Rocket Guidance

[Atlantis]

What are the federal regulations regarding active rocket stabilization? I've looked on various websites, but I haven't gotten any clear answers.

 

[CPUTommy]

That would be a big negative.. Dont even attempt...

 

[chadrog]

I can't comment on the fed's reg's, but as far as hobby rocketry goes, tis not allowed.

 

[GregGleason]

When you say "active stabilization", what do you mean by that?

Have you seen the link below:

https://www.ninfinger.org/rockets/gcsfaq.html

Greg

 

[bobkrech]

What are the federal regulations regarding active rocket stabilization? I've looked on various websites, but I haven't gotten any clear answers.

There are no laws prohibiting active hobby rocket stabalization. Period.

However you are prohibited from putting a targeting system into a hobby rocket that would make the rocket into a weapon delivery system.

Bob

 

[sodmeister]

Did John Pursley not use vectored nozzles in his scratch Vanguard rocket ?

Paul T

 

[GregGleason]

Did John Pursley not use vectored nozzles in his scratch Vanguard rocket ?

Paul T

https://www.accur8.com/NARAM41_Vanguard.html

Greg

 

[sodmeister]

Indeed Greg ,I found the article also in my favorites.

What a great looking rocket ! I`ve always looked up to John and his great skills !

Paul T

 

[Atlantis]

Thanks for your help guys!

I'm working with one of the organizations at my university that is working on an orbital launch vehicle that utilizes a balloon assisted rocket launched from Antarctica to fly a 1kg cubesat. We are also planning to do subscale flight testing here in the states.

This comes as a follow on to another organization's cubesat project, which is still waiting for a ride with NASA.

 

[luke strawwalker]

Hate to sounds like some others around here but does ANYBODY ever use the search function?? This topic has come up at least twice in the past year or so...

There's no rule prohibiting active guidance systems in rocketry, that I'm aware of anyway. Bob said there isn't, and since he's on NAR S&T (AFAIK) I think you can pretty well take that to the bank. The rules prohibiting using rockets from being "weaponized" to deliver explosives on target, but there's already a fistful of Federal laws covering this sort of thing anyway, so hobby rocketry rules/laws are the least of one's worries in this regard.

John Pursley has done a number of active guidance projects, including the Vanguard, Mercury Redstone, and Saturn V (IIRC-- I know he used RC controlled ejection in one of his NARAM Saturn V's....) The system basically consists of a slightly reworked off-the-shelf model airplane anti-crash system with a ring of sensors placed horizontally in the top of the rocket or nosecone, looking out to the cardinal points of the compass (pitch and yaw), basically... each senses the horizon by seeing the world in it's field of view as either black (atmosphere/sky) or white (horizon and ground). The controller constantly compares views of the sensors on opposing sides-- if the rockets tilts over, one sensor will see mostly white, the other mostly black, and the controller then automatically sends a proportional signal to the appropriate servo on the motor gimbal, steering the motor the correct direction (if you set everything up right) to push the bottom of the rocket back under the nose, bringing the rocket 'upright' in its flight path. When both sensors on opposite sides of the rocket both see equal amounts of "black and white" then it neutralizes the servo position back to center. Of course after burnout, the corrective forces are GREATLY reduced (almost nil, but not completely zero) due to the small amount of gas being vented through the nozzle from the delay train in the motor. So a certain amount of static stability IS required for the rocket to continue to fly stably after motor burnout. (The Vanguard with it's mid-body small-to-larger transition tends to move the CP back, making it easier to stabilize despite being finless... an Ares I "hammerhead" type design though is the COMPLETE OPPOSITE and it would take some VERY careful design and a ton of noseweight to make an Ares I with this method). Scale Redstone fins are plenty with motor gimbaling to be stable, because by the time the motor burns out, the propellant weight has reduced from burning and shifted the CG forward, improving stability.

I asked John about gyro stability, since that was the first idea *I* had when I started considering active stability... (and it seems that here on the fora at least once or twice a year someone comes up with the same idea and starts discussing it). Small RC heli gyros were his first idea too, quickly discarded. Gyro drift makes them practically worthless for the purpose-- there's nothing to compensate for the gyro drift, meaning the gyros lose track of "which way is straight up" rather quickly. The heli gyros are there to sense rates (sudden changes in movement) and compensate for it by trimming the input of the tail rotor, and since the heli is under active control and constantly receiving input into its flight path, so long as the pilot knows which way is up, the gyro's don't have to... make sense?? A rocket isn't being controlled this way, and relying entirely on the gyro to know which way is up simply doesn't work (right now with off-the-shelf gyros anyway). People always say "I don't believe that-- I'm gonna build a gyro-based engine gimbal stability system anyway", but you NEVER see one of them posting their results or showing it working. Besides, why would you want to when the anti-crash model airplane sensor is an "off-the-shelf" ready system???

It's also been done very well by George Gassaway with his "sun-seeker" rocket, which used photocells to detect the angle of sunlight coming in from outside, and then swiveling steering fins to center the spot of sunlight on the photocell(s). This is particularly good system for front or midbody finned rockets like the Sidewinder or AMRAAM.... even works quite similar to infrared guidance used on anti-aircraft missiles that home in on the target aircraft's exhaust heat... I read about a roll stabilization system for camera rockets on the Brittain Fraley website that used an opposed pair of small steering fins, controlled by a series of photocells arranged radially around the rocket, with one set as the "Center" cell and the others set to have lower voltage values in the controller, so if the rocket rolls to either the left or right of the center cell, those cells are more directly in sunlight, generate higher voltage than the center cell, and the controller automatically generates a signal to the steering servos, which moves the fins in opposite directions, turning the rocket in the roll axis until the "center cell" is again in direct sunlight with the surrounding cells in deepening shadow around the rocket... he flew a camera with the system off, typical "spin-cycle video" with the ground spinning rapidly beneath the rocket, then he reloaded and turned the system on, and the ground stayed rock-solid centered all the way to apogee, never straying more than a few degrees clockwise or counterclockwise...

BTW, the horizon sensor and sun-sensor systems were designed strictly for pitch and yaw control (keep the rocket flying straight up). There was NO roll control provided by the guidance system. Brittain Fraley's system was STRICTLY for ROLL control, to make the videos returned from rockets clear and smooth... it has NO pitch/yaw control over the rocket...

Later! OL JR

 

[jadebox]

It's also been done very well by George Gassaway with his "sun-seeker" rocket, which used photocells to detect the angle of sunlight coming in from outside, and then swiveling steering fins to center the spot of sunlight on the photocell(s). This is particularly good system for front or midbody finned rockets like the Sidewinder or AMRAAM....

The biggest problem with this scheme is that you're limited to launching around noon on certain days of the year. We don't normally have to deal with "launch windows" when we fly.

I read about a roll stabilization system for camera rockets on the Brittain Fraley website that used an opposed pair of small steering fins, controlled by a series of photocells arranged radially around the rocket, with one set as the "Center" cell and the others set to have lower voltage values in the controller, so if the rocket rolls to either the left or right of the center cell, those cells are more directly in sunlight, generate higher voltage than the center cell, and the controller automatically generates a signal to the steering servos, which moves the fins in opposite directions, turning the rocket in the roll axis until the "center cell" is again in direct sunlight with the surrounding cells in deepening shadow around the rocket...

That's cool. I spent some time researching roll control because I didn't like what the rocket's spin did to my videos (and what watching the videos did to me). I never got to the point of actually building anything because I found that larger rockets with carefully aligned fins were an easier solution than active roll control. But, it's such an interesting challenge that I may take it on sometime.

-- Roger

 

[GregGleason]

Here is the video link for the roll control comparison from Brittain Fraley's Rocketry Site:

https://www.deltavrocketry.com/video/MK SHEA - Stabilizer Comparison - Onboard.mpg

It's worth looking at if you haven't seen it.

Greg

 

[N2O4]

My roll stabilized rocket actually did use a helicoptor gyroscope to drive the servo for the the stabilization fins. There was no optical sensor and the duration of the flight is short enough (<2 min to apogee) that the drift of the piezoelectric gyros is minimal.

Brittain

 

[New Ocean]

The paranoia! Sure you can to this, it has been done before. Steerable canards or long burning motors are both possible ideas.

 

[GregGleason]

My roll stabilized rocket actually did use a helicoptor gyroscope to drive the servo for the the stabilization fins. There was no optical sensor and the duration of the flight is short enough (<2 min to apogee) that the drift of the piezoelectric gyros is minimal.

Brittain

Welcome to the The Rocketry Forum!

That was a very cool technical achievement by the way.

Greg

 

[georgegassaway]

Regarding Sunguidance:

The biggest problem with this scheme is that you're limited to launching around noon on certain days of the year. We don't normally have to deal with "launch windows" when we fly.

Oh, I've flown it on various times of day and in winter too. It doesn't have to fly vertically, just take off within 30 degrees of vertical. Indeed my favorite trick to do with it is to angle it 30 degrees from vertical, AWAY from the sun, so when it flies it has to pitch itself towards vertical, then goes past vertical, and locks onto the sun at whatever elevation the sun happens to be.

Several of those have been below 45 degree sun elevation.

One thing I've not done is to try to fly it so late in the day that the sun would be at say 10 degrees above the horizon. THat would be risky.

BTW - Here's a page on my website about Sunguidance, and a link to a video.

https://georgesrockets.com/GRP/RandD/Sunguidance.htm

https://georgesrockets.com/GRP/video/VidFiles/Sunguidance_Web.mov

- George Gassaway

 

[jadebox]

Oh, I've flown it on various times of day and in winter too. It doesn't have to fly vertically, just take off within 30 degrees of vertical. Indeed my favorite trick to do with it is to angle it 30 degrees from vertical, AWAY from the sun, so when it flies it has to pitch itself towards vertical, then goes past vertical, and locks onto the sun at whatever elevation the sun happens to be.

Several of those have been below 45 degree sun elevation.

One thing I've not done is to try to fly it so late in the day that the sun would be at say 10 degrees above the horizon. THat would be risky.

BTW - Here's a page on my website about Sunguidance, and a link to a video.

https://georgesrockets.com/GRP/RandD/Sunguidance.htm

https://georgesrockets.com/GRP/video/VidFiles/Sunguidance_Web.mov

Thanks George. You've done some really cool stuff!

I was, of course, being a little flippant. But, there was a serious point to my comment. You can't use a sun seeker to make the rocket fly straight up (which might be your goal) unless the sun is straight above.

Combining an angled sun seeker with another form of guidance, such as the ones used to prevent rolling, might work, though. But, I'm pretty sure that would be very challenging.

-- Roger

 

[luke strawwalker]

My roll stabilized rocket actually did use a helicoptor gyroscope to drive the servo for the the stabilization fins. There was no optical sensor and the duration of the flight is short enough (<2 min to apogee) that the drift of the piezoelectric gyros is minimal.

Brittain

COOL! For some reason I thought it used light sensors... that's the first time I've heard of a heli gyro being used for guidance! I talked to Pursley about it and he had researched it but said the gyro "drift" wasn't suitable for rocketry... of course though his system was actively guided in pitch/yaw, with no control in roll...

I presume your system is strictly in roll, with passive "fin" stability in pitch and yaw, which would cause FAR less concerns about gyro drift since a gentle roll is tolerable...

Later! OL JR

 

[WizardOfAZ]

Maybe this would be too complicated, archaic, or just wouldnt work, but I imagined a purely pneumatic stabilization system where a weighted pendulum is attached to a manifold pressurized by CO2 cartridge, and ported to four sides of the rocket body. Much like an aviation gyroscope in a small airplane, when the rocket tipped out of vertical, the pendulum seeking gravity would then open the manifold and direct a small blast of air to the appropriate direction to achieve vertical. I suppose with the thrust force and inertia of the pendulum might be pinned aftward and this concept be flawwed, but certainly it could be though through and in some way amended. I dont know. Im just thinking as i type, and imagining something like the Saturn launch in Apollo 13, where you see the active gas thrusters stabilizing the rocket during ascent.

 

[jadebox]

... when the rocket tipped out of vertical, the pendulum seeking gravity ...

Once the rocket left the ground, the pendulum wouldn't work. A simple thought experiment will illustrate why. Imagine that you're in the rocket and it has no windows or sensors to tell yu which way is "down." When the rocket is on the ground, you drop a ball. It falls straight down, so you can use it tell which way is "down." Then, the rocket motor fires. You feel the intense pressure of 6Gs of force. Assuming that you are not flattened to the floor and don't pass out, what happens if you drop the ball again? It still travels straight down to the floor. Unknown to you, however, the rocket has arced into the wind. The direction the ball travels is not down toward the center of the earth. But, the ball will still appear to fall straight down from your perspective. So, there's no way for you to tell which way is down using something as simple as a dropped ball (or a hanging pendulum).

But, you're in good company. Robert Goddard made the same mistake.

-- Roger

 

[Rex R]

something I wonder about. motor produces 7 gee thrust and accellerates at 6 gee (you feel 7gee straight down), if the rocket goes off vertical wouldn't the gavity component remain the same?

 

[jadebox]

something I wonder about. motor produces 7 gee thrust and accellerates at 6 gee (you feel 7gee straight down), if the rocket goes off vertical wouldn't the gavity component remain the same?

The only reason you feel gravity on the ground is that the ground keeping you from falling toward the center of the earth. You're actually experiencing -1G sitting or standing on earth. In free fall (assuming no air resistance), you'd feel 0G as you are accelerated at 1G toward the earth.

Once it leaves the earth, everything in the rocket is accelerated at the same rate. If you were in the rocket, while the motor is burning, you'd feel the acceleration from it. You, the rocket, and the pendulum would be accelerated at the same rate. The pendulum would point straight towards the source of the accerleration, the motor.

After burnout (assuming no air resistance on the rocket), you and the rocket would experience 0G. The rocket and everything in it would be in free fall and would accelerated at the same rate (1G). So, you'd float around in the payload section. The pendulum would float, too. It wouldn't point towards the earth.

-- Roger

 

[Rex R]

so when I'm in my car and leaving a stop sign, I'm just imagining that I'm just being pulled down into my seat?

 

[jadebox]

A much better explanation than mine of why a pendulum won't work is given in the Guidance and Control System FAQ.

Pendulums don't react to gravity.

Pendulums react to any moment (torque, couple) that appears between the weight and the pivot. When a pendulum is stationary this moment would be created by the force of gravity on the weight and the reaction force from the ground applied through the pivot. Now, what happens in a rocket (or other free body)? When the system is in free fall, the mass is pulled down by the force of gravity so it is accelerating downwards at 1 gee. Also, the pivot (and attached rocket) is pulled/accelerating downwards at 1 gee. Net result? Nothing. The force due to gravity is equal to the force required to accelerate the masses (inertia) at 1 gee so there is no net moment acting between the pendulum and the pivot.

The pendulum would react to external forces (thrust, drag...) but even with these present, it will still not react to gravity. That is, pendulums react *only* to forces applied to the pivot that are not applied to the weight. A pendulum inside a free body will exactly align it self with the forces that are applied to that body but not to the weight.​

-- Roger

 

[jadebox]

so when I'm in my car and leaving a stop sign, I'm just imagining that I'm just being pulled down into my seat?

I didn't say that. You feel the effect of gravity because the seat is preventing you from falling. If your car was falling at 1G, and was also being accelerated horizontally, you'd only feel pressure on your back.

-- Roger

 

[1tree]

The thing about the pendulum is that it will seek "trim." aAnd in the case of the rocket during boost the majority of the g-force (or trim) is felt down the length of the rocket. If you have ever been in a plane you have experienced a similar thing. The plane tips I relationship to the horizon, but you only feel a pull straight into the seat. (Well assuming the pilot keeps the plane in trim.) The end result is that under 6G acceleration your pendulum will only deviate 1/6 of what it will under no acceleration.

I think you could do it. But it would be much more complex because you have to account for the different rates. You would also have to deal with the oscillations that would naturally setup as the rocket corrected itself.

As for gyroscopes, they have progressed a lot and I personally don't buy that drift is an issue. The issue I would expect is that you might have to use raw input. In older RC helicopters the gyro ran independent of the radio control. And it had to ignore input from the pilot. Modern systems integrate the control. Also the cost and size has dropped dramatically.

I think if you combined the gyro with an accelerometer you could come up with a system that locks to what is considered vertical at the moment of launch and held the rocket to very close to that attitude. But you would have to develop the software. You would also need to experiment with how aggressively the fins moved.

The issue now is someone doing it. The cost could remain below the cost of some motors I have seen.

 

[WizardOfAZ]

A much better explanation than mine of why a pendulum won't work is given in the Guidance and Control System FAQ.

Pendulums don't react to gravity.

Pendulums react to any moment (torque, couple) that appears between the weight and the pivot. When a pendulum is stationary this moment would be created by the force of gravity on the weight and the reaction force from the ground applied through the pivot. Now, what happens in a rocket (or other free body)? When the system is in free fall, the mass is pulled down by the force of gravity so it is accelerating downwards at 1 gee. Also, the pivot (and attached rocket) is pulled/accelerating downwards at 1 gee. Net result? Nothing. The force due to gravity is equal to the force required to accelerate the masses (inertia) at 1 gee so there is no net moment acting between the pendulum and the pivot.

The pendulum would react to external forces (thrust, drag...) but even with these present, it will still not react to gravity. That is, pendulums react *only* to forces applied to the pivot that are not applied to the weight. A pendulum inside a free body will exactly align it self with the forces that are applied to that body but not to the weight.

-- Roger

Right...

 

[jadebox]

The end result is that under 6G acceleration your pendulum will only deviate 1/6 of what it will under no acceleration.

As explained earlier, gravity will accelerate the pendulum and the pivot it is hanging from equally. So, gravity will have no effect on its angle.

-- Roger

 

[georgegassaway]

A lot of good explanations of why a pendulum system won't work for rockets.

Here's another bug, that does not involve why gravity won't make it work.

It's lateral acceleration, or "sideways" G-forces relative to a rocket that for the sake of this beginning properly, at the launch is (was) pointed vertical.

Assuming a perfectly smooth vertical launch, the flight acceleration G force is perfectly straight down the length of the rocket, a line from the nose tip thru the center of the engine nozzle.

But then, a little bit of wind causes the rocket to have a very tiny angle of attack to the airflow, and therefore a tiny little lateral (sideways) G-force.

It's now doomed to rapidly force itself off-vertical and either hit the ground or do loops in the sky.

Note that the little lateral G-force in this case is not gravity-related, it's due to the airflow at a very slight angle of attack and therefore a slight lateral g-force. The rocket could be perfectly vertical but a wind gust could cause this lateral G-force.

The Pendulum guidance responds to this by producing a small corrective deflection to make it "turn" the other way to counter it the presumed off-vertical tilt, but that very corrective force causes its own G-load..... in the same direction as the original disturbance.

The Pendulum system then responds to its own self-generated lateral G-load by moving the control surfaces even more, causing even more of a lateral G- force, and in response to that more corrective "control" which feeds on itself rapidly until the Pendulum system reaches maximum control surface deflection and locks up completely, making the rocket do loops around and around until burnout (and then a few more loops till it slows down enough).

Actually what I described in the above paragraph would happen in a fraction of a second, from first tiny control response attempt to maximum lock-up.

So, if anyone does not quite believe that gravity won't make a free-flying pendulum work, then ignore the gravity part and note the above as to why it won't work for vertical guidance for rockets, it's own control response forces would lock itself up into a loop (if it did not hit the ground first).

- George Gassaway

 

[N2O4]

COOL! For some reason I thought it used light sensors... that's the first time I've heard of a heli gyro being used for guidance! I talked to Pursley about it and he had researched it but said the gyro "drift" wasn't suitable for rocketry... of course though his system was actively guided in pitch/yaw, with no control in roll...

I presume your system is strictly in roll, with passive "fin" stability in pitch and yaw, which would cause FAR less concerns about gyro drift since a gentle roll is tolerable...

Later! OL JR

You are correct that the basic rocket stability is achieved via passive aerodynamic stabilization. As a matter of fact I actually made the canard fins small enough where they couldn't significantly affect the stability of the rocket even if something happened to the control system. I used a simulator to determine the maximum flight path deflection if the movable fins both went to full deflection in the same direction.

The biggest issue I had to work out was not gyro drift, but the speed of the servo. The gyro could produce a signal fast enough, but I had to have a control system that could make corrections quickly enough so the rocket would not start oscillating. I had to use one of the high speed servos to do this. I also had to set the pivot point of the fins right at the aerodynamic center of pressure to reduce the force required from the servo to move the fin and allow the servo to operate at it's highest slew rate. I used a little baby homemade wind tunnel to find the actual center of pressure. On subsonic flights this worked great as the CP doesn't really move as you go faster. On the supersonic flights I actually had to go to a special planform for the canards as a normal fin's CP will shift significantly rearwards when going from subsonic to supersonic. I spent a fair amount of time on the stability of the control system.

It was an interesting project that produced lots of interesting little learnings.