From: henry@zoo.toronto.edu (Henry Spencer)
Subject: Re: Photon Engine
Organization: U of Toronto Zoology
Date: Sun, 25 Nov 90 05:31:25 GMT

In article <9011212202.AA13397@cmr.ncsl.nist.gov> roberts@CMR.NCSL.NIST.GOV (John Roberts) writes:
>Theoretically, power consumption can be modest, acceleration gentle, and even
>some degree of steering possible. Perhaps Jordin Kare could describe the
>proposal in more detail (it's been a while).

Since Jordin hasn't spoken up, herewith my recollections of what he and
others have said...

There are a variety of schemes using lasers to transmit power from big
fixed power plants to spacecraft (and aircraft).  One problem that does
appear is that rocket engines are ferociously powerful, and really
enormous lasers are needed to transmit enough power to run a big one.
(The Saturn V first stage power output at launch was circa 35 gigawatts.)
Another difficulty is building a "combustion" chamber with a very highly
transparent window in it.  (It is possible to transmit the beam up the
nozzle, but this has its own problems.)

Both of these problems can be minimized if you are using the system for
satellite maneuvering rather than boost to orbit, since you can then use
a small, low-thrust engine which doesn't put massive demands on chamber
materials.  This is a very promising idea.

However, Earth-to-orbit is what we'd really like to do.  And it looks
viable, if you change some of the assumptions.  The power needs can be
brought within reason simply by scaling down the size of the payload,
on the assumption that higher launch rates will make up for small size.
You can do a useful launcher with a few megawatts per kilogram, if I've
remembered the numbers correctly.

As for the chamber problem... one scheme for a solid-fueled laser rocket
was basically just a stick of propellant with a nozzle around one end,
with the nozzle sliding up the stick as the laser wore away the end.
You can go one better, if your laser can generate a pair of closely-spaced
short pulses.  The first pulse vaporizes a thin layer of the surface.  The
second starts a "laser-supported detonation wave" in the vapor, heating it
to very high temperatures.  Put the two pulses close together, and the
vapor is still a thin layer on the surface of the propellant when the
second pulse arrives.  Now you don't need a nozzle, because the vapor
expands mostly at right angles to the surface.  So your spacecraft is
very close to the ideal: a block of propellant with a payload glued on top.

There are several other advantages to this approach.  For one thing, the
thrust is perpendicular to the surface, not the beam, so the beam can be
coming in at almost any angle.  This also means you can steer the thing
with the beam, varying the power distribution across the beam to rotate
the spacecraft.  It is probably possible to literally have nothing but
"propellant, payload, and photons", although in fact a bit of cooperation
from the spacecraft makes things like beam pointing rather easier.  If
you are concerned about effects on the ozone layer, or whatever, you
can just turn off the beam and let the spacecraft coast while it passes
through sensitive regions.  Range safety is easy, because the spacecraft
has no independent maneuvering ability and its trajectory is very
predictable.  The accelerations are a few gees, low enough that a big
system could probably be man-rated.

Could you use air as propellant?  Maybe.  A reflective plate with parabolic
hollows carved in it will focus an incoming pulse (provided it's pretty much
perpendicular) to a set of hot spots near the plate, where the air will
break down and absorb the beam, producing miniature thermal explosions
that will push on the plate.  It's been tried in the lab; it works.

Could you build one today?  Maybe a small one.  Both lasers and optics are
beyond the off-the-shelf range, but there are contractors who could build
the laser as a routine custom engineering job.  It helps that this system
works fine, in fact better, at relatively long wavelengths, where almost
everything is easier.  The right thing to do would be to build one with
a payload of, say, one kilogram, as a test system.  There are still a lot
of unknowns in the detailed engineering.  Once the test system proved
feasibility, a few hundred million dollars could build one with a yearly
payload to orbit equalling the (theoretical!) payload of the entire
shuttle fleet.  Costs depend on how intensively you use it, because the
capital costs of construction tend to dominate the power bill.  A man-rated
system would be really huge because of the sheer mass needed; better stick
to sending up cargo in small pieces for now.  You can do an awful lot with
20kg pieces delivered cheaply to orbit in large numbers.

Even the test system would be useful, actually, since it could vaporize
small pieces of space debris and de-orbit larger ones, if it were equipped
to deal with completely uncooperative targets.

Would it be useful as a weapon?  Against satellites, maybe, depending
on how much cooperation it needs from the target.  Against smaller and
faster targets, not very.  The long wavelength works against it in a
weapons application, where doing maximum damage quickly is essential.

Is it being worked on?  Yes, in a minor way.  Jordin is head of a small
group that is looking into things like the laser-propellant interactions.
They are assuming that SDI will push laser and optics technology far
enough, and are working on the non-weapon-relevant aspects.

What's the holdup?  Money.
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