Nukes versus chemical

Jay,

Of course the 80% mass fraction applies to a typical chemical propulsion system.

Most of us on this list know how nuclear propulsion systems work, such as NERVA, Orion, Timberline.
Most of us understand that the oxidizer mass fraction is 0% for those systems.

My apologies if this was not clear from my post, on this list it is typically not necessary to explain some of those details.

It seems that you are not alone in missing my point of the 80% mass fraction number, so I shall ellaborate.

Specifically, I think it is clear that our nuclear discussions are focussed on NERVA type systems of late, which would use H2 reaction mass.

If we assume that unlimited oxygen is available in LEO from lunar/NEO sources, then we can ignore the mass fraction of O2. The comparison between nuclear and chemical then reduces to a comparison of the Isp of chemical versus nuclear, for a given mass of hydrogen or hydrocarbon fuel.

I must admit I forget the respective Isp numbers, NERVA is I believe something like 1000 compared to maybe
200 for a chemical system ? I am sure somebody will post better numbers.

The nuclear rocket has a slight overhead disadvantage (versus chemical) in terms of the weight of the nuclear reactor and associated hardware, for a large load of hydrogen this becomes insignificant.

The performance of nuclear systems is certainly several times higher than for chemical systems. And the capital cost of the infrastructure required to manufacture oxygen is significant. Actually, getting oxygen from water is not so difficult, simple electrolysis will suffice, so if water is available on Luna or NEO's then we are in great shape for some major developments.

Of course, the investment for extracting oxygen from silicates is a lot higher.

In the near term there are no oxygen facilities in space currently. This means that for a one or two off mission to Mars a nuclear system is really essential, which IMHO means it probably will not happen, because of the political problems.

That means that the Mars folks should join us in trying to develop oxygen production facilities in space, for that is our best hope of getting to Mars as well as for achieving a spacefaring civilizstion.

Cheers,

Charles R.