# [ExI] extropy-chat Digest, Vol 87, Issue 17

Fri Dec 10 23:19:07 UTC 2010

```On Fri, Dec 10, 2010 at 2:46 PM, Keith Henson <hkeithhenson at gmail.com> wrote:
> The key concepts are mission velocity and rocket exhaust velocity.  To
> get into LEO takes around 9km/sec delta V.  Orbital is only 8 km/sec
> but you burn somewhat more because of the time gravity is accelerating
> you downwards and from air drag.
>
> In simple terms, going to twice the exhaust velocity (which is what
> you have to do with LH2/LOX), the mass of the fuel has to be about 85%
> of the takeoff mass.  A reusable rocket is about 15% structure, which
> leaves no payload using LH2/LOX.
>
> Laser heated hydrogen at 3000 deg K has an exhaust velocity of 9.8
> km/sec.  That means the structure plus payload can be 1/3rd of the
> takeoff mass.  I.e., a 300 ton vehicle could reach orbit with 50 tons
> of structure and 50 tons of payload.
>
> To get in excess of one g takes around 6 GW.  The only way this makes
> economic sense is if the lasers are run 90% of the time.

If it's just a matter of exhaust velocity, what about ejecting stuff
at 100+ km/s?
You'd need to make sure it wasn't impacting anything, but that can be done by
accelerating straight up, then once high enough, shutting the engine down
briefly, pointing the rear along a path confirmed to be clear of anything, and
restarting the engine.  (Yes, mid-air engine restart is tricky, but far from
impossible, especially if you're ejecting discrete chunks - which, at that
exhaust velocity, you might be - instead of doing a constant burn.)

```