# Re: Physic help

```>> I personally find it not a stange number at all, it shows very clear what
>> part of the initial mass can be converted to energy (for anti-matter&matter
>> mixture f=1, or said differently, all mass can be converted to energy).
>
>True, after you do the calculation, but initself not a very helpfull number.
> In this case, its sort of an intermediate step in a calculation, rather than
>a usefull result in itself.  Also kind of confusing.

It is certainly not an intermediate number in a calculation. I think it is
one of the most important numbers when one compares fuels needed for
interstellar space travel. (Compare this to the density of different
substances, it is a fundamental property and not just some number that turns
up after you devide the weight by the volume.)

-=- -=- -=- -=- -=- -=- -=- -=- -=- -=- -=- -=- -=- -=- -=- -=- -=- -=- -=-

>The lit programs I used were the ones that calculated the specific impulse
>when given a exaust velocity, and the ones for calculating the terminal
>speed/fuel mass ration/specific impulse.  Give 2 of the three and it figures
>out the third.  There were actually 2 of the latter programs (both for staged
>and single stage craft) and the numbers were close.

That program uses the classic rocket equation that is also used for chemical
rocket engines:

M=M0 Exp(Vend/Vexh)

This formula assumes that all fuel can be used as reaction mass, as long as
you use chemicals the error is no very significant, but when using fusion
fuel a measurable part of the fuel is converted to energy and thus cannot be
used as reaction mass.
It is mainly this that causes the rather large error, besides that
relativistics also plays a small role that probably causes another small
deviation.

>I don't see how energy content of the fuel would effect the calculations,
>since that wasn't an imput.  Only the exaust velocity or specific impulse.

That's exactly what I mean, the program didn't even bother to figure out
what kind of fuel was used.

Timothy

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