Re: Performance needed for planetary visit


chornedsnorkack@xxxxxxxxxxxx wrote:

>So, what would be the performance constraints of an
>engine that has the capability of:

>Landing on a planet with an atmosphere and gravity well
>comparable to that of Earth

If we assume availability of that atmosphere, we can use
aerobraking (or whatever you want to call it) and
parachutes/wings/rotors to do the landing part with little
cost. Alternatively, we can use vertical jet thrusters...

>Climbing out of the same gravity well without any refueling
>and without abandoning any parts - only fuel and air/food
>may need replacing before repeating the mission

This is a big one. With an Earth-like atmosphere, everything
else you want can be done with a rather conventional
aircraft. But Single Stage To Orbit? A bit harder...

Well, you want maybe 12+ km/s of delta-v to get out of
an Earth-like gravity well. Assuming a 1000sec Isp solid
core rocket, the exhaust velocity is around 10km/s.
No problem! Make your cryogenic hydrogen tank big enough
to store twice as much hydrogen as the empty mass of the
spacecraft, and you've got it. Right? Not so fast...

>Staying below 2 g throughout the mission - the landing
>and takeoff should require no special training for
>passengers

....That's going to require more fuel. For a ballpark
figure, I'm going to assume the worst--you thrust straight
away from the planet instead of curving away into an orbit.
You want to accelerate to about 10km/s at 1gee (since 1gee
is "lost" fighting gravity). That takes 1000 seconds,
so the actual delta-v you needed was 1000 seconds * 2gees,
or 20km/s. Ouch! That requires a mass ratio of 7.4,
so you need 6.4 times as much hydrogen propellant as the
empty mass of your rocket.

So your rocket is mostly fuel tank. No big deal...but...

....Unfortunately, there's the mass of the reactor to
consider. A plausible solid core nuclear thermal rocket
reactor has a thrust/weight ratio of maybe 5:1. That
means that if you want a 2gee capability, your reactor
needs to be 2/5 of your ship's fully laden weight, or
40%. That's a problem...

The solution which comes to mind is to make the ship
air-breathing. Thrust from a solid core nuclear thermal
rocket is low because of the use of hydrogen propellant.
If you funnel heavier nitrogen and oxygen through the
reactor, then you can get much greater thrust levels.
Furthermore, you're not consuming your propellant tanks!

So, you just need air intakes for a multi-mode nuclear
rocket. This gets you to thinner upper atmosphere air,
where you go faster and faster as the thinning air
allows it.

With an air-breathing nuclear jet engine, you could hover
in the air for days or months, so your other requirements
are straightforward.

BTW, I hope you don't really care about the environment of
your "enemy" planet, because there's going to be a hellacious
amount of radiation and radioactives dumped into the
atmosphere. If you want something less Chernobyl-like
you'll need to assume some pretty fancy advanced technology.

Isaac Kuo

.

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