Physics Help Forum Ion Propulsion Rocket Momentum

 Equilibrium and Elasticity Equilibrium and Elasticity Physics Help Forum

 Mar 24th 2016, 10:07 PM #1 Junior Member   Join Date: Mar 2016 Posts: 1 Ion Propulsion Rocket Momentum Hey, I've been trying to solve this problem for awhile, and my book is no help since it only lists the rocket equations. An ion propulsion system uses ejected ions to change the momentum of a spacecraft. If 1x10^20 protons are ejected per second at 3x10^5m/s relative to to a 100 kg spacecraft that is originally going 0 m/s, how many days will it take for the spacecraft to reach 9x10^3m/s? I've tried using the rocket equations: Rvrel=Ma (I don't know R - the rate of fuel consumption) and vf-vi=vrel(ln)Mi/Mf which I'd need vrel for and I can't find that without R. I also thought to use the change in velocities (vi being 3x10^5 and vf being 9x10^3, but I'd need time to find acceleration. It seems like this has an obvious answer, but I just cannot figure it out! Can someone give me a nudge in the right direction? A useful equation maybe?
Mar 25th 2016, 05:00 AM   #2
Senior Member

Join Date: Apr 2015
Location: Somerset, England
Posts: 708
 change the momentum of a spacecraft.
This is the key phrase that tells you how to do this one.

What is the momentum of the craft plus the ions at the beginning and at the end?

What, therefore is the total change of momentum of the system?

 Tags ion, linear momentum, momentum, propulsion, rocket

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