Re: Surface -- Orbit traffic

Some ideas:

First, don't be so quick to discount conversion air-rams. Sure, you
get aerodynamic drag, but you also get thrust. At high altitudes, you
get less thrust because you scoop up less air, but you also get less
drag, so the two effects should (to first order) cancel out, letting
you get up to the outer regions of the atmosphere without using any on-
board propellant at all. Scramjets have been demonstrated to work at
~3.5 km/s, and expected top speeds for technologically mature scramjet
programs range from 4 km/s to 8 km/s. LEO velocity is about 7.6 km/s,
so if you are zipping along in the rarefied regions of the upper
atmosphere at 6 km/s, it should only take another couple of km/s to
boost you up into an exo-atmospheric orbit.

A conversion air-ram has important advantages over a scramjet. First,
it does not need to take any fuel along with it for combustion with
the atmosphere - the conversion catalyst gets you your energy for free
by zapping the air (if you need to use heavy elements to reduce
neutron irradiation, you do need to take along a minuscule amount of
heavy metal pellets for your conversion reactor). Second, it should
be more reliable, since it doesn't need to keep supersonic combustion
going. Third, it is much more energy rich than chemical combustion,
leading to much greater thrust (and thus higher speeds at which drag
matches thrust). Note that an on-board conversion reactor is not
necessarily an environmental threat. You can shield against the
neutron radiation (and use the heated shielding to help heat the air
for your air-ram), and neutron radiation does not travel very far in
an atmosphere anyway. If you need heavy metals, you will likely be
releasing small quantities of neutron-rich fragments that will beta
decay on a time scale of seconds to years - this could be a serious
concern if you have regular traffic unless the nuclear fragments can
be contained on-board and not simply ejected into the exhaust. If you
are converting baryons in atmospheric nuclei, you end up with just a
bunch of alpha particles, deuterons, and He-3 - but maybe a small
amount of tritium. We can live with some tritium in the environment,
but if the radiation dose from the tritium significantly exceeds that
from cosmic rays, radon, internal decay of C-14 and potassium-40, and
the like, you might start to get worried. It helps that most of the
tritium will be dumped above the troposphere, where it will decay into
He-3 before ever venturing back down into the air we breathe, the
water we drink, or the food we eat.

For that last little boost up out of the atmosphere, drizzle some
water into the nuclear fire of your conversion reactor, and use the
resulting plasma as a propellant for a rocket.

Also, you don't need to reach orbit, only get out of the atmosphere.
As Wayne pointed out, you have high acceleration drives on your
interplanetary/interstellar spacecraft. One of these can easily get
into LEO, expend the 7.6 km/s to come to rest with respect to the
planet and rendezvous with a shuttle that has boosted out of the
atmosphere and has reached the top of its trajectory. Then the
spacecraft puts out another 7.6 km/s to get back into orbit, either
carrying the shuttle, or dropping the shuttle but taking the cargo on
board. This only costs the shuttlecraft about 3 km/s of delta-V on an
earth-like planet, and all of that could come from its air-ram. If
needed, the spacecraft that is to rendezvous with the shuttle can even
hover for a while as it waits for the shuttle - the drive plume will
likely put on a light show for the folks on the ground as it impacts
the atmosphere, but it will not be dangerous.

Since you are operating in a very energy-rich setting, you can have
launch lasers to move the power supply off of the shuttlecraft. The
laser station is powered by a big honkin' conversion reactor, and then
beams that power to shuttles as they boost out, either to heat the air
for a laser air-ram, or to give that last bit of boost with a laser-
water rocket. This may be much cheaper for commercial traffic,
although explorers and the military and people operating in remote
areas where the locals have not built a laser launch station would
probably prefer to use an on-board conversion reactor for their launch
energy.

For re-entry, aerobraking is good if you are not going too fast. If
you are going too fast, either fire up your conversion-water rocket,
or request some laser time from traffic control for your laser-water
rocket. If your shuttle is simply taking a hop up out of the
atmosphere, it will not be going too fast, and can simply glide back
down, like Scaled Composite's Space Ship One. The same is also true
if the shuttle is just dropped from a spacecraft from just above the
atmosphere at sub-orbital velocities (or even zero velocity with
respect to the planet).

Luke
.

Relevant Pages

  • Re: Paper on early space programs
    ... >of useful hardware that would have made the conversion rather easier. ... >reluctance to mess with the basic shuttle hardware. ... station concepts to be a threat to the "Freedom" space station program. ...
    (sci.space.history)
  • Re: Skylon Assessment Report
    ... impossibility, you don't support your position by making the claim. ... enough temperature so that conversion of N2 into NOx is not possible? ... prototyped hybrid engine that needs tonnes of atmosphere to go along ...
    (sci.space.policy)
  • Re: Socialism in SF
    ... the planet's atmosphere because one team of scientists had been ... working in metric measures and another in imperial, and a conversion ... show that physics is not a hard science, in case you missed the bulletin ...
    (rec.arts.sf.written)
  • Re: Socialism in SF
    ... the planet's atmosphere because one team of scientists had been ... working in metric measures and another in imperial, and a conversion ... or do understand it and cannot refute the claim. ...
    (rec.arts.sf.written)