5
$\begingroup$

With the heated side facing the sun and the rest in its own shade, could two contra-rotating Vacuum Stirling Engines and flywheels, in tandem, accumulate torque, to power a reaction wheel? Can the heat from an ion engine be used to provide forward momentum this way?

$\endgroup$
2
  • $\begingroup$ If the first will work, then why can't the heat from an engine also provide forward momentum instead of sunlight? I know the spinning wheel doesn't move you but the acceleration and deceleration of two contra-rotating wheels can. $\endgroup$
    – user34405
    Feb 2, 2020 at 18:12
  • $\begingroup$ No, adding up torques can't produce a linear acceleration, just a net torque on the whole ship. $\endgroup$ Feb 3, 2020 at 3:58

1 Answer 1

14
$\begingroup$

Could Stirling Engines work on sunlight alone? With the heated side facing the sun and the rest in its own shade, could two contra-rotating Vacuum Stirling Engines and flywheels, in tandem, accumulate torque, to power a reaction wheel?

Yes.

Can the heat from an ion engine be used to...

power a Stirling engine? Yes!

...provide forward momentum this way?

No.

Let's assume your spacecraft start out with zero translational momentum and zero angular momentum. That's pretty much the end of the story.

You can rotate your spacecraft since it has moving internal parts like cats do. For more on that see Destin's stunt cat Gigi, as well as cats in simulated zero gravity do it in the YouTube video Slow Motion Flipping Cat Physics | Smarter Every Day 58 viewable below.

Even though your spacecraft's net angular momentum about its center of mass remains at zero, parts can move with respect to other parts, and those movements include relative rotations.

You can not propel your spacecraft forward or backward. Spinning wheels will not move you anywhere, unless you are on a road, or in an atmosphere.

However you will likely start to move very slowly because the photons you are reflecting, absorbing and re-radiating have a small amount of momentum. This is not related to any particular motion of any internal parts moving parts. You've simply become a very weak solar sail. For every "hot" photon from the Sun you'll radiate many "cooler" photons. Conservation of energy means $E_{in} = E_{out}$ and the momentum of a photon is just $E/c$, so if you radiate into a hemisphere you won't be able to completely cancel your absorbed momentum, and of course some light will be reflected back adding even more momentum.

$\endgroup$
12
  • 9
    $\begingroup$ @Xaddell total momentum of a system is a conserved quantity. You can't convert pure energy in a change of momentum, you can use energy to eject some propellant at high speed in order to accelerate you in the opposite direction, but the total momentum of the combined centre of mass (you+the propellant) will be unchanged. In a standard rocket, the fuel is both a source of energy and the reaction mass; with a different source of energy you still need to eject some reaction mass in order to accelerate. $\endgroup$
    – Peteris
    Feb 2, 2020 at 18:44
  • 6
    $\begingroup$ Fundamentally, a reaction wheel moves one side of the craft "forwards" by moving the other side "backwards", i.e. using it as reaction mass. But both sides remain coupled together. To get forward momentum, you have to use something else as reaction mass, and let go of it, i.e. lose it. $\endgroup$ Feb 2, 2020 at 19:29
  • 1
    $\begingroup$ @uhoh en.wikipedia.org/wiki/Pioneer_anomaly a heat source in the spacecraft can do exactly that $\endgroup$
    – Antzi
    Feb 3, 2020 at 5:00
  • 2
    $\begingroup$ @Antzi In "Can the heat from an ion engine be used to provide forward momentum this way?", the term "this way" refers to a Stirling engine powering a reaction wheel. So no, I really don't think so. $\endgroup$
    – uhoh
    Feb 3, 2020 at 5:58
  • 2
    $\begingroup$ @CJ Dennis: Maybe for an idealized spherical cat :-) $\endgroup$
    – jamesqf
    Feb 4, 2020 at 16:52

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge you have read our privacy policy.