# Reply to post: Re: Blue whales.

### NASA lights humongous rocket that goes nowhere ... until 2019

#### Re: Blue whales.

Unfortunately this also makes NASA look like a wind-up, because horsepower is, well, power, and power is distance moved per unit time

And therefore the power in the exhaust is calculable: you're moving known mass at known velocities. In public discussions, this often leads to incredible numbers about how many horsepower some rocket engine develops. In engineering discussions about nuclear and electric rocket analyses, the jet power becomes somewhat important because the available input power (from the reactor, solar panels, or hamsters in wheels) imposes limits on the possible output power, and therefore maximum possible thrust and exhaust velocity.

If my memory hasn't completely failed me, I recall the equation being like:

Power = 0.5 x [exhaust velocity] x [thrust force] / efficiency

An ion engine with a specific impulse of 3000 (exhaust velocity of ~30,000m/s) that develops 1 pound of thrust (4.5 Newtons) via a mechanism 50% efficient (typical for electric rockets) requires an electrical input power of:

0.5 x [30,000m/s] x [4.5N] / 0.5 = 135,000 Watts

While its exhaust has 62,500 Watts and the engine's radiators are handling a similar 62,500 Watts.

In the late 1950s, the US sketched out a number of nuclear-thermal rocket designs rated by their gigawatts of output, like the 12-gigawatt and 14-gigawatt rockets considered for the Helios and Hyperion rockets. The uranium in the engines could only produce some much power to warm and expand the exhaust, and it was critical to understand what that value was.

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