# Specific Impulse - I just don't get it.

1. Feb 27, 2013

### StrayCatalyst

I'm working on a hard sci fi story, so I'm trying to get the technical details right - but I don't actually have a background in space travel, astrophysics, orbital mechanics, etc, so I'm running up against my own ignorance. I've Googled it, read the wiki, read NASA's site, and I'm still clueless.

A Fission-fragment rocket engine has a specific impulse (estimated) of 1,000,000 seconds. There are some significant practical problems (like feeding wire through the reaction chamber at 1km/s to allow it to react without simply vaporizing from the heat). The wire in question might be replaced with CNT or similar, which might allow the wire to remain within the reaction chamber for longer without melting - the papers I've read predated CNTs by a few years.

Dusty plasma engines appear to have the same impulse, without the problem of having to keep a wire cooler than melting temp while inside the reaction chamber.

Both of these depend on the price of 242Americium being cheaper than \$1500 a gram in order to be viable, but that's not my question.

So, given a specific impulse of 1,000,000 seconds on a ship of 50 tons weight, what sort of thrust could this engine produce, for how much fuel consumed?

2. Feb 27, 2013

### Staff: Mentor

Thrust is proportional to specific impluse (Isp) and mass flow rate ($\dot{m}$). There is a conversion factor involved in the product to get the actual thrust.

http://www.grc.nasa.gov/WWW/k-12/airplane/specimp.html

The higher the Isp, the lower the mass flowrate for a given thrust. But Isp requires energy/power.

BTW - this is an aerospace engineering matter and not so much astrophysics.

3. Feb 27, 2013

### StrayCatalyst

Thank you for moving this to the correct part of the forum, I'll take a look at the link and see if that clears things up.