I could use just a little guidance

[jderimig]

Spinning the whole rocket doesn't require extra mass and length unlike using an internal gyroscope that's why.

And its cheaper, more reliable and the simplest solution. If you don't want video.......

 

[TonyL]

jderimig,

spinning the whole rocket or just an internal wheel to achieve a particular angular momentum does not alter the gyroscopic effect. Also spinning a rocket at a few Hz does not magically straighten it out, it still cones for the reasons you mention [gyroscopic effect, plus the usual others], but if done correctly, the dispersion pattern is smaller than without spinning. One program a number of years ago wanted very slow rockets, so they spun them up at 3000rpm and launched them [fin-less I believe] to make them go straight. The only unguided, finless rockets that I am aware of [not counting the ones with sticks], though the big spinning wheels in Thailand are functionally similar.

br/

Tony

 

[dhbarr]

And its cheaper, more reliable and the simplest solution. If you don't want video.......

Despinning video in post should be fairly straightforward as long as { something something roll rate vs. frame rate }. I'm a bit surprised we haven't seen that white paper yet

 

[jderimig]

jderimig,

spinning the whole rocket or just an internal wheel to achieve a particular angular momentum does not alter the gyroscopic effect.

Tony

Correct but spinning the rocket helps to null out any thrust or lateral aerodynamic forces on the airframe.

 

[Nytrunner]

Is this the “Tony” from IREC?

Wrong Tony. He's here though you'll have t'fish around if you want to find him.

 

[TonyL]

Correct but spinning the rocket helps to null out any thrust or lateral aerodynamic forces on the airframe.

I would like to see the math on that.

 

[gbh]

Guys, you are over thinking the problem. 99+% of our rocket fly straight because they are designed to be aerodynamically stable. The aerodynamic shape and the center of gravity coupled to the forces generated by the fins direct the rocket to minimize the angle of attack of the airflow over the fins. A perfectly balanced rocket do not always ascend vertically due to the simple fact that the axial motion of the rocket and the perpendicular velocity of the wind produce a non vertical minimal angle of attack trajectory solution. And once you are in a non-vertical trajectory, a gravity turn is initiated and the effect will increase as a function of time, further deviating the trajectory off vertical.

The true trajectory solution is a 6DOF problem: linear velocities and accelerations in the x, y, and z directions, and rotational velocities and accelerations in the x-y (roll), y-z (yaw), and x-z (pitch) planes. In aerodynamically stable unguided rocket flight, the principal forces are generated by the thrust of the motor, the aerodynamic drag of the atmosphere and the gravitational force of the earth. All other motions are cross axis couplings due to wind and imperfections in the rocket shape and mass distribution resulting in non-vertical flight.

The major instability in most high altitude attempts is coning, or roll-pitch coupling. This coupling causes the base of the rocket to react with an undamped, increasing amplitude oscillation that ultimately upsets the rocket by causing it to turn sideways in the airflow and disassemble. Conversely by controlling and modifying the roll and pitch of the rocket vertical flight can be maintained. Alternatively, one can control the yaw and the pitch of the rocket maintain vertical flight however this method does not stabilize roll, which may or may not be an issue.

Most of the pitch changes in a rocket flight that result in a non-vertical trajectory occur in the first few seconds where the axial velocity is low and the influence of the crosswind is greatest. Active roll-pitch or yaw-pitch control can prevent this from occurring during boost, or the correction can be applied during coast after boost. The former is preferred if maximum altitude is desired, but the lower energy solution is during coast.

The average natural angular pitch and yaw velocities of an aerodynamically stable rocket are very low, a few degrees per second or less, due to the large moment of inertial along the axis of the rocket. The rotational velocity of a rocket however can be several orders of magnitude higher, up to over several thousand degrees per second (rotation of 1 Hz is 360 degrees per second.) To control pitch and yaw you do not need a very high frequency control loop. To control roll you need a faster response however the inertial forces are lower and the torque requirements will be lower so from a controls perspective is may be a wash.

Note that the axial velocity and acceleration is not a significant player. The effect of velocity is that the power that must be developed by the servos is approximate proportional to the mach number cubed (aerodynamic drag forces) however since the motions are slow, the AOA corrections do not have to be large amplitude so the power requirement is not excessive.

the point I am making is that control actions are not the issue in a properly designed system. The problem remains determining true vertical. With a 6DOF inertial sensor, you should be able to maintain vertical fight with 5 valid sensor inputs, so if the z-axis accelerometer is overanged, vertical determination is still possible. While the correction may be less accurate during motor burn than during coast, it is still better than no correction, and once the motor burns out, you return to full 6 sensor data and have the maximum accuracy.

FWIW

Bob

This is a very interesting thread! Quite a difficult problem to overcome and very impressive results. I was not aware of the term "gravity turn" due to less than vertical orientation at launch. The analogy of a balancing telephone pole makes me realize that some of my less than vertical flights are due to not having a vertical start to the launch. My approach to spin stabilization is a little crude, and it is counter productive to a high performance flight. I use relatively massive components for my rocket construction and my fins are massive and wide. In my case it is all about rotational inertia. no yo-yo de spin needed as it starts out not wanting to spin.

 

[UhClem]

I would like to see the math on that.

If the rocket doesn't spin, any thrust misalignment will cause it to pitch over. A slight misalignment of the fins can also cause it to pitch over.

With some rotation those forces will get distributed about the circle and if not null themselves out, at least greatly decrease the net effect. This is a well known way of reducing dispersion:

"During flight, all launch vehicles are imparted with a spinning motion to reduce potential dispersion of the flight trajectory due to vehicle misalignments." [url=https://snebulos.mit.edu/projects/reference/launch_vehicles/NASA/SRHB.pdf]Sounding Rocket Program Handbook pg. 28[/URL]

An interesting bit of information in that document is Table 3.3.1 which shows the expected dispersion (3 sigma) for two available guidance systems. The system most like what Jim is using has a dispersion of 7.5% of apogee. (down and cross range)

 

[TonyL]

UhClem,
Yes indeed it is an effective method [and thanks for the link], but you have not quantified the relative merit of it versus just using an equivalent angular momentum. Ultimately I don't particularly care which is better, just that it would be interesting to understand how much contribution the gyroscope effect makes.

I particularly like how in fig. 3.2.1.1 on page 31, every two stage far exceeds 100km.

br/

Tony

 

[JimJarvis50]

I'm going to start over on documenting the LDRS flight I'm working on here in the "guidance" thread, where it belongs, and not in the three-stage thread. To review, the project will use my single stage test rocket (with a spin can) for the sustainer. The booster will be the booster that I used on my two-stage test rocket, which I flew to test out having the stabilization section located between the stages. The flight would be with Gorilla motors (M1665 to M745) to about 25K with stabilization at the top of the sustainer and active for the entire flight.

The main challenge so far has been to design an interstage coupler that doubles as the bearing surface for the spin can. The coupler has to be strong enough for the flight profile and to transmit the booster and sustainer motor forces to the air frame. It turned out to be a relatively simple piece to design, although it took a fair amount of thought to get everything oriented correctly. The spin can spins just fine.

Jim

 

[Nytrunner]

Fascinating

Is your theory that the canard flow won't act on the booster fins since they are now not directly behind the guidance section?

 

[JimJarvis50]

Fascinating

Is your theory that the canard flow won't act on the booster fins since they are now not directly behind the guidance section?

Well, the canards will be pretty far away from the booster fins. Also, the controls will have a relatively low gain for this flight. Finally, the sustainer fins might help to straighten out the flow a little. I think it will be OK, but we'll find out.

The spin can that I used on the first stage of the three-stage flight can only be used with a 98mm 6XL case, and I don't want to use a motor that large for this flight. Ideally, the entire flight will be sub-sonic.

Jim

 

[Not Quite Nominal]

A few thoughts regarding vortex-inducing control reversal (a bit late to this thread)

Tip vortex / induced drag is inversely proportional to the square of span. Material strength permitting, a higher aspect ratio canard (and consequent lower AoA) will generate a weaker tip vortex for the same control force.

The spin can is probably the more clever solution though.

 

[JimJarvis50]

A few thoughts regarding vortex-inducing control reversal (a bit late to this thread)

Tip vortex / induced drag is inversely proportional to the square of span. Material strength permitting, a higher aspect ratio canard (and consequent lower AoA) will generate a weaker tip vortex for the same control force.

The spin can is probably the more clever solution though.

I had seen that concept before. I was a little unclear if the canard span has to be greater than the fin span, or just a higher aspect ratio, but your comment on the lower AoA makes sense. Generally, there would be an issue with maintaining a stability margin for my rockets, as I would never fly the equipment I have with an inherently unstable rocket.

Not sure the spin can is more clever, but it is fun and quite inexpensive.

Jim

 

[Not Quite Nominal]

I had seen that concept before. I was a little unclear if the canard span has to be greater than the fin span, or just a higher aspect ratio, but your comment on the lower AoA makes sense. Generally, there would be an issue with maintaining a stability margin for my rockets, as I would never fly the equipment I have with an inherently unstable rocket.

Not sure the spin can is more clever, but it is fun and quite inexpensive.

I don't think that canard span to fin span ratio would come into effect until the fin is so big that it is hit by both sides of the tip vortex.

Regarding the tip vortex itself: In the simplest terms, lift is caused by a pressure differential between the two surfaces of a wing. The air that flows chordwise is useful airflow that produces lift. The air that loops around the wingtip is useless airflow that produces drag.

Greater wingspan decreases drag because a larger proportion of the air is productively flowing chordwise and a smaller proportion is flowing spanwise (great simplification)

Lower angle of attack decreases drag by decreasing the pressure delta between the two surfaces, which decreases the tendency for air to "escape" by circulating around the wingtip.

In your case, greater canard span has a third benefit: by extending the span, the Cp of the canard moves outward, which means that for the same desired roll torque, a smaller force is needed.

A small increase in span (on the order of 50%) could have an big reduction in tip vortices.

As to the spin can: It's a clever solution that allows for yaw and pitch stability without having any effect on roll. I'm considering making one with an ACME can just for amusement value. (ACME can and MMT on the bottom with a free-spinning fuselage that goes into it. I'll spin up the fuselage with a drill right before launch for stability and amusement)

 

[JimJarvis50]

Once I get the spin can modifications done, I'll take a look at the stability and the added weight that would be required in the nose and see what the options are. I'll report back.

You simply must post a video of your spinning fuselage.

Jim

 

[JimJarvis50]

So, here's the configuration with my "medium" canards. I have three sizes with spans of 2.5", 1.875" and 1.125". For the medium canards, the sustainer stability is 1.5 calibers with no weight added to the cone. The large canards require 2 pounds added to the nose and the smaller canards are overstable at 2.2 calibers. Given the higher velocity of the flight and the fact that the canards are well away from the CG, I think I need to restrict the motion to no more than 2° or 3°. Not really much flexibility, but then again, it's pretty much exactly what I want.

Jim

 

[Not Quite Nominal]

I suppose that is an inherent drawback. More canard area up front means less induced drag for the same control force, but that necessitates an equivalent movement of the CP forward.

Is it possible to move the canards aft? Distance of the canards to CG dictate the lever arm for yaw/pitch control, but roll control is independent of distance to CG. The question now is how much AoA you're using for pitch vs. roll control.

The nice thing about the pitch/yaw inputs is that they are symmetric and don't seem to cause roll reversal. Larger canard, further aft would provide the same control force (assuming the canard area increases by the same proportion as the lever arm decreases). If you're using less control for pitch than roll (which would be my guess, given that your roll lever arm is measured in inches and your pitch lever arm in feet), you might be able to move them even further back.

Larger canard, same location: less vortex in roll, more pitch/yaw, less stability

Larger canard, moved proportionally aft: less vortex in roll, same pitch/yaw, smaller effect on stability.

Larger canard, moved more than proportionally aft: less vortex in roll, slightly less pitch/yaw authority, perhaps zero effect on stability.

Not having your numbers in front of me I'm obviously spitballing here, but I think the yaw/roll/stability tradeoff can be worked to a good solution.

 

[Nytrunner]

Since the control fins are actively making adjustments to keep the vehicle heading vertical, is static margin and CP shift really as much of a concern here as it is in a normal, passive, unguided rocket?

Looking at this staged configuration of your guidance test bed rocket (guidance section up top), it reminds me of actual tactical missiles with forward control surfaces that are Intentionally less stable so that they can course correct more efficiently. Admittedly, you aren't trying to seek a moving target ("up" isn't trying to get away from you ), so your use case is a bit different.

Is your prime concern retaining stability in case the system doesn't engage pitch/yaw correction?

 

[JimJarvis50]

Is it possible to move the canards aft? Distance of the canards to CG dictate the lever arm for yaw/pitch control, but roll control is independent of distance to CG. The question now is how much AoA you're using for pitch vs. roll control.

In theory, it might be possible to move the control section downward to sit on top of the altimeter bay rather than on top of the main chute bay. It would be difficult to get much vertical stabilization there though. You probably noted that the way I have done two-stagers to this point is to have the control section between the stages, and then it drops off when the sustainer lights.

It's difficult to describe how much AoA is used for pitch versus roll. Pitch is pretty easy. A typical flight might arrange for the canards to turn equal to the angle of the rocket up to about 7 degrees and then stay at 7 degrees for higher angles. Roll is harder to visualize, but basically, the angle of movement is up to 7 degrees for rotation rates of 500 degrees/sec. But, only one-third of the servo range is allocated to roll control. It gets more complicated than that though because it is set up to send control movements to the canards with the lowest angular changes. So, if a canard is maxed out for pitch control, more control action would occur for the canards that are not maxed out. The idea of this was to keep the canard angles similar. But I have videos where you can see one canard moving and the adjacent one still. It all seemed like a good idea at the time and it seems to have worked.

One thing that I have noted is that with canard angles similar to the angle of the rocket, it takes nominally 5 seconds or so for the rocket to move from an angle to vertical. I have four flights showing much the same timing. One variable that I think is important, but that I have been unable to quantify, is how overstable the rocket is to begin with. I can easily calculate the effect of the canard torque on the roll rate, but I haven't been able to do that on the yaw/pitch rate.

Jim

 

[JimJarvis50]

Since the control fins are actively making adjustments to keep the vehicle heading vertical, is static margin and CP shift really as much of a concern here as it is in a normal, passive, unguided rocket?

Looking at this staged configuration of your guidance test bed rocket (guidance section up top), it reminds me of actual tactical missiles with forward control surfaces that are Intentionally less stable so that they can course correct more efficiently. Admittedly, you aren't trying to seek a moving target ("up" isn't trying to get away from you ), so your use case is a bit different.

Is your prime concern retaining stability in case the system doesn't engage pitch/yaw correction?

I'm not an expert, but I doubt that the system I use is fast enough to keep a rocket under control that is not inherently stable. That said, I think setting up the sustainer to be a little less overstable might be a good thing - getting the yaw/pitch control a little more in balance with the roll control. I have been toying with the idea to add a little weight to the back of the rocket to accomplish perhaps a stability right about 1 caliber or a touch less. That's about as far as I'm willing to go.

Jim

 

[Not Quite Nominal]

Since the control fins are actively making adjustments to keep the vehicle heading vertical, is static margin and CP shift really as much of a concern here as it is in a normal, passive, unguided rocket?

The problem here is that if the control surface is moved back to increase static stability, the lever arm for the control surface shortens, making it less effective.

If the control surface is left forward, it is more effective at control, but decreases static stability.

In theory, it might be possible to move the control section downward to sit on top of the altimeter bay rather than on top of the main chute bay. It would be difficult to get much vertical stabilization there though. You probably noted that the way I have done two-stagers to this point is to have the control section between the stages, and then it drops off when the sustainer lights.

In that case moving the control surface lever arm is not possible, which means that my previous solution of moving the control surface back to compensate for larger size won't work for this configuration.

IIt's difficult to describe how much AoA is used for pitch versus roll. Pitch is pretty easy. A typical flight might arrange for the canards to turn equal to the angle of the rocket up to about 7 degrees and then stay at 7 degrees for higher angles. Roll is harder to visualize, but basically, the angle of movement is up to 7 degrees for rotation rates of 500 degrees/sec. But, only one-third of the servo range is allocated to roll control. It gets more complicated than that though because it is set up to send control movements to the canards with the lowest angular changes. So, if a canard is maxed out for pitch control, more control action would occur for the canards that are not maxed out. The idea of this was to keep the canard angles similar. But I have videos where you can see one canard moving and the adjacent one still. It all seemed like a good idea at the time and it seems to have worked.

That makes a lot of sense. What speed range are you most interested in for pitch control? Watching your video of the booster hitting apogee nearly vertical makes me think that at least for the booster configuration, you've got excess pitch authority for most of the boost.

Also watching the video it looks like you're not getting huge opposing deflections, so you might be able to reduce your roll control deflection limits? You might be under-controlled at the very beginning and end of the flight where you are slow, but you'd have enough during the critical middle part of the boost, and you'd reduce your AoA and possibility for control reversal?

One thing that I have noted is that with canard angles similar to the angle of the rocket, it takes nominally 5 seconds or so for the rocket to move from an angle to vertical. I have four flights showing much the same timing. One variable that I think is important, but that I have been unable to quantify, is how overstable the rocket is to begin with. I can easily calculate the effect of the canard torque on the roll rate, but I haven't been able to do that on the yaw/pitch rate.

Roll rate seems to be an easier problem, because there aren't all the aerodynamic effects of all of the rest of the rocket pitching and yawing when a pitch/yaw control input is put in.

 

[TonyL]

Hi Jim,
while active stabilization is certainly a norm in tactical vehicles, it does bring with it the burden of 'having to work', so your plan for static stability is probably the best approach unless you want to tackle 'system identification' so you can quantify your servo loop stability margins. If that sounds like a lot of jargon to folks, it is. However it would also all be relevant for active stability control. I like your proportioned 'control sharing' between pith and roll, I still think you could allocate more to pitch if you wanted. For estimating the pitch performance, commanding an s-shaped path [ideally a 'chirp'] would be a simple way to back out the canard angle to vehicle response relationship from recorded data [at least over a small part of the flight regime, which is a start]. Coming up with a torque estimate is more complicated but not necessarily critical as vehicle response is what you care about most and scaling it a modest amount for different vehicles should work.

I have been corresponding with Chuck at RASAero, and a static margin of at least 2 over all conditions seems like a good place to be for supersonic flight. That can mean way over-stable at other times. Your vertical autopilot can reduce the necessary margin, but to quantify just how much probably means trial and error if you don't delve into system identification.

br/

Tony

 

[OverTheTop]

Your vertical autopilot can reduce the necessary margin, but to quantify just how much probably means trial and error if you don't delve into system identification.

The main assumption with this statement is that the autopilot is working all the time. I know my system is only active during boost, and I think Jim also only has active control for part of the flight envelope. That means we are stuck with having a decent margin of around two as a starting point if the flight is going to delve into the supersonic region. Personally I have not pushed mine to that regime yet.

 

[Nytrunner]

Unknown if Jim will code in this direction, but moving the control surfaces to the nose of the sustainer as he has done could allow him to maintain active control throughout the whole flight.

 

[JimJarvis50]

Unknown if Jim will code in this direction, but moving the control surfaces to the nose of the sustainer as he has done could allow him to maintain active control throughout the whole flight.

The plan would be to have it active through the flight. The current profile, and with the addition of a few external cameras, will keep the maximum speed somewhere around 1000 ft/s. So, I need to set things up with this speed in mind, which means pretty minimal control surface movements. I do plan to reduce the roll response relative to yaw/pitch. The rocket will be overstable on boost by quite a bit, and the sustainer would go from 1.5 to 2.7 calibers or so with the motor burn (although I might reduce this initial margin a little).

Jim

 

[Nytrunner]

Just saw this Wallop launch video of a two stage with active control canards. reminded me of your videos

I like watching it freak out in the higher atmosphere where it has less control authority



*facebook and youtube sanity rule applies: Don't read the comments

 

[TonyL]

I have to say I did not expect free spinning canards, very interesting. They are managing guidance while spinning, but for a half million guidance module one should expect that

 

[OverTheTop]

I have to say I did not expect free spinning canards, very interesting.

I am going to make a guess that they are using brushless DC motors or servomotors, with encoders.

We have a neat drive on one of our products that has an encoder and the resolution is 1/6000000 of a revolution, absolute. 4ms movement time too (accelerate, move, decelerate) .

 

[JimJarvis50]

I have to say I did not expect free spinning canards, very interesting. They are managing guidance while spinning, but for a half million guidance module one should expect that

Interesting video! It would be really nice to be able to do the guidance while spinning.

Speaking of spinning, for the LDRS flight, I'm going to reduce the roll gain by about a factor of three. Nothing quantitative about it except for feel, and it's moving in the right direction.

Jim