In message - Bradford Liedel
writes:


I find this stuff very interesting. I'm curious to see if (within the
next 30 years) space travel actually becomes a consumer industry
rather than a government only industry. With backstreet boys being
launched into space, towers into the atmosphere, corporations
competing on new shuttle designs, etc...who knows what this will all
bring.


I often see this type of thinking: "if only we'll start launching on
commercial scale things will be cheap". Well, things are not that rosy:
physical limits come to play. The classical rocket equation:

dv = Ve * ln((final mass) / (initial mass))

whe

Ve = exhaust velocity.
dv = change in velocity

The exaust velocity is more or less constant for chemical fuels. In F-1
engines of the Saturn-V first stage it was around 2.9 km/Sec in vacuum
(2.6 km/Sec at sea level). The required dv is about 8 km/Sec (to LEO).
Substitute the figures into the equation and you'll get that final mass
is only about 5% of the initial mass. That means: 95% of the rocket mass
is fuel and the WHOLE structure and payload and engines has the meager
5% of the mass budget. That'll dictate you engineering decisions very
uncomfortable to live with:

1) You can't make the spacecraft "sturdy as a buttleship", in fact
you'll be forced to make its structure rather flimsy (forget
about "belly landing" with shuttle) and therefore you'll have
to very thoroughly inspect it before EVERY flight to make sure
absolutely nothing is damaged and probability of slight damage
requiring repairs will be quite high. Such inspection by an army
of technitians adds cost.

2) The cryogenic fuels (LH2+LOX or Kerosine+LOX or other similar stuff)
are much mode dangerous to handle than ordinary jet fuel, therefore
in almost any event of unexpected pre-launch maintenance you'll need
to drain the tanks and refill them again and it's not as simple
as dealing with jet fuel - again you'll need many more people
which again adds to the cost.

3) Because of the tight mass budjet (5%) every equipment must be
on the cutting edge in terms of mass (materials used) which makes
it expensive to build and maintain.

I'm not saying you can't make launches cheaper than NASA does (if Shuttle
launch costs $19,000/kg and is equal to Saturn-V launch cost per kg than
clearly NASA missed something implementing the "reusable cheaper than
expandable" attitude) but there are inherent technical problems which can't
be solved in a cheap way when you'rr constrained by the 5% mass budget.

However, if you'll use nuclear propultion - that really opens the road to
cheap space access. All you need is LOTS of R&D money to restart programs
USAF conducted in 50-s and 60-s (and got as far as having working prototype
of nuclear rocket on a test stand) and solve the problems of engine life,
radioactive exhaust, worst case launch failure survivability of the
reactor, etc. And of corse, you'll need to re-educate the public (voters)
to allow polititians to make such decisions.


************************************************** ****************************
* Arie Kazachin, Israel, e-mail: *
************************************************** ****************************
NOTE: before replying, leave only letters in my domain-name. Sorry, SPAM trap.
___
.__/ |
| O /
_/ /
| | I HAVE NOWHERE ELSE TO GO !!!
| |
| | |
| | /O\
| _ \_______[|(.)|]_______/
| * / \ o ++ O ++ o
| | |
| |
\ \_)
\ |
\ |
\ |
\ |
\ |
\ |
\ |
\_|