[Question] 2 Stage Rocket (over)Stability

Matthew Tan

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Hello!

I have a question regarding the over-stability of a 2 stage rocket. The rocket starts with a static stability margin of 1.36cal. It launches fine, but as the booster motor burns out, it reaches a stability caliber of 3.25. Then once the stages separate, the booster carries on with a stability of about 2.25 but the sustainer jumps up to 5.75 calibers of stability (no more tail weight from the booster). Upon the motor burning, it reaches a peak stability of 6.5cal. The booster is 4in and the sustainer is 2.1in in diameter. Is this too over-stable? Will the rocket (especially the upper stage) cone in flight? If it is over stable, how can I address this?

Thanks!

PS: The OR file is attached. The configuration in question is the M1939 to the J570.
For ease of viewing, the stability plot is here:
Screenshot from 2023-05-31 23-50-16.png
 

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Titan II

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...but the sustainer jumps up to 5.75 calibers of stability (no more tail weight from the booster).
That does not make any sense. The stack has a CP and CG. Following seperation, the sustainer has its own CP and CG that is not related to any booster factors. See page 112 in the document I have attached. Observe the CP and CG of the stack in figure 129. Then observe the CP and CG of just the sustainer in figure 130.

 

Matthew Tan

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Right, good point. What I mean is the stability of the stack goes to a stability of 3.25. Then, upon separation, the now separate sustainer has a stability starting at 5.75 and burning to 6.5 cal. Are these ranges of stability ok for the sustainer stage? (as they seem high but perhaps it's ok?)
 

Charles_McG

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My two stage scale sounding rockets sim as overstable. With a 8’ launch rail, using a boost thrust:weight of roughly the same numeric value as the crosswind seems to keep the flight straight. Assuming no hard wind shear at altitude. Caveat: I’m flying G-I range motors to ~3000 feet.
 

T-Rex

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I should probably just keep my mouth shut but.......... (Please hang around and let us help you with your rocket. I'm probably going to be accused of scaring you away)

There is so much wrong in your sim file that I can't begin to figure out what is going on.

Having erroneous mass points just plugged into the sim is messing with the simulation ("real world CG correction" mass of 1.85oz? Putting it in as you have changes the length of the sustainer, and what is it????). Centering rings with 0 for an inner diameter? No MMT tube? Motor blocks? A drogue in the middle of the booster av-bay? What is a "Motor retention bulkhead" (in the middle of the booster motor)? 14oz worth of flight computer in the sustainer? A booster av-bay with no electronics or bulkheads? 15" shock cord in the sustainer? etc, etc........... Fix the sim to match the real world and maybe it will work better.

If you have already built the model (seriously doubt it), take a mass and CG measurement (without motor) of each stage and put them in as stage overrides. Just MAYBE that will answer your question.
 
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Matthew Tan

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Yeah, that's my bad. The sim is very much all over the place. The "real world CG corrections" were put in to just get the sim to match the actual rocket (which is exactly what you recommended).

I ended up adding some ballast to the lower stage (once again overriding the masses and CG for each stage) which brought the stability of the upper to around 3.5cal. I think that should solve the problem.
 

Titan II

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Is this yours? It was on reddit with the exact same question.


Have you run a sim on the sustaner without the booster?

question-2-stage-rocket-stability-v0-ls0jlfhukf3b1.jpg
 

Matthew Tan

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Yes! That was mine. I have run the sim on just the sustainer, which showed nearly the same thing. I suspect the difference was from the starting velocity being 0 as it was not boosted.

I suppose the most pressing question (as we're leaving for the launch today, FAR 51025) is how high of a stability margin can be tolerated on the upper sustainer stage?
 

Titan II

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I ended up adding some ballast to the lower stage (once again overriding the masses and CG for each stage) which brought the stability of the upper to around 3.5cal.
Sorry Matthew. I do not understand that statement, as the sustainer stability is completely independent of the booster following seperation. Thus, how does adding ballast to the lower stage change the stability of the sustainer? To coin a famous phrase, "What we've got here is failure to communicate." Regardless, I primarily have two stage rockets and have enjoyed the dialog. I would not be overly concerned if the sustainer has a static stability of 3.5. I would think there are some folks at FAR with some good input. Please let us know the results.
 

waltr

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My take on answering OP's question.

Just off the Rail is the lowest velocity -- when it can be most effected by cross wind.
This is when Stab Margin is lowest so less likely to wind cock.

At motor burn-out the Stab Margin is higher (CG moved forward). However, this is also at the highest velocity so any cross wind has very little effect.
Now the Booster drops (separation charge) and the sustainer should still have a high velocity (little effect from cross wind).
Get the sustainer motor going when there is still plenty of velocity and Stab margin should be fine.

Side note:
The OR sim of my 2-stage has Stab margin on stack go from 1.8 to 2.7 until separation. Then the Sustainer stab margin goes from 1.8 to 2.0.
 

Titan II

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Matthew...I believe FAR 51025 was over on the 4th. How did it go?
BTW...are you with Stanford?
 

cerving

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If your two-stager is properly simmed in OR, you can look at the graphs for either of the stages or both together, until they separate. If you don't see that option, you need to do some more work in OR... find someone that's done this before.
 

Matthew Tan

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Hi everyone!

Still catching up after the launch (which was on the 4th!). Unfortunately, the sustainer was abandoned in the desert, but the lower went up to 11k, which is a little under what the OR sim said it was supposed to do on the motor we ended up using. The lower stage was successfully recovered but because the upper which held the altimeter was abandoned, we couldn't compete in the competition. Good lessons learned all around.

I am with Stanford.

We did have an interesting instability (or overstability, not sure but probably instability) develop off the rail before flying straight (see grainy pictures from drone below)

1686186163530.png
and the full trajectory is attached. The angle looks really steep here (and it is) but the rocket flipped around quickly and kept going on it's way (see video).

Any ideas as to what happened here? The sustainer did stay on the rocket during these... aerobatic orientation maneuvers.
 

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Matthew Tan

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Here's the OR stability plot for the rocket that flew in the competition (see pictures and videos above). Perhaps that low off the rail stability is what caused the big maneuver (loop? cone? rapid yaw?)
1686186334578.png
 

Titan II

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Sorry it did not work out as planed, but you gave it a good try. Stanford seems to have an excellent program.

So the sustainer fired? Could you see the trajectory? Since our conversation was regarding the sustainer, that is my primary interest. On another note, your diagram shows a CG of 203 cm and a CP of 217 cm. Was the 203 cm CG theoretical or measured? I am assuming the latter, but just checking.

Thanks for getting back with us.
 

Matthew Tan

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Thanks! We're not too upset; this was the 2nd flight of the 2 stage rocket (and my 4 flight overall) so it was a lot of good experience building.

We're not sure if the sustainer fired, though based on the altitude at which the booster apogeed, the altitude lockout on the upper igniter avionics should have permitted it to do so. We watched both come down on drogue and the sustainer seemed to take a really really long time to come down, so my guess is that it fired all the way to nearly 25k (but probably not over as the rocket pitched downrange a bit after the "maneuver").

The CG was the measured CG (using the shoddy method of putting in a dummy mass on the OR to match the weight and cg of each stage).
 

Handeman

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I don't want to be a downer here. I understand FAR pretty much lets you launch anything and they have the space and bunkers to let things fail without consequences. Did you have an experienced 2 stage TRA or NAR mentor helping you?

You had a simulation you didn't trust or understand and you still launched?

This seems to me like a typical college team that is great at engineering components but has little practical experience with rocketry and have failed flights because they only have that theoretical engineering and not the practical experience, and what practical experience the team does tends to get, leaves with the graduating class.

I understand the colleges want to gain practical experience for their student by doing these rocketry projects. I think they need to bring in systems engineering and project management students to oversee and run these projects to help eliminate some of these types of failures. There really should be flight readiness reviews done with a decision authority that says Go-No Go and that authority should be someone that understands the whole process, including the practical aspects of flying rockets, not an engineering professor that has a invested interest in the flight going forward. My opinion is, you should have never flown that rocket.

/rant
 

Matthew Tan

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Good points all around, and I appreciate the feedback!

I had a few of the more experienced members in our program take a look at/advise this build (we have some individuals who have had years of launching rockets, even before university, L3s, etc). The rocket did fly successfully once before this event on smaller scale models to validate the system and stability (so there was a engineering validation process of sorts).

I understand exactly what the sim is saying and doing. Of course, dynamic stability is one where the math is often less helpful than experience (I don't have an accurate matrix of stability derivatives for this rocket), which is why I brought it here to get more feedback on acceptable ranges. I was looking more for that "practical experience with rocketry" to go with the sim. (on a side note, those practical experiences have yielded answers online anywhere between .5 to 3 to "I've launched my rocket up to 10 cal and it was fine!" to even "overstability is overrated," paraphrasing of course. It's so system dependent!)

At least on our team, often the professors aren't making the go-no-go calls. Rather, it's the experienced students in the program, program leads, and the FAR guys.

Out of curiosity, and personal learning, why should I have not flown the rocket? At least from my perspective, it was a rocket that had flown before, was simulated to be stable, and was checked off my multiple people more experienced than I. In the pursuit of developing my own go-no-go conditions, what would people have improved knowing before calling "go" on this launch?

Thanks and looking forward to learning!

edited for grammar
 

Titan II

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Matthew...

I was wondering when someone was going to make a post like that. Handeman has no clue what he is talking about and it is the type of garbage that we frequently see here constantly putting down the college teams. Narcissism tends to rule on this forum. I looked you and the Stanford operation up and found out you have a lot of experience at hand and available. I wish you success in the future.
 

Handeman

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@Titan II, you can have your opinion, but at our launch site, we have restricted all college teams to 4000 ft limits until they can prove their designed recovery systems work 100% as designed because of past experience. The teams tend to be excellent with engineering components and rockets and getting them to fly, but they don't have the practical experience to safely fly rockets and their recoveries tend to be over engineered, or too under engineered, and tend to fail. We will no longer risk our launch site by allowing college teams to launch large rocket to high altitudes without a previously proven recovery system. We already had a failed recovery land on a home and car. We don't won't allow that again.
Our experience is this is much less of an issue when the teams work closely with an experienced club member mentor than when they come in with college professors as mentors.

My personal experience, being a systems engineer, the college teams lack an over all systems engineering and program management oversite. They don't have someone that is looking at the whole rocket and it's launch as a system and making sure everything from the rocket systems, ground systems, launch site requirements, etc. are checked and verified with equal intensity.

My opinions are based on experience.
 

Handeman

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I had a few of the more experienced members in our program take a look at/advise this build (we have some individuals who have had years of launching rockets, even before university, L3s, etc). The rocket did fly successfully once before this event on smaller scale models to validate the system and stability (so there was a engineering validation process of sorts).
We have found that a lot of the college L2 fliers have two flight, their L1 cert flight and L2 cert flights. It's great that you have more experience mentors. Did your smaller scale rocket fly successfully on smaller motors and successfully recover? That is usually something you would want to do before risking the full up model. Did your full up model fly on smaller motors to lower altitudes and successfully recover?

I understand exactly what the sim is saying and doing. Of course, dynamic stability is one where the math is often less helpful than experience (I don't have an accurate matrix of stability derivatives for this rocket), which is why I brought it here to get more feedback on acceptable ranges. I was looking more for that "practical experience with rocketry" to go with the sim. (on a side note, those practical experiences have yielded answers online anywhere between .5 to 3 to "I've launched my rocket up to 10 cal and it was fine!" to even "overstability is overrated," paraphrasing of course. It's so system dependent!)
I hear where you are going. You didn't quite trust the results of your sim so you came here for clarification and didn't get any. The responses were all over the board, from it's great to it's bad. It sounds like you never got a good answer to your question, from here or anyone on your team. That probably should have been a red flag.

At least on our team, often the professors aren't making the go-no-go calls. Rather, it's the experienced students in the program, program leads, and the FAR guys.
Did you ever have an actual Flight Readiness Review of the rocket, all of it's systems, procedures, launch site requirement, etc. where there was one decision authority that said Go-NoGo for the launch? Or was it kind of a group thing where everyone put in an opinion and the majority decided to just go ahead? When individuals want to launch large rockets, it's pretty much up to them, with RSO approval. That can work for a college teams too, sometimes, but if you really want to learn what commercial engineering is about, you should do the mile stone gates with the entry and exit criteria and that includes a flight readiness review with a competent and knowledgeable decision authority.

Out of curiosity, and personal learning, why should I have not flown the rocket? At least from my perspective, it was a rocket that had flown before, was simulated to be stable, and was checked off my multiple people more experienced than I. In the pursuit of developing my own go-no-go conditions, what would people have improved knowing before calling "go" on this launch?
Unless I mis-understand, the full up rocket had never flown before (The rocket did fly successfully once before this event on smaller scale models) , only a scale model. You were asking here about stability, so what was simulated as stable? You weren't sure? From what I understood, it was the stability of the sustainer that was questionable. Was there a difference between the smaller scale model and the full scale model? My opinion is that if you were not sure of the stability of the whole rocket, the booster and the sustainer, then it shouldn't have been flown.

My opinion is that if your team had included the system engineering processes in the whole building and flying of this rocket, many things, including the stability questions would have been brought forward and solved well before the flight had take place.

Have you had a post-flight review? Has everyone gotten together and determined a cause of the flight failure?

I'm sure you'll have many more successful flights.
 

Matthew Tan

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Did your full up model fly on smaller motors to lower altitudes and successfully recover?
We didn't fly a small scale model. We flew the actual full scale rocket (the exact same one that flew on the comp) on smaller motors. It flew perfectly straight, had a drag separation, successful second stage ignition, and successful recovery of both stages. I've attached the video here :)

where there was one decision authority that said Go-NoGo for the launch
Our team had a CDR with each other, the experienced members, and the team leads. I was the single authority for Go-no-go call based on the input from the experienced members and the RSO (all giving go. The way I ran it was if anyone said "no-go" the call would be no-go. Thoughts on this method?).

My opinion is that if you were not sure of the stability of the whole rocket, the booster and the sustainer, then it shouldn't have been flown.
It was just the overstability of the sustainer that we were worried about. We knew the rest of the rocket (booster and sustainer together) were stable and within range, and the booster alone was stable (see first OR stability plot).

Have you had a post-flight review? Has everyone gotten together and determined a cause of the flight failure?
We've all watched the video but haven't had a meeting to discuss yet (have to finish finals). It's really just the loop, or cone, or whatever that maneuver it does just after takeoff that we were concerned about. The rest of the flight looks to have completed nominally, and we watched both stages come down on drogue (we successfully recovered the booster but had to abandon the sustainer due to running out of time to hike and search).

Thanks for the feedback! We do often let these projects get pushed towards the later limits of their development schedules. For future projects, I'll be making sure to have more thorough checks earlier on so we avoid these questions the day before launch.
I looked you and the Stanford operation up and found out you have a lot of experience at hand and available. I wish you success in the future.
Thanks so much for the help and support Titan! We really appreciate it!
 

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