# Advantages of a Solid Fueled Upper Stage

I was recently looking at the specs of the Antares launch vehicle and I was surprised to find out that they were using a solid fueled motor as their upper stage.

I've always learned that in the upper stage of an LV the ISP is the most important parameter to design for, but solid fueled motors aren't know for their high ISP numbers.

So my question is, why did they do it in the case of the Antares vehicle?

• Just speculation here, but: Orbital ATK, the operator of the Antares, is a solid-rocket company. The entire (liquid-fueled) first stage appears to have been designed and manufactured by a Ukrainian company. I'm not entirely sure why Orbital didn't decide to use one or more solids as (a) lower stage(s) (where thrust, and not just ISP, are very important), and purchase a liquid-powered upper stage (which would also allow engine relights, needed for some satellite deployment missions). I expect the answer involves costs, though. – CBHacking Dec 27 '17 at 2:56

Solid fuel does make sense for a top stage, a so-called kick stage, the most notable example being the Star family. A kick stage has only smaller or similar gross mass as the payload it propels, which means that the weight savings from a high-ISP fuel are limited – you're in either case only getting a small bit of extra Δv. In other words, you're within in the nearly-linear range of the Tsiolkovsky equation. By “nearly linear” I'm referring to $$e^x \approx 1+x$$ for sufficiently small $$x$$. The infamous “tyranny of the rocket equation” only strikes when $$x$$ approaches 1 or larger, in this case meaning $$\Delta v\gtrsim v_0$$.