3
$\begingroup$

How to quantify power output of a statite?

It's a kind of (non-)satellite, that remains immobile in relation to the central body (e.g. hovering over one of the poles of a planet) through continuous counteracting the gravitational pull through its propulsion. (the linked article suggests solar sail, but other means have been proposed; ion propulsion based Sun statite could maintain position for months.)

If we take work as force over distance, since statite remains immobile, work understood that way is zero. This approach is impractical here.

Yet indubitably fuel is burnt, propellant accelerated, a certain energy output over time is required to maintain status quo.

How should I calculate this work, energy and power, knowing the local gravitational acceleration and mass of the statite?

(let's assume the statite mass change over time of the experiment is small to avoid rocket equation problems).

$\endgroup$
1
$\begingroup$

While the statite remains stationary, it has to eject mass (like a rocket motor) in order to maintain altitude. The work done is computed by considering the momentum exchange needed per unit time. If the force of gravity you are counteracting is $F$, then the momentum of matter accelerated per unit time is also $F$ (since $\Delta P = F\Delta t$, it follows that $F=\frac{\Delta P}{\Delta t}$)

Now the energy contained in the ejected mass is $\frac12 m v^2 = \frac{P^2}{2m}$. It follows that the larger the ejected mass, the lower the energy needed. This is why the Sikorsky challenge was won by a human-powered helicopter with a giant span - it could move a very large amount of air (mass) very slowly (low energy needed).

So - you cannot answer your question without considering the mechanism used for providing the force. And there's "no free lunch". Using photons to maintain altitude is (energetically) expensive...

Also - note that the above negates your "assume negligibly small statite mass change..." comment.

$\endgroup$
  • $\begingroup$ Would knowing exhaust velocity (proportional to specific impulse of the engine) while defining mass flow as "sufficient" be enough for a full answer? $\endgroup$ – SF. Feb 7 '17 at 15:48
  • $\begingroup$ @SF. you need to know two things. Exhaust velocity and mass flow, exhaust velocity and thrust, or mass flow and thrust. $\endgroup$ – Floris Feb 9 '17 at 21:51

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.