Altitude control problem. Help!

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ArchitectOfSeven

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Okay so I have a really interesting problem that I am dealing with in one of my classes at school. We are designing a rocket but because of a series of misunderstandings I'm trying to get a minimum diameter rocket with a Contrail M1491 motor to exactly 10,000ft. Obviously that sounds like extreme overkill because it is but I still have to solve the problem. In OpenRocket it sims to roughly 25,000ft when reasonably optimized. If I set the surface quality to the roughest setting it takes a huge penalty to altitude which is progress. That in addition to a pair of camera pods should get me to my target so I think I may be going the right direction with this.

So my question is this: Have any of you used surface roughness to control altitude before? If so how did it go? Also, what analytical method did you use to determine your surface imperfection size?

I was thinking of coating the rocket in some specific size of sand or some other particle if I could figure out what it needed. Generally speaking skin temperature shouldn't be too much of an issue because its mostly aluminum. Also NAR and TRA rules do not apply.
 
Control system feedback or feed forward and try using the altimeter to control grid fins or a actuator spoiler to increase drag like an aircraft speed brake function. If that's your senior project that prof is f---ing evil. I mean... I would first try to design the rocket to get 10,000 ft then try a control system to increase drag. Or rearward eject chute deploy suddenly before 10k ft. That's bullsh*t problem to solve man your not dumb. You might wanna research how those SLI rockets are hitting target altitudes.
 
Flat nose cone? Make it heavier?

Heavier is not really practical because I can't add a meaningful amount of weight without turning it into a kinetic energy weapon. Besides, it's already carrying a 5lb water tank to qualify for the competition. Flattened nose is an option, just need to figure out what that does to stability.
 
Go use your grad student contacts. They have like Mach 25 wind tunnel CFD sh*t... They can look at particle effects on surface of a 3D model from exhaust plumes and shocks. They eat open rocket for breakfast with a hand calculator, and call supersonic trivial. They might have more ideas.
 
Control system feedback or feed forward and try using the altimeter to control grid fins or a actuator spoiler to increase drag like an aircraft speed brake function. If that's your senior project that prof is f---ing evil. I mean... I would first try to design the rocket to get 10,000 ft then try a control system to increase drag. Or rearward eject chute deploy suddenly before 10k ft. That's bullsh*t problem to solve man your not dumb. You might wanna research how those SLI rockets are hitting target altitudes.

Okay there is no way I'm going with an active altitude control system lol. My plan is legit just to coat the body with epoxy and coat it with sandblasting grit of some yet to be determined size. All I need is to figure out how to work that into the C_d calculation and at least implement a decent Ansys simulation to back it up. One thing about that that bothers me is if that develops a thicker than normal boundary layer that nullifies the effectiveness of the fins. Or worse, if it has some other effect that destroys the stability entirely.
 
Mechanically increase the plate area of the rocket. Either chute or mechanism to skin surface by fluid mechanics analytical plate drag method to stop rocket in flight. You'd have to time it to account momentum if they make you stick with min diameter. Anyways, out. I don't think any ablative or change the skin friction crap is feasibly doable unless it's a on paper assignment only, sounds hard enough that they want you to fail no matter what you try.
 
Okay there is no way I'm going with an active altitude control system lol. My plan is legit just to coat the body with epoxy and coat it with sandblasting grit of some yet to be determined size. All I need is to figure out how to work that into the C_d calculation and at least implement a decent Ansys simulation to back it up. One thing about that that bothers me is if that develops a thicker than normal boundary layer that nullifies the effectiveness of the fins. Or worse, if it has some other effect that destroys the stability entirely.
I had a hypersonic professor with 25 years experience do supersonic CFD with not the 3D modeling solidworks but with his own dedicated code he developed for his research. For a SEDS build. Guys like that will find you any sanding roughness needed theoretically, they might need 4-6 weeks. They'll use the programs to see the boundary layer effects beyond your understanding after they say inviscid and list enough assumptions to make you puke.
 
Mechanically increase the plate area of the rocket. Either chute or mechanism to skin surface by fluid mechanics analytical plate drag method to stop rocket in flight. You'd have to time it to account momentum if they make you stick with min diameter. Anyways, out. I don't think any ablative or change the skin friction crap is feasibly doable unless it's a on paper assignment only, sounds hard enough that they want you to fail no matter what you try.

How is changing skin friction not feasible? I know I can figure out the drag with CFD and easily manufacture it. The big question I'm trying to put out here is if anybody has done something like this and if so, how well did it work? How did it affect the fins and stability?
 
So in theory you change your Cd enough you massive increase drag I get that concept. But how the ---- would you manufacture the surface change so precisely and uniform to not cause asymmetric drag forces by manufacturing defects? You have a theory idea to solve the problem. When I put a SEDS rocket into ANSYS a nuclear engineer student showed me the import but it has like f600.133..... On all the surfaces and even the graduate prof's basically said give us your solidworks files we hate ANSYS too. It was a cumbersome program but I am mechanical engr student. There were 60 boundary layers on multistage simple rocket and program won't tell you want surface the ANSYS boundary is on. I screwed with it eight hours then basically asked a research center on campus.
 
Your not going to get a MD M powered rocket that sims at 25K down to 10K by a skin coating. You will need drag - big drag which means surface area that don’t inhibit the stability too bad.

We had a college team that had flaps that deploy at X altitude to do the same thing you are asking about but at an L motor and s not a MD - Notre Dame I believe (you may be able to search for that).

I can say I have a 4” QCC with the intakes that work as air brakes and I can barely get that above 5k almost no matter what I put in it - so I’m not an OR master - but I think you can play around with surface area and Sims - then test from there.
 
Okay so I have a really interesting problem that I am dealing with in one of my classes at school. We are designing a rocket but because of a series of misunderstandings I'm trying to get a minimum diameter rocket with a Contrail M1491 motor to exactly 10,000ft.

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 get the feeling that the Contrail M1491 is not really a "requirement" as much as "this is the motor we have, and we don't have the money to get something different"
 
Is the assignment requiring a min diameter rocket? This is what I can't wrap my head around he starts with MINIMUM cross sectional area which is minimized plate drag at any velocity then he complains it sim'd 25k!?? Literally confused. I designed a SEDS airframe from scratch and I had to ask others to get a simple drag coefficient at Mach 1.7 (graduate people found the drag coefficient so I could use fluid mech analysis on drag force) and that was a three week nightmare itself but SEDS let us maximize altitude up front.
 
I get the feeling that the Contrail M1491 is not really a "requirement" as much as "this is the motor we have, and we don't have the money to get something different"
They could easily make a non min diameter airframe with vastly increased drag force to reach ten thousand feet. This guy sounds like a junior without a clue of fluid mechanics let alone compressible gas dynamics and there's like a legit impossible budget or skillsets needed to finish. And they said well here's the old team's rocket make it work. Hahaha. Glad I started a SEDS chapter, school actually gave us money to develope our own rockets. Granted I still had to take a internship to help pay for it. Then we had to teach the newer team members.

Hey OP is contest okay with exceeding 10kft then deploying chute at 10kft after apogee!????! Or must it have velocity = 0 at the 10,000ft mark.
 
It's a hybrid.....won't reach the claimed altitude anyway, none of them do..... Just sayin......

Not true, my Contrail's 54mm always exceeds the sim. Here is why..
Contrail certified their 54mm motors at 95F outdoor temp with warm N20. Liquid density was low. Motors filled at 70F will have about 20% more impulse than the certified value.

The double edge of that sword is that hitting an altitude target with a hybrid requires you to control the N2O tank temperature (fill and motor). Otherwise you will easily have up to a +/- 20% uncertainty in motor impulse flight to flight.

Edit: A better approach to solve your motor over-power problem is to under-power it by filling it with less N2O. You can achieve that with putting a scale in your launch system and fill to a predetermined weight, OR, put volume reducing filler pieces in the motor tank.
 
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How is changing skin friction not feasible? I know I can figure out the drag with CFD and easily manufacture it. The big question I'm trying to put out here is if anybody has done something like this and if so, how well did it work? How did it affect the fins and stability?
You won't have enough drag force to slow it down. This is like asking Prius brakes to stop two freight trains but with aero forces not calipers. Do you realize motor it burns for a few seconds usually under 5-10s then it has enough mass and velocity to get to 25,000 ft???? You have to plan to accelerate it in the opposite direction vector to stop at a predetermined altitude. You need a rear deploy chute, a spoiler, a non min diameter rocket, or a retro burn solid motor. The drag force on a min diameter rocket is so low compared to parachute drag force it's not worth arguing. You can dimple a rocket and save 2% off of drag if you dimple the entire rocket without causing structural failure. Your crapping yourself dude. Add a bunch of tube fins radially then pack rear deploy drive chutes altimeter control those. That's the horrible idea that I got to save your doomed project assignment. If you put a sewer cap on the top of it you still won't have enough drag force.
 
Not true, my Contrail's 54mm always exceeds the sim. Here is why..
Contrail certified their 54mm motors at 95F outdoor temp with warm N20. Liquid density was low. Motors filled at 70F will have about 20% more impulse than the certified value.

The double edge of that sword is that hitting an altitude target with a hybrid requires you to control the N2O tank temperature (fill and motor). Otherwise you will easily have up to a +/- 20% uncertainty in motor impulse flight to flight.

Edit: A better approach to solve your motor over-power problem is to under-power it by filling it with less N2O. You can achieve that with putting a scale in your launch system and fill to a predetermined weight, OR, put volume reducing filler pieces in the motor tank.
if they could throttle it that is a oblivious easy way to adjust thrust. In theory hybrids throttle.
 
if they could throttle it that is a oblivious easy way to adjust thrust. In theory hybrids throttle.

I would assume that throttling a hybrid is beyond the team's capability for this year's competition. Putting bricks in the tank is KISS.
 
Get a 10,000 foot spoil of wire. Tie one end to the rocket.


There, I’m a helper!
 
Reviewing some of my model test data, you would not be able to find a grain size that does not significantly affect the mass of the rocket to achieve the desired increase in drag to meet your requirement. You will be better off adding mass using some combination heavier materials for your air frame and fixed ballast.
 
What happens to stability CG moves a lot as mass increase? Overstable as f--- if masses go in normal payload pay area. Plus where will chute go? I don't think they could put enough mass in it to do that. Hey maybe it'll go horizontal.
 
Attach a large flat plate or very broad transition aft of the fins. That's going to increase drag in a more understandable way than surface roughness.
 
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.
 
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