Re: Performance needed for planetary visit


IsaacKuo wrote:
> 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.

With problems.

> 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.
>

Radiation and radioactives are a real problem. As is relying on
parachutes. Or fixed wings.

After all - what would be necessary for a reasonable-sized ship to land
in a forest, pick up a spy, or an injured person and return, even to
low orbit?

Assuming no "beaming up".

Reactionless drives are nice, if available.

What are the feasible mass/thrust ratios of nuclear reactors which do
NOT emit large amounts of radioactivity or radiation, because all the
fission products are caught and accumulated in the reactor?

Can nuclear reactors have the power for a propeller plane to take off
from a runway?

What about a nuclear-powered flying boat?

Mind you, a landplane is pretty useless as a lander. You need a runway.
So, you have to first land a lot of people and machines to build a
runway. Something else is needed for a lander!

Flying boats are better, but still limited. They require lakes or
harbours. They cannot land inland if the area happens to lack lakes of
appropriate size. Nor do present flying boats appear to possess the
power to operate from ordinary ocean swell!

Could a nuclear reactor provide the power density for a helicopter?

Would it be possible to have a device which at low airspeeds has huge
air intakes, like those of great
turbofans, and then on the larger speeds covers those main intakes up
with something aerodynamic and heatproof, and launches using high-speed
jets?

.

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