Author Topic: Capability of SpaceX cold gas thrusters in the Roadster 2  (Read 22026 times)

Offline Ludus

  • Full Member
  • ****
  • Posts: 1198
  • Liked: 605
  • Likes Given: 299
https://electrek.co/2018/06/10/tesla-roadster-elon-musk-thruster-backseat/

This seemed close enough to topical for a try.
Itís not the kind of issue that auto forums can handle well.

This seems like itís intended to be serious. What sort of performance boost can pressurized air deliver?

Iíd assume itís all expended in any use and takes awhile to repressurize. What can it potentially contribute to acceleration or stopping?

Offline GreenShrike

  • Full Member
  • *
  • Posts: 180
  • Liked: 182
  • Likes Given: 542
Re: Capability of SpaceX cold gas thrusters in the Roadster 2
« Reply #1 on: 06/11/2018 03:01 am »
Well, assuming 50kg of nitrogen with an ISP of 60, fired from a Roadster with a total mass including nitrogen of 1350kg...

9.807*60*ln(1350/1300) = ~22.2m/s = ~80km/hr.

So 50kg of nitrogen could bring a Roadster to a stop if it wasn't going all that fast, or get it up to sub-highway speeds if it was standing still.

The period of time is determined by the thrust of the cold gas jets, and so sized accordingly.  To accelerate 1300kg of mass at 1g of acceleration takes:

9.807*1300 = ~12.7kN of force.

Somewhat larger than the thrusters on a satellite, I think it's safe to say. The largest I found in a very quick search of Moog's website was 4N... ;-)

One g of acceleration would apply 22.2m/s of delta V in...

22.2m/s / 9.807m/s^2 = ~2.26s

About two and a quarter seconds. So you could float the car for that long, if the thrusters could keep the car balanced.

Useful? I haven't a clue.

And, hey -- it's gotta be on topic if you can bring the rocket equation into it... ;-)
« Last Edit: 06/11/2018 03:01 am by GreenShrike »
TriOptimum Corporation            Science
                                      Military /_\ Consumer

Offline johnfwhitesell

  • Full Member
  • ***
  • Posts: 319
  • Liked: 105
  • Likes Given: 198
Re: Capability of SpaceX cold gas thrusters in the Roadster 2
« Reply #2 on: 06/11/2018 03:41 am »
The important question is how big up a pump would I need to hover indefinitely?  Would the pump fit in the car or would I need a truck to carry by air supply?

Offline bocephus419

  • Member
  • Posts: 45
  • Liked: 7
  • Likes Given: 2
Re: Capability of SpaceX cold gas thrusters in the Roadster 2
« Reply #3 on: 06/11/2018 04:29 am »
The important question is how big up a pump would I need to hover indefinitely?  Would the pump fit in the car or would I need a truck to carry by air supply?

Well, just riffing off GreenShrike's work, the flow rate required would be:

50 kg / 1.165 kg/m^3 = 42.92 m^3
42.92  m^3 / 2.26 s = 19 m^3/s

Online Cheapchips

  • Full Member
  • ***
  • Posts: 300
  • UK
  • Liked: 169
  • Likes Given: 321
Re: Capability of SpaceX cold gas thrusters in the Roadster 2
« Reply #4 on: 06/11/2018 06:51 am »
What duration hover would you get on Mars?  Enough to survey the route ahead?

I'm sure a drone is easier, but Mars needs more sports cars...
« Last Edit: 06/11/2018 12:16 pm by Cheapchips »

Offline speedevil

  • Senior Member
  • *****
  • Posts: 3213
  • Fife
  • Liked: 1661
  • Likes Given: 1973
Re: Capability of SpaceX cold gas thrusters in the Roadster 2
« Reply #5 on: 06/11/2018 10:01 am »
I speculated on the other thread (which now seems to have been deleted) if this could be referring to capability for hopping, 'last mile' landing on Mars/Moon.

Take Tesla S shell, battery, replace the wheels, add MLI for thermal insulation and a block of ice to transpire for thermal control, make minimal other mods, throw ten out from BFS at a hover 50m up, before heading for earth.
BFS can do 20 tons to near the moon surface, with refuelling in orbit, tests for which are already needed, and if you're filling up the BFS fully in orbit, you can do this 'for free'.

It can be frustratingly hard to tell between prescient silly metaphors - 'catchers mitt'/MrStephen and off-the-cuff jokes never intended to go anywhere. :)

« Last Edit: 06/11/2018 10:05 am by speedevil »

Offline rpapo

  • Cybernetic Mole
  • Full Member
  • ****
  • Posts: 1216
  • Michigan, USA
  • Liked: 666
  • Likes Given: 499
Re: Capability of SpaceX cold gas thrusters in the Roadster 2
« Reply #6 on: 06/11/2018 10:02 am »
The last Roadster they launched into space had a flaw: no RCS.  They had to fix that for next time.
« Last Edit: 06/11/2018 10:03 am by rpapo »
An Apollo fanboy . . . fifty years ago.

Offline Eerie

  • Member
  • Full Member
  • ****
  • Posts: 780
  • Liked: 128
  • Likes Given: 7
Re: Capability of SpaceX cold gas thrusters in the Roadster 2
« Reply #7 on: 06/11/2018 11:37 am »
Roadster 2 - the perfect getaway car? Use thrusters to rapidly accelerate, jump over fences, avoid police...

Online Chris Bergin

There's a risk (understatement) this could become party thread level ;) However, it is fair game for a thread on here as he mentioned SpaceX COPV thrusters. I'm 90 percent at thinking he's trolling the media again to see who would write it up, but it's Elon's 10 percent that always catches people out, so - a fun experiment.

But let's not go silly on here.

PS He said "Yes" to it being real, but he also trolled The Verge with that over Falcon Heavy news, or something! ;D
« Last Edit: 06/11/2018 11:44 am by Chris Bergin »

Offline rpapo

  • Cybernetic Mole
  • Full Member
  • ****
  • Posts: 1216
  • Michigan, USA
  • Liked: 666
  • Likes Given: 499
Re: Capability of SpaceX cold gas thrusters in the Roadster 2
« Reply #9 on: 06/11/2018 12:32 pm »
From what I can gather, he wants to use nitrogen compressed by the car itself, and so replenishable.  Just how long it would take to replenish we have no idea.  In a sense, it is like the boost mode you occasionally see on hybrid cars.  It drains the battery severely for a moment, and it takes a while to recover, but gives you quite an extra push for a bit.

Practice with ISRU...
An Apollo fanboy . . . fifty years ago.

Online Robotbeat

  • Senior Member
  • *****
  • Posts: 28701
  • Minnesota
  • Liked: 8773
  • Likes Given: 5698
Re: Capability of SpaceX cold gas thrusters in the Roadster 2
« Reply #10 on: 06/11/2018 01:11 pm »
I speculated on the other thread (which now seems to have been deleted) if this could be referring to capability for hopping, 'last mile' landing on Mars/Moon.

Take Tesla S shell, battery, replace the wheels, add MLI for thermal insulation and a block of ice to transpire for thermal control, make minimal other mods, throw ten out from BFS at a hover 50m up, before heading for earth.
BFS can do 20 tons to near the moon surface, with refuelling in orbit, tests for which are already needed, and if you're filling up the BFS fully in orbit, you can do this 'for free'.

It can be frustratingly hard to tell between prescient silly metaphors - 'catchers mitt'/MrStephen and off-the-cuff jokes never intended to go anywhere. :)
Meh, I donít think itís hard to tell when heís joking. If what heís saying is possible (but just barely), heís sincere, although not always like one would expect.
Chris  Whoever loves correction loves knowledge, but he who hates reproof is stupid.

To the maximum extent practicable, the Federal Government shall plan missions to accommodate the space transportation services capabilities of United States commercial providers. US law http://goo.gl/YZYNt0

Offline testguy

  • Full Member
  • **
  • Posts: 260
  • Clifton, Virginia
  • Liked: 226
  • Likes Given: 214
Re: Capability of SpaceX cold gas thrusters in the Roadster 2
« Reply #11 on: 06/11/2018 01:16 pm »
The thrusters will use compressed  air not nitrogen.  Yes, air is mostly nitrogen (79 percent nitrogen and 21% oxygen) but the gas properties are different.  The on board compressor will replenish the COPV's with compressed ambient air.

Offline Ludus

  • Full Member
  • ****
  • Posts: 1198
  • Liked: 605
  • Likes Given: 299
Re: Capability of SpaceX cold gas thrusters in the Roadster 2
« Reply #12 on: 06/11/2018 04:09 pm »
The thrusters will use compressed  air not nitrogen.  Yes, air is mostly nitrogen (79 percent nitrogen and 21% oxygen) but the gas properties are different.  The on board compressor will replenish the COPV's with compressed ambient air.

This seems pretty clear. The mythbusters rocket science question is given that it has to fit in the backseat of the roadster2, can COPV pressurized air provide significant enough thrust to contribute usefully to performance?

Itís good enough I suspect no matter how long it takes to repressurize the COPV after a use, if it can produce enough thrust to be meaningful.




Offline gaballard

  • Member
  • Posts: 9
  • Los Angeles
  • Liked: 8
  • Likes Given: 26
Re: Capability of SpaceX cold gas thrusters in the Roadster 2
« Reply #13 on: 06/11/2018 04:12 pm »
At this point in time, if Elon says he's going to do something, it must be assumed - no matter how absurd the idea - that he is 100% serious about it.

Offline cferreir

Maybe he is thinking more along the lines of what Bosch is testing... Check this link

http://www.thedrive.com/motorcycles/21012/jet-thrusters-could-help-prevent-motorcycle-crashes-in-the-future


Offline RedLineTrain

  • Full Member
  • ****
  • Posts: 817
  • Liked: 522
  • Likes Given: 653
Re: Capability of SpaceX cold gas thrusters in the Roadster 2
« Reply #15 on: 06/11/2018 04:25 pm »
I wonder how Tesla's going to get it past ITAR.

Offline johnfwhitesell

  • Full Member
  • ***
  • Posts: 319
  • Liked: 105
  • Likes Given: 198
Re: Capability of SpaceX cold gas thrusters in the Roadster 2
« Reply #16 on: 06/11/2018 04:49 pm »
PS He said "Yes" to it being real, but he also trolled The Verge with that over Falcon Heavy news, or something! ;D

IIRC the story you are talking about is him saying that Falcon Heavy would launch a roadster.  Didn't they end up doing that?

At this point in time, if Elon says he's going to do something, it must be assumed - no matter how absurd the idea - that he is 100% serious about it.

He sure does joke about being a super-villain a lot... does anyone know how to apply to be a hench?
« Last Edit: 06/11/2018 04:50 pm by johnfwhitesell »

Online envy887

  • Senior Member
  • *****
  • Posts: 5010
  • Liked: 2903
  • Likes Given: 1497
Re: Capability of SpaceX cold gas thrusters in the Roadster 2
« Reply #17 on: 06/11/2018 04:56 pm »
I wonder how Tesla's going to get it past ITAR.

Design the thruster system so that it's not practicably usable in a spacecraft or launch vehicle.

Offline chipguy

  • Member
  • Posts: 92
  • Ottawa Canada
  • Liked: 94
  • Likes Given: 30
Re: Capability of SpaceX cold gas thrusters in the Roadster 2
« Reply #18 on: 06/11/2018 04:57 pm »
Well, assuming 50kg of nitrogen with an ISP of 60, fired from a Roadster

A cold gas thruster with ISP of 60 has ~600 m/s exhaust velocity.

Safety concerns for surrounding pedestrians, vehicles and property
renders this entire thing a very late April 1st joke IMO. Also not a
thing I want driving through my neighbourhood at 2 AM.

Offline gaballard

  • Member
  • Posts: 9
  • Los Angeles
  • Liked: 8
  • Likes Given: 26
Re: Capability of SpaceX cold gas thrusters in the Roadster 2
« Reply #19 on: 06/11/2018 05:13 pm »
Well, assuming 50kg of nitrogen with an ISP of 60, fired from a Roadster

A cold gas thruster with ISP of 60 has ~600 m/s exhaust velocity.

Safety concerns for surrounding pedestrians, vehicles and property
renders this entire thing a very late April 1st joke IMO. Also not a
thing I want driving through my neighbourhood at 2 AM.

This is the same guy who just sold a bunch of flamethrowers... it could have been the Roadster Flamethrowing Edition.

On second thought, I'd love to see that...

Tags: