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Help requested on simple thruster calculation

Discussion in 'SpaceX' started by George S. Bower, May 15, 2019.

  1. George S. Bower

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    I am trying to figure out how much thrust a certain cold gas thruster would produce.

    Assume specific impulse=60 seconds and a mass flow rate of 20 lb/sec

    on earth at sea level

    What is the calculated thrust???

    Thx,
    George
     
  2. bxr140

    bxr140 Active Member

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    Assuming basic competency in referencing SMAD, T = Isp*mass flow*gravitational constant
    Assuming basic competency in my maths, thrust is 5349N, or 1202 lbf in Freedom Units.

    That's a huge cold gas thruster...
     
  3. George S. Bower

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    Thx bxr
    I got 1200 lbf
    so our math agrees. I used 1200 for the roadster 0-60 time calc w/ thrusters.

    Roadster 0-60 times calculated with cold gas thrusters

    Do you know how many lb thrust one of falcon nine first stage thrusters is that they use for landing the booster?
     
  4. Yggdrasill

    Yggdrasill Active Member

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    81,000-212,000 lb, depending on throttle setting.
     
  5. George S. Bower

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    I was referring to the thrusters that are by the grid fins.
     
  6. Yggdrasill

    Yggdrasill Active Member

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    Ah, I don't think there are any official numbers on those.
     
  7. George S. Bower

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    Agreed I can't find it on a google search. BXR140 above commented that the thruster I calculated in the OP is a big one. Maybe the 1200 lbf thruster I'm looking at IS bigger than current tech.
     
  8. miimura

    miimura Active Member

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    Where did the 12 lb/sec figure come from? That mass flow rate is equivalent to about 9,412 cfm at STP.
     
  9. bxr140

    bxr140 Active Member

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    What’s your application?

    The reason cold gas isn’t usually used in higher thrust applications is because the Isp is so low you need a ton of propellant to do anything...and at that point its usually more efficient to use more reactive propellants with much higher Isp.
     
  10. George S. Bower

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    I assumed 100 lb of air (mostly N2) discharged in 5 seconds= 20 lb/sec
     
  11. George S. Bower

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    Tesla Roadster
     
  12. bxr140

    bxr140 Active Member

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    Ah, didn’t click the link up thread.

    Bear in mind that the pressure vessel is probably going to weigh quite a bit, and take up a ton of space. For instance, a typical k bottle of compressed air weighs 133lb empty...and 150lb full. Maths that out and you’re packing 782 lbs of pressure vessels that need maybe 20ft^3 of space in the vehicle to get 100lbm of ‘propellant’.

    Certainly that’s far from the most efficient packaging, but you get the point.

    So is this just a clever thought experiment? Or is there more to it?
     
  13. ecarfan

    ecarfan Well-Known Member

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    Without intending to speak for the OP, Elon has said there will be a “SpaceX option” for the coming new Tesla Roadster, using compressed gas thrusters to improve handling. Really. I will believe it when I see it!
     
    • Like x 1
  14. mongo

    mongo Well-Known Member

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    100 lbs is about 1335 cubic feet (dry air), or 9 149 cuft scuba tanks at 3442 psi. Tanks would weigh 423 pounds, so just over 500 pounds when full. Tanks are 8x8x30 so a 3x3 array would be a 24x24x30 or a 10cuft rectangle.
    Elon mentioned a CPOV in the back seat, so I expect it will be lighter and smaller.
     
  15. George S. Bower

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    Here's the numbers from the performance model prediction for acceleration with a 1200 lb (20 lb/sec) thruster and a 2400 lb (40 lb/sec) thruster.

    BXR140 commented that my 20 lb/sec thruster was a big one and I'm starting to believe him after doing some searching. I found some 1000 lb thrust vernear thrusters that were used on early rockets but they used fuel and oxidizer, they weren't cold gas thrusters. Are the falcon nine thrusters used on the first stage (located up by the grid fins) cold gas thrusters?? I don't know.

    Point being that 2 cold gas thrusters producing 2400 lb thrust might be just too BIG.
    So maybe the thrusters Elon is talking about are more about handling on a race course.....around turns??? Then again why not just use vectoring thru the wheels.

    The other situation where the thrusters might be useful is off the line. If the Roadster is traction limited off the line then that would be a great application for the thrusters.

    Slide2.JPG
     
  16. George S. Bower

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    Finally found Musk quote. It's more like 10 thrusters placed around the car.

    "SpaceX option package for new Tesla Roadster will include ~10 small rocket thrusters arranged seamlessly around car. These rocket engines dramatically improve acceleration, top speed, braking & cornering. Maybe they will even allow a Tesla to fly…"

    Next-Gen Tesla Roadster will hover like the flying DeLorean from Back To The Future, no joke says Elon Musk
     
    • Like x 1
  17. Matias

    Matias Active Member

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    My WAG is, that that thruster would make huge noise.
     
  18. Uncle Paul

    Uncle Paul Active Member

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    Reminds me of the Vacuum cleaner car that Chapparel used on some of their competition cars. Had an axillary snowmobile motor in the rear that would pull air out of the skirted bottom of the car, sucking it down in the corners, but not used in straight line. Gave them a tremendous advantage and dominated where entered.

    Soon banned!
     
  19. bxr140

    bxr140 Active Member

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    Yeah, that was an active evolution of passive ground effect side skirts (not that it matters much, but it was also the Brabham). I could imagine the thruster solution on the roadster actually being used to create virtual sideskirts. Then, assuming there's some air compressor on board to refill the thruster 'propellant', that could suck from under the car similar to the 'fan car'.

    I could also see the thrusters being used in other aerodynamic configurations, not dissimilar to how formula one cars routed their exhausts [a few years ago] to facilitate better airflow under the rear wing. The thruster plume would effectively change the aerodynamic shape of the vehicle, and one can imagine the benefits of a dynamically sized bulge here or there, closing off or opening up a vent, etc...its sort of the evolution of active aerodynamic surfaces/features.

    Last, and probably least clever, one could imagine a thruster improving downforce, which of course will improve acceleration, cornering, and [when turned off or re-directed] top speed.

    I find it hard to imagine a thruster being used directly to control a car, as I find it hard to reconcile the forces required. Presumably a ~3000lb roaster in a 1g turn is reacting with 3000lb of side load across the four tires. In my brain you'd want at least many hundreds of pounds of thrust on each corner to be effective.
     

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