🚀 go-pugleaf

In article <74006@cup.portal.com> BrianT@cup.portal.com (Brian Stuart Thorn) writes:

>    Of course, the Shuttle can't jettison the ET to to lighten the
>    load for TEI. Then again, even a single SSME has an order of
>    magnitude more thrust than the Apollo SPS engine.

...

>    Previous paragraph about SSME thrust versus Apollo SPS thrust
>    applies here as well. Of course, no reason to limit Lunar Shuttle
>    to using only a single SSME...

The *thrust* is not a problem, after getting to LEO. If you have less
thrust, it only takes a little longer to get to the Moon. In fact, you
could make both TLI and LOI with only one OMS (or even RCS) engine, if
you had enough fuel to it and it lasted a long continuous firing. The
problem is the amount of fuel needed.

There is a simple equation to calculate the amount of velocity change
for a given amount of fuel and mass of vehicle:

	dv = ln (m/m0) * I

where
dv = velocity change
m  = initial mass (with full tanks)
m0 = mass without fuel (empty tanks & payload)
I  = the impulse of the engine used

With chemical rockets, I is some thousands of m/s. For example, the impulse
of SSME is about 4400 m/s. With SRB's it is lower.

For a complete trip to moon and back, you must sum the velocity changes 
needed in it's various parts together and use that to calculate your 
fuel requirements. Unfortunately, I don't have the exact requirements
for TLI, LOI, TEI and EOI on hand (maybe someone can help me here), but
even LEO requires a total velocity change in the order of 9000 m/s.

Thus, to only get to LEO with a single stage booster equipped with SSME's,
you had to have the mass ratio of about 7.73:1. That means that of the
initial mass of the vehicle, 87% had to be used as fuel just to get to
LEO, and the remaining 13% would include all the tanks and engines for
the fuel.

If someone has the velocity change requirements for the rest of our trip
to moon, we can continue this little number game.

	Pauli
--
Disclaimer fault - lawyers dumped