Beamed power to space vehicles

What is the maximum flux levels that can be beamed to space craft by
lasr or microwave?

The specific application would be to beam power from a SSPS (power
available: 5GW) to a space vehicle using an electric drive (e.g.
VASIMR). The spacecraft would be descending from High Earth Orbit to
swing by Earth, and the aim would be to give it the maximum boost
whilst it passes Earth (for Gravity assist).

My thinking is that microwaves are the way to go because:

- They are more eficient than solar panels (even single wavelength
panels) and therefore there is less waste heat to dump.
- The wire mesh collector can be lighter than a solar array.

Trying some numbers, if the collecting area were 1km2, or 10km2, what
power could it collect?

Could such a system approach chemical engines in terms of thrust?

A later application would be to boost electric powered vehicles as
they swing by the sun at some 600km/s. This would obviously involve
transmitting power over much longer distances.

In article ,
(Alex Terrell) wrote:

What is the maximum flux levels that can be beamed to space craft by
lasr or microwave?

The specific application would be to beam power from a SSPS (power
available: 5GW) to a space vehicle using an electric drive (e.g.
VASIMR). The spacecraft would be descending from High Earth Orbit to
swing by Earth, and the aim would be to give it the maximum boost
whilst it passes Earth (for Gravity assist).

My thinking is that microwaves are the way to go because:

- They are more eficient than solar panels (even single wavelength
panels) and therefore there is less waste heat to dump.
- The wire mesh collector can be lighter than a solar array.

Trying some numbers, if the collecting area were 1km2, or 10km2, what
power could it collect?

Could such a system approach chemical engines in terms of thrust?

A later application would be to boost electric powered vehicles as
they swing by the sun at some 600km/s. This would obviously involve
transmitting power over much longer distances.

You might want to ask these questions at Dr. Benford's website.

http://home.earthlink.net/~jbenford/Activities.html

He has done beamed power work for many years.

Regards,

Tom Billings

--
Oregon L-5 Society

http://www.oregonl5.org/

"Rodney Kelp" writes:

On that line wouldn't it be more feasable to slow a space craft down before
reentering the atmosphere thereby eliminating the heating effect of reentry?

Totally impractical --- It takes the same mass-ratio as putting the vehicle
into orbit in the first place. It is not practical to have a single stage
with a mass ratio greatly in excess of 10, and under those conditions,
"Single-Stage To Orbit" is just barely marginal with known chemical
propellants and materials, but "Single-Stage To Orbit And Back Again"
is utterly hopeless, without advanced nuclear rockets that the EPA will
_NEVER_ allow to fly in the Earth's atmosphere.

Therefore, unless you have a propellant-dump in orbit and you are willing to
launch 10 expendable propellant freighters for every vehicle you bring back,
what you propose is simply not practical.


-- Gordon D. Pusch

perl -e '$_ = \n"; s/NO\.//; s/SPAM\.//; print;'

(Gordon D. Pusch) wrote in message ...
"Rodney Kelp" writes:

On that line wouldn't it be more feasable to slow a space craft down before
reentering the atmosphere thereby eliminating the heating effect of reentry?

Totally impractical ---

If we're limited to chemical rockets, yes. But with other propulsion
technologies, it merely becomes inefficient. With very high
efficiency rockets aerobraking is impractical.


It takes the same mass-ratio as putting the vehicle
into orbit in the first place.

Correct. Which for chemical rockets is impractical. For nuclear or
laser rockets, or laser light sails, this becomes merely an
inefficiency.

For example, if we're discussing a delta vee of 11 km/sec (escape
velocity) and we have a chemical rocket that has an exhaust velocity
of 4.5 km/sec and a nuclear rocket that has an exhaust velocity of 45
km/sec - we can compute the propellant fraction using the rocket
equation as follows;

For the chemical rocket;

u = 1 - 1 / EXP(11/4.5) = 0.9132

We need 91% of the vehicle mass to be propellant - to merely do this
once. So, its impractical to do it twice.

For the nuclear rocket;

u = 1 - 1 / EXP(11/45) = 0.2169

We need 22% of the vehicle mass to be propellant - so to do this twice
requires 39% of the vehicle mass to be propellant. An inonvenience
perhaps, but not an impossibility.

At the exhaust speeds grow higher, propellant fractions grow smaller.
For a super rocket with an exhaust speed of 4500 km/sec (300,000
km/sec = light speed);

u = 1 - 1 /EXP(11/4500) = 2/10ths percent = 0.002441

Such a 'hot' rocket containing say 5% propellant (the same mass ratio
as a typical automobile) could execute 25 delta vee burns each 11
km/sec. Rockets this efficient containing the same propellant
fractions as an airliner or freighter ship, could blast across the
solar system at constant gee crossing the spaces between the planets
in days - and not use atmospheric braking - since the propellant to
accelerate the structure needed to make use of such braking far
exceeds the ability of that propellant to change the speed of the
spaceship. The speed changes imparted by the structure by comparison
are small.

In this latter case the use of aerobraking would be considered
impractical, since it take away precious propellant that can be used
far more efficiently to impart delta vee.

[snip]

William Mook