vertical stability with thrusters

Discussion in 'High Power Rocketry (HPR)' started by Chad, Jan 24, 2020.

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  1. Jan 24, 2020 #1

    Chad

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    There's a couple resources out there re: using gas thrusters for, say, reducing roll. The conclusion is they're very heavy and expensive to get to work. The most successful systems seem to be canards like what is discussed in the "I could just use a little guidance thread" in the staging, airstarts, and clustering forum.

    Like a song stuck in my head, i keep thinking about using pressurized water for a thruster. Imagine a 12oz, thick, plastic water bottle with a schrader valve on one end. You use a bike pump to pressurize it and then use something like a diy pinch or poppit valve controlled by a microcontroller for release. What you have is basically a small but powerful water gun. Given the density of water and good old F=MA you should get much more force compared to say co2 or N2. You have to lift the water but at least it's not in a heavy pressure tank.

    So, why won't this work? Surely it's been tried before. I have the programming and electronics experience necessary for a prototype but I don't want to invest the time and money if it's already been done and shown to not work.
     
  2. Jan 24, 2020 #2

    crossfire

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    Go for it.
     
  3. Jan 24, 2020 #3

    blackjack2564

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    that's only half of it....then ya need to design the controller and codes to tell the thrusters what to do.
     
  4. Jan 25, 2020 #4

    jderimig

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    Liquid water is not going to generate alot of force. Low velocity. Now add a rapid heating element to your system and convert the water to steam and you may have something there.
     
  5. Jan 25, 2020 #5

    jlabrasca

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    Or maybe a suitably combustible chemical mixture packed into an open-ended tube of some sort?

    There is like two quarters of mechanics and fluids and thermo to unpack here -- but briefly, it might help to stop thinking in terms of Newton's second. Think, rather, in terms of the first law of thermodynamics. Water, being (near enough) incompressible, the work of moving the rocket will have to be by the conversion of energy "stored" in the compressed air (or other gas) above the water in the tank and/or in the elastic deformation of the tank itself. The system will have a low energy density.

    You might also think about the specific impulse of liquid water as a propellant.

    https://www.grc.nasa.gov/WWW/K-12/airplane/specimp.html


    edit: You know what, never mind that last bit. Too many things to fiddle with. I'll stop with work-energy equivalence .
     
    Last edited: Jan 25, 2020
  6. Jan 25, 2020 #6

    mbeels

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    I think it would be simpler to think in terms of F = d/dt ( mv ). Or force is proportional to rate of momentum change. A heavier fluid (liquid water) moving at a relatively slower velocity could exert the same force as a lighter gas moving at a relatively higher velocity. (A water rocket goes higher than a rocket with pressurized air alone.) But I think that the control portion would be extraordinarily difficult.
     
  7. Jan 25, 2020 #7

    jlabrasca

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    Seriously don't want to get into specific impulse. The constraint is the energy per unit weight for the system. A water rocket thruster will deliver less work per newton of weight than would a chemical rocket.

    https://www.airplanesandrockets.com/motors/Jetex-Motors.htm

    But what are you trying to correct with this active stabilization? The OP mentioned roll, but in case he meant pitch -- the correcting torque will increase with the distance between the COM and the nozzle. Put the thrusters in the nose to get the maximum correction for the delivered thrust. You're still lifting batteries (or capacitors) and servos /actuators and valves and water in a tank to do the job that can be done by fins (or by the conservation of angular momentum if you can to spin the rocket)
     
  8. Jan 27, 2020 #8

    Chad

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    thanks everyone for your replies.

    I found some little 12 volt 1/4" solenoids that work up to 115 psi on amz, i can control it with a spare DC motor controller breakout board i have and my trusty Adafruit Feather. I may try and knock something together and use a digital scale to see what kind of force I can get and for how long.

    In my mind i'm imagining little squirts every so often to keep the rocket pointed mostly straight. It's not like i'm trying to fly the rocket and do acrobatics, just a little nudge here and there. One of my other electronics projects is a tilt lockout mechanism so I'm well on my way as far as control electronics/software goes.

    you know what would be really neat? Some sort of squib type setup with blackpowder, there's a lot of energy per unit mass available there. It would be cheap, but time consuming, to setup an array of tiny squib charges. I make my own ejection charge canisters from centrifuge tubes and nichrome wire (which is like soldering hair btw :mad: ). I can visualize how to put it together but it would take forever.
     
  9. Jan 27, 2020 #9

    Nytrunner

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    However that's a fantastic idea for making a functional scale PAC-3 Patriot missile
     
  10. Jan 27, 2020 #10

    jadebox

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    Instead of squibs ...

    How about a ring of small model rocket motors around the body tube or ringing the main motor at the base of a larger rocket? :)

    -- Roger
     
  11. Jan 27, 2020 #11

    heada

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    MMX motors? small, fast burning, low thrust and a known item.
     
  12. Jan 27, 2020 #12

    jlabrasca

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    Yes. What could possibly go wrong? >smile<
     
  13. Jan 28, 2020 #13

    jderimig

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    Why not just make a tiny hydrogen peroxide motor and feed it will a small solenoid pump?
     
  14. Jan 28, 2020 #14

    cerving

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    ^^ This... ^^ :)
     
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  15. Jan 28, 2020 #15

    jadebox

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    Yes. I wasn't totally serious, but thinking a little more about it .....

    It would be quite a challenge to determine which one to ignite and to get them to ignite quickly enough, but on a larger rocket the effect could be small enough to be safe and, possibly, large enough to be somewhat effective.

    The smaller motors could be in rings at different offsets from the center giving the controller more .. uh ... control.


    Handling small motors would definitely be safer than squibs.
     
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  16. Jan 28, 2020 #16

    Tim51

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    Modern HPR 2 describes Kimberley Harms and the Washington Community Space Programme using G motors to roll their full size Honest John during flight in this fashion, but the motors ringed the upper 'warhead' section I believe.
     
  17. Jan 28, 2020 #17

    Serac

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    If you are going to pressurize the water with something (CO2, black powder, etc), my thought is that you should use that, and not water, for the reaction. Yeah, you would get momentum transfer with water. But, at the expense of the weight of the water, and the weight of the water handling machinery. You should design a simple controller on paper, and estimate how much water you may need. My gut is that to use a relatively small amount of water, you are going to need high pressure, and the efficiency savings will be moot.

    This sounds really hard and complex to me. If you're going this route, you should just bite the bullet and design a hydrazine monopropellent attitude thruster....
     
  18. Jan 28, 2020 #18

    Chad

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    yeah water is heavy, no weigh( ;) ) around that.

    Even if the physics all worked out the trigger valve question is out there, it would have to be an instant on / instant off type thing. I emailed a very specialized airgun manufacturer here in DFW about the idea and how knock-open valves work in airguns. We'll see how crazy they think the idea is.
     
  19. Jan 28, 2020 #19

    jadebox

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    Have you seen what Joe Barnard has been working on? (Well, one of the things he is working on.)

     
  20. Jan 28, 2020 #20

    jlabrasca

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    What, exactly, are you trying to achieve? Since you mention the tilt lock-out, I am guessing this is for a staged rocket. How long is the burn/coast that you expect to have time for active control to be effective?
     
  21. Jan 28, 2020 #21

    Chad

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    that's pretty amazing! I emailed him and told him to check out the valves on .50 caliber airguns.

    at the end of the day, i want to fly very high. The way we do it now is with high impulse, very high thrust motors and accelerating to way over the speed of sound at the very bottom of the atmosphere and coasting up. It would be better to use a very high impulse, moderate thrust motor and take our time going through the bottom of the atmosphere. The problem is the longer the rocket is under thrust the more likely it is to arc over and, once it starts, it only gets worse.

    for ref: https://www.apogeerockets.com/Peak-of-Flight/Newsletter483
     
  22. Jan 29, 2020 #22

    jlabrasca

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    Okay. I would point out that this is a problem that has been solved a bunch of different times, a bunch of different ways, for guided missiles -- but this is a hobby and its fun to try to innovate. Still, prior art might give you some inspiration?

    Man, I just got rickrolled. Watching Joe Barnard do anything makes me want to reach through the computer screen and take the tools out of his hands. I cannot always stop myself from shouting when he explains things. I do get why folks find his stuff interesting, even admirable, but I teach physics to engineering students for a living.
     
  23. Jan 29, 2020 #23

    georgegassaway

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    Best to use aerodynamic control. Far simpler. A rocket-science problem with RCS for vertical guidance is that the normal aerodynamic stability of the rocket becomes stronger and stronger the faster it goes, as a square of the velocity (forces on a model going 400 mph are 4 times greater than a rocket going 200 mph, not double).

    So, then it needs a massive amount of RCS force to attempt to be effective. Even at a relatively slower speed. The only good use for RCS would be near apogee if you wanted to try to keep the payload section or rocket pointing vertical all the way to apogee, in very thin air. But that RCS would be terrible for the early launch phase.

    And yes I know of Joe Barnard's ground experiments. I advised him of the same problems. Interestingly he's not done any updates on that for a very long time, which may be more due to the technical problems than the practical flight problems, or maybe he did realize it wasn't going to have the power to steer a HPR rocket well enough. I personally thought of it as a likely dead-end, neat idea but aerodynamic steering far more practical. Of the video tests I did see, it looked like the thrust responses from the RCS thrusters were nowhere near what they would need to be.

    I will say that ages ago, around 1988 or 1989, I did a bit of ground testing using a Freon-type roll control "thrusters", in a proof of concept thruster roll test for my Sunguidance system (sensors rigged to sense sunlight from the side, to keep one side rolled towards the sun. To be launched vertically with the sun at 45 degree elevation or less. It was simply two pieces of brass tubing bent 90 degrees, "nozzles down", rigged to a servo to rotate like ailerons on a plane, to produce roll control. I hung the rocket vertically by the nose on a string, and simply used a freon-ish replacement can I held externally, with a hose that connected to a "Y" joiner for flexible tubing that ran to the brass tubes for the roll control system. I thought of it sort of like an Atlas Missile's "vernier" thrusters, except it only worked for roll, nothing else. Anyway, the ground testing went OK, it would maintain roll. But I never tried to come up with an internal tank system that would make it practical to test fly. Also, no valves, it would have started (likely unplugged) for liftoff, both brass tubes pointed straight down, with continuous thrust until the tank was empty. As it started to roll, then the servo would move them (aileron-style) to correct the roll.

    While I never test flew the "vernier roll thruster" method, I did do some roll control sunguidance tests using aerodynamic aileron-ish control vanes at the middle of the model. It sorta worked but had some issues with overcontrol, as it needed to be dialed in better (smaller vane area and/or less servo travel).
     
    Last edited: Jan 29, 2020
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  24. Jan 29, 2020 #24

    Chad

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    Thanks again for all the feedback, this forum is a bright spot on the internet. One of the other things i'm working on is an orientation and state software/hardware stack that is as accessible and hackable as possible. Regardless of what type of stability system used the orientation data is a pre-req. I'm going to focus on that software for the time being and come back to this down the road.
     

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