't just melt off (a potential concern because all that energy has to go somewhere). Anyway, we had another chat with the professor and we managed to change the order for the injector components so we can build the L369 instead which makes this much easier. We can also manipulate the impulse a bit when we design and make the nozzle as well. Crisis averted.

Now seriously though if anyone has actually tried using skin friction to stay inside of a waiver or whatever please tell about it. I'm genuinely interested in hearing how that went. Concerning the air brake method, I had thought about that one but didn't have space or budget left to implement it so it got tossed out pretty quickly.
 
You could just open up the nozzle on the Lxxx motor to knock down the impulse. Or add a longer dip tube in the motor so you end up with less liquid nitrous and more gas. Lots of possibilities.

Why the hybrid?

Edward
 
You could just open up the nozzle on the Lxxx motor to knock down the impulse. Or add a longer dip tube in the motor so you end up with less liquid nitrous and more gas. Lots of possibilities.

Why the hybrid?

Edward

The class project is to build and fly a customized 75mm contrail motor. Professor made that decision
 
Seriously some of you guys are acting like complete jerks. Calm down and maybe we can get something more productive out of this other than making me quit this hobby because the people act like assholes. If you would like a little context for the dilemma my class group started out designing a rocket for the FAR 1030 competition with the intended goal of 30k. We submitted our proposal, got it accepted, budgeted out the whole damn thing and made the material order. A few days later, the professor "informs" us that we are required to go for 10k instead which incited a bit of a panic as we literally already blew the entire budget and can't just spit out an additional $120 for 5.5" blue tube or whatever would make it go closer to target. At that point I started to look for ways to bleed kinetic energy quickly post burn and mathematically, boosting the skin friction can work, providing it doesn't just melt off (a potential concern because all that energy has to go somewhere). Anyway, we had another chat with the professor and we managed to change the order for the injector components so we can build the L369 instead which makes this much easier. We can also manipulate the impulse a bit when we design and make the nozzle as well. Crisis averted.

Now seriously though if anyone has actually tried using skin friction to stay inside of a waiver or whatever please tell about it. I'm genuinely interested in hearing how that went. Concerning the air brake method, I had thought about that one but didn't have space or budget left to implement it so it got tossed out pretty quickly.

You are clearly not thinking about this correctly. There are people who have covered rockets in fur and/or carpet to make them really draggy. This is the truth, I kid you not.

Your issue is that you have a rocket and motor combination that are so mismatched for your stated altitude goal as to be essentially impossible unless you get serious about adding drag, and a lot of it.

As to folks being assholes there are some who like to babble on and use foul language every post. They might also claim vast knowledge that is not backed up by practical been there and done that.

If John Demar gives you advice you should probably listen to what he has to say, think about what he is saying, and then ask questions. He is, and has been, an extremely knowledgeable person in this hobby for a long time.
 
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