Flywheels / NEO Retrieval

Charles Radley wrote:

> Nobody has addressed the amount of propellant which would eb needed for
> retrieving an asteroid.
> It is very large.

Not that large. Consider the rocket equation:

deltaV = exhaustVelocity * ln(initialMass/finalMass)

For a probe weighing 500kgs, attached to a rock weighing 1 tonne
the final mass is 1.5 tonnes. Let's assume we use a Hall thruster
with an ISP of 1600 seconds (exhaustVelocity is 9.81 * 1600 =
~16,000km/s).

This means that for a deltaV of 3 km/s [sounds ok, maybe even high
if we slingshot around the moon AND aerobrake at the earth] so the
initial mass before you head for home is:

1.81 tonnes i.e. ~310 kgs of fuel.

That's not all that much. Sure its going to be slow, you'd have to
wait many months, and you'd have to add more fuel for the way out,
but on the way out the probe is going empty so guesstimate for
doubling the fuel load and it probably comes out about right, i.e.
1.4 tonne initial vehicle drags back 1 tonne of rock plus itself,
i.e. you end up with more mass than you started. You could screw on
some new thrusters, refuel it, and send it back out again.

You can play with the numbers some more, bigger rock, more fuel,
more power, more thrusters, but that gives you the basic idea. You
might be able to start in LEO, or atleast MEO- if you do, don't
forget to add the delta-v to get to escape- it's ~4 km/s from LEO,
plus twice whatever the delta-v to get to the asteroid is.

Anyway, that's the basic idea; go away and play ;-)

Ok, here's the data for a Hall thruster:

Propellent: xenon [expensive...]
Thrust: 0.083 N [not Indy racing]
ISP: 1600 secs [c.f. 450s for the space shuttle that does
~9km/s deltaV]
Power: 1350 watts [hmm. quite a big solar panel...]
Mass flow: 5.3mg/s [very small]
Total Impulse: 1000,000 Ns [not bad...]
Mass: 3.5 kgs [I'll have 20!]
size: 15x22x12.5cm

So, from the total impulse and the thrust I calculate the rated
life of the thruster is 139 days.

For 1350 watts you can accelerate 1 tonne by 1km/s in that time.

Doubling up the power and adding many more thrusters would multiply
up the time you can thrust for and double the thrust levels.

Hall thruster are supposed to work with other, cheaper,
propellents, like oxygen, but I don't have performance figures for
that. The cost of xenon is huge, IRC, it comes from liquifying air
so there's no theoretical limit to supply. Hall thrusters can be
run on oxygen with 10% of something else [probably xenon or argon],
that might be another approach for the propellent.

I think this may compare quite favourably with going to the moon.
In LEO you can probably stuff the rock in the back of a Space
Shuttle or something or send it to the ISS for analysis.

Comments welcome.

--
- Ian Woollard (ian.woollard@...)

"Is a planetary surface the right place for an expanding
technological civilization?"
- Gerard O'Neill