Active Stabilization with the help of Thrust Vector Control (TVC)

[Raikhan Kushekova]

Hello, everyone!
I'm currently a member of a rocket club in my university.
I've been given a task to do the literature research on active sabilization using thrust vector control.
My work is to find the current state of art and to make a summary on how to basically do it.

Now I've been searching everywhere and found some interesting papers, usually it's bachelor diplomas and NASA reports, but I haven't seen exact control algorithms and full documentation on that.
I do understand that it's quite challenging and requires understanding of many fields like: control theory, aerodynamics etc and there are great books like "Advanced Control of Aircraft, Spacecraft and Rockets by Ashish Tewari", what would you recommend in my position? What books or already achieved projects are there to look at?

My team is using gimble for thrust vectoring and trying to build a testbed for hardware and software development.

Thank you so much!!

 

[mpitfield]

BPS https://bps.space/

 

[ihbarddx]

It works only during boost. Unless you have a very low thrust/long boost system, you should consider what you will do after cut-off.

 

[cerving]

Most people that are doing active control are using canard fins so they can still correct the attitude after burnout. If you have four of them controlled independently you can also provide for spin correction.

 

[Motocrossman24]

You could probably just implement a multirotor flight controller using arducopter or inav and use 4 canards at the nose locked into a position hold, it would just attempt to keep a level attitude as long as there’s air moving over the surfaces, it should be very accurate.

 

[plugger]

I could be wrong but I'm not sure anyone has actually done a comparison regarding TVC and the "gains" made by staying vertical vs the standard methodology (aka no TVC) when it comes to peak altitude (with all other things considered equal). Sure, it would be a sliding scale dependent on how "off vertical" the non TVC flight is but given TVC places more mass where you don't want it (the aft end of the rocket) I do think this sort of thing is often a lot of work for little if any actual benefit from a peak altitude perspective.

Besides BPS have a look at

https://www.reddit.com/r/amateurTVC/

It seems like every second twenty-something year old interested in rocketry is looking to build their own TVC system.

 

[Wallace]

I could be wrong but I'm not sure anyone has actually done a comparison regarding TVC and the "gains" made by staying vertical vs the standard methodology (aka no TVC) when it comes to peak altitude (with all other things considered equal). Sure, it would be a sliding scale dependent on how "off vertical" the non TVC flight is but given TVC places more mass where you don't want it (the aft end of the rocket) I do think this sort of thing is often a lot of work for little if any actual benefit from a peak altitude perspective.

Besides BPS have a look at

https://www.reddit.com/r/amateurTVC/

It seems like every second twenty-something year old interested in rocketry is looking to build their own TVC system.

Until things become much lighter there will not be an apogee advantage, takes to much "stuff" to make it work. Just cool to create and that's why...

 

[Wallace]

Unless..It involves staging. That's an entirely different game.

 

[plugger]

Unless..It involves staging. That's an entirely different game.

How does staging change the equation?

 

[Wallace]

Dunno..

 

[georgegassaway]

The most interesting use for TVC is for long-burn motors, on S-L-O-W boosts. For say, a Saturn-V that takes 8 seconds to boost to 300 feet, or 17 seconds to boost to 500 feet.

For the purposes of high altitude, and/or fast boosts, aerodynamic control is the way to go. TVC sucks for high speed, the aerodynamic stability makes the TVC less and less effective the faster the rocket goes... and at some point makes the TVC totally worthless (and that point of worthlessness would be around 100 mph to 200 mph, depending on various parameters).

Doing TVC on a fast boosting rocket model would be foolish.

BTW - Joe Barnard (BPS.space) may be a 20-something, but he is INCREDIBLY impressive in what he has been able to do. And even more so ALL the things he has learned from scratch in the last 3-4 years. There's no 20-somethings, 40-somethings, or ANY-somethings who have done as much to advance the state of the art of TVC for model rockets as he has. So sez this 60-something, who's been doing onboard rocket guidance for 30-something years (1988 Sunguidance, and 1989 Gimbaled Thrust Guidance).

 

[Wallace]

Mr. Jarvis would be the one to ask if you'd like that answered in a less than fng fashion.

 

[DaveW6DPS]

Thrust vectoring is only active when there is thrust. For most rocket flights that is 4 seconds or less, followed by 10 to 20 seconds of coasting. Vectoring thrust for 10 to 20 percent of the ascent doesn't make as much actual difference as using airfoils that can provide correction force as long as there is significant forward speed.

I have been toying with a mock up for a vertical trim system based on a simple Arduino controller.

 

[OverTheTop]

How does staging change the equation?

If you have TVC control on the booster (or aerodynamic active stabilisation for that matter) at separation you have, in ideal flight, zero tilt from vertical. That means that after the coast the rocket is still almost vertical when the sustainer motor is lit. The gravity turns in a staged flight can get really epic, just because of the extended times of boost and coast phases, and cost significant apogee altitude.

 

[plugger]

Mr. Jarvis would be the one to ask if you'd like that answered in a less than fng fashion.

Active stabilisation =/= TVC, at least in respect to Jim's active guidance system.

 

[plugger]

If you have TVC control on the booster (or aerodynamic active stabilisation for that matter) at separation you have, in ideal flight, zero tilt from vertical. That means that after the coast the rocket is still almost vertical when the sustainer motor is lit. The gravity turns in a staged flight can get really epic, just because of the extended times of boost and coast phases, and cost significant apogee altitude.

TVC on the booster would only be active for the booster burn and has numerous issues as called out by George above. Also, TVC effectively implies a non-MD vehicle which is suboptimal if someone is staging for altitude so it's inclusion in a booster would be more for show than substance.
As for active stabilisation via aerodynamic methods, I'm still far from convinced. The reason I say this is for multiple reasons; the added drag of the airfoils used for active guidance, the length penalty that arises from incorporating the system in the airframe, and the mass penalty that comes with its inclusion. As stated initially, I know this would be a sliding scale in terms of the less vertical a non-guided flight is and how that corresponds to the guided system from an active guidance perspective but I'm still not convinced.

 

[OverTheTop]

Whatever system you have you take a hit in the mass fraction for the flight. What you do gain is almost guaranteed vertical at separation which is a big factor in getting max altitude out of the sustainer. It takes a bit of luck out of the equation.

 

[plugger]

Whatever system you have you take a hit in the mass fraction for the flight. What you do gain is almost guaranteed vertical at separation which is a big factor in getting max altitude out of the sustainer. It takes a bit of luck out of the equation.

Exactly. But let's be honest, most stage seps ideally occur immediately after booster burnout. I suspect you'd get better results in application by angling the launch rod by 1 degree off vertical with the wind for every 7kph of wind at ground level. There's more than one way to try to get your rocket to fly as straight as possible, especially during boost.

 

[Wallace]

Exactly. But let's be honest, most stage seps ideally occur immediately after booster burnout. I suspect you'd get better results in application by angling the launch rod by 1 degree off vertical with the wind for every 7kph of wind at ground level. There's more than one way to try to get your rocket to fly as straight as possible, especially during boost.

Did you mean to say away from the wind?

 

[JimJarvis50]

I think my three-stage flight made a few points worth considering. The stabilization module was between the first and second stages. It added 8 pounds, and probably brought down the simulated altitude from around 200K to around 185K on a straight flight. However, in practice, it straightened the angle of the rocket at second stage ignition from what would have been around 8 degrees to around 3 degrees at the point of ignition. However, that change is the difference from landing a few miles away to landing much further away. To the best of my recollection, 8 degrees is something like 12 miles out, and 20 degrees is like 30+ miles out. The actual flight landed 3 miles out. That's why I use stabilization, and at some angle, it helps on altitude too. I think thrust vectoring could accomplish much of this benefit, but having stabilization continue through the coast period helps too.

Jim

 

[Wallace]

Thanks Jim, my response would have been much less eloquent. It is difficult at best to consider all potential factors. The 3 stager is orders of magnitude over my head. I have a "sense" of what may or "may not" work gained from life experience mostly, and alot of reading. But...Always best to get the answer from one who's actually been there. I have a lot more respect for people that actually do things, pass or fail/win or lose, than I do for anyone with a piece of paper that "claims" they're qualified..

 

[boatgeek]

Here's a wild notion: What about using a TVC type system with a small-ish motor as a first stage on a 3-stage rocket. The goal is to get the stack to 100-200 ft/s pointed vertically with no more altitude than is needed to do that. The second and third stages would then fly just like they would have under a normal launch, except that they are vertical at initial firing. You don't lose a lot of drag to a large diameter booster because you aren't going very fast. Of course, that depends on having a TVC system that could handle a long 54mm motor like say a K300. Bonus if you could fit it into a 4" first stage so that the first and second stages could be similar diameters.

 

[Nytrunner]

Did you mean to say away from the wind?

Same thing

With the wind = away from the wind

Against the wind = into the wind

 

[Wallace]

Got it

 

[plugger]

I think my three-stage flight made a few points worth considering. The stabilization module was between the first and second stages. It added 8 pounds, and probably brought down the simulated altitude from around 200K to around 185K on a straight flight. However, in practice, it straightened the angle of the rocket at second stage ignition from what would have been around 8 degrees to around 3 degrees at the point of ignition. However, that change is the difference from landing a few miles away to landing much further away. To the best of my recollection, 8 degrees is something like 12 miles out, and 20 degrees is like 30+ miles out. The actual flight landed 3 miles out. That's why I use stabilization, and at some angle, it helps on altitude too. I think thrust vectoring could accomplish much of this benefit, but having stabilization continue through the coast period helps too.

Jim

Your case is different from others imo Jim as you were flying a three stage stack. You had two staging events to consider and having active stabilisation would have given you the best chance to light the third stage with as much verticality as possible. And please correct me if I'm wrong but from my memory I believe you had your active stabilisation "parked" during motor burn and the high velocity portion of your flight?

Conversely when people are staging with a high altitude designed two stage stack the only way active stability would make sense imho is for it to be incorporated into the sustainer. The reasoning in my mind for this is that the booster is ideally only coupled to the sustainer for a few seconds at most (motor dependent obviously) so the gains of having active stabilisation on the booster would be minimal if at all. Plus the added penalty from a mass fraction and drag perspective would most likely negate any gains the system provided.

Just as an FYI to everyone, I'm a HUGE fan of BPS and I'm incredibly impressed with the amount of people who are flying their own home built fin based active stabilisation systems. But in the end I don't see myself going down this path as I'm just not convinced from a high altitude two stage staging perspective that they provide any true benefit. And that's what I'm most focused on when it comes to the hobby. I also think in the long run cold gas thrusters are the way to go as realistically fin based stabilisation is really only useful for flights to say 100k ft. I can't think of a better example to support this than your flight Jim where your sustainer was effectively tumbling end to end as it was nearing apogee.

 

[JimJarvis50]

Your case is different from others imo Jim as you were flying a three stage stack. You had two staging events to consider and having active stabilisation would have given you the best chance to light the third stage with as much verticality as possible. And please correct me if I'm wrong but from my memory I believe you had your active stabilisation "parked" during motor burn and the high velocity portion of your flight?

Conversely when people are staging with a high altitude designed two stage stack the only way active stability would make sense imho is for it to be incorporated into the sustainer. The reasoning in my mind for this is that the booster is ideally only coupled to the sustainer for a few seconds at most (motor dependent obviously) so the gains of having active stabilisation on the booster would be minimal if at all. Plus the added penalty from a mass fraction and drag perspective would most likely negate any gains the system provided.

Just as an FYI to everyone, I'm a HUGE fan of BPS and I'm incredibly impressed with the amount of people who are flying their own home built fin based active stabilisation systems. But in the end I don't see myself going down this path as I'm just not convinced from a high altitude two stage staging perspective that they provide any true benefit. And that's what I'm most focused on when it comes to the hobby. I also think in the long run cold gas thrusters are the way to go as realistically fin based stabilisation is really only useful for flights to say 100k ft. I can't think of a better example to support this than your flight Jim where your sustainer was effectively tumbling end to end as it was nearing apogee.

I think my comments would apply equally to two or three-stage rockets - it's just that my example happened to be a three stage. On that flight, the system provided roll control from launch and then roll and vertical control during the coast period. It dropped off the bottom of the second stage just when that stage was fired. I do it the same way on two-stage flights where I have used the system.

The biggest problem with having canards along for the entire time is just the range of speeds that they would have to operate at. I'm going to try the system at LDRS attached to the top of a two-stager, but the speed range is limited to within the range of previous experience. Granted, that flight is not about getting to the highest altitude.

Jim

 

[Wallace]

F

 

[Wallace]

Everything aside; Pretty tough to argue with someone that's been there.

 

[plugger]

Everything aside; Pretty tough to argue with someone that's been there.

In all seriousness Wallace I don't consider this an argument. It's a discussion, nothing more. I doubt I've offended Jim, but if I have that wasn't my intention. Jim's raised points I've not thought about regarding the distance a rocket travels from a cylinder perspective and that's not something I've ever considered. And yes, Jim's had more than one high alt staging flight whereas I've not. But Jim doesn't have a monopoly on those rarefied altitudes; other people have gone as high and higher than he has. And yet Jim's the only one in that small club that's even leveraged active stabilisation in his successful attempts (at least to my knowledge). So there is evidence to highlight that active stabilisation isn't a requirement for high altitude staging despite being used successfully in that realm.

And I still remain unconvinced in the net benefit of such systems when it comes to a peak altitude AGL perspective. Even Jim wrote above "That's why I use stabilization, and at some angle, it helps on altitude too." which meshes quite well with my earlier comment of "I know this would be a sliding scale in terms of the less vertical a non-guided flight is and how that corresponds to the guided system from an active guidance perspective."

 

[TonyL]

plugger,
You are correct that one does not have to use active stabilization to reach high altitudes, however it sounds like you have never had to do a dispersion analysis? [the statistical estimation of potential flight trajectories due to rocket/launcher/environment variations]

Anything on the rocket I think should be there for a purpose. Jim's stated intent has been to reduce landing dispersion [sounds good to me], and it has been demonstrated successfully.

If one is going for maximum altitude, one can always do the university style 'hail-mary' shot and eventually one of them will likely work [or not].

However, if one wants to get there [whatever altitude that is] on the first attempt, with a higher probability of success than random dispersion provides, then one needs to do something different than just 'fling it into the sky'. A vertical autopilot on the booster is the very best place for it [all else being equal] as controlling the rockets initial direction will maximize vertical delta-V. Correcting direction after direction errors have been allowed to accumulate is very unlikely to be as efficient [assuming the same control system] as there is no free lunch when it comes to drag. Correcting under boost can be harder, so compromise in order to improve likelihood of success is always a candidate.

You are correct to consider that the added mass will potentially reduce the maximum achievable altitude, but you are neglecting flight trajectory dispersion, which will limit the probability of actually reaching the theoretical altitude. We have seen many flights with very high theoretical altitudes that were never realized. The much fewer number that have been successful are there through a combination of hard work and luck. The right answer is the the one that is expected to meet one's objectives.

One will always have the highest theoretical altitude from the most minimized/optimized design, however realizing it is another matter

br/

Tony