NEO Mining Question

victoriatangoman wrote:

> If I'm reading your answer correctly, then I think you've mis-
> interpreted what I wrote. I didn't mean to imply the standard usage
> of a mass driver with respect to NEO, which is to use the partial
> mass of the NEO to propel the NEO to earth-space. What I'm proposing
> is that the orbit of the NEO not be changed. The mass driver uses a
> small SPS traveling in the orbit with the NEO, and that electrical
> energy is used to shoot payloads continuously to earth-space.

No. I got that. I still don't think its a good idea in most
cases.

> Granted, the traditional use of a mass driver would be very
> inefficient with respect to specific impulse. But the delta-v that
> can be imparted into the payload can be very significant. It would
> be a function of the acceleration, energy used, and track length.
> With track length being fixed, the accelration and energy used are
> the variables used to impart different delta-v.
>
> The mass driver doesn't suffer from the limitations of the rocket
> equation, where it has to start the voyage with all of its fuel and
> accelerate that fuel.

Yes, but quite a lot of NEOs are far less than the exhaust
velocity of the rocket away, and hence the rocket equation is
still very linear at that part of the curve. Accelerating
the fuel doesn't hurt nearly so much when the payload has
similar mass as the fuel.

> I don't claim expertise on Hohlmann orbits, but my understanding is
> that such an orbit is one in which the minimal delta-v is required
> to reach the destination, but the trade-off is time in transit.

A Hohmann orbital transfer is the lowest delta-v orbital
transfer for getting between two orbits. It assumes
theres nothing at either orbit. In the case of a NEO, the
earth and the moon, that's not correct, but the Hohmann
gives a stake in the ground you can try to improve upon.

In practice though you can often do much better than Hohmann-
for example a non Hohmann orbit that uses a reverse slingshot
around the moon can kill quite a bit of speed relative to
the earth, and can drop something into high earth orbit
which otherwise would go sailing past if the moon wasn't
there; and this manouever uses no fuel in principle.
In practice you're never exactly lined up so some small
thrusters are needed to do this. Also, aerobraking is
often suggested, and works well in the earths atmosphere.
These things are very non Hohmann; but they don't interwork
with a mass driver.

> Further, my understanding is that Hohlmann orbits are not the ONLY
> orbits that can be plotted. If the mission planner is willing to
> consider higher delta-v, then more windows are opened, and further,
> if delta-v does not become a consideration, then there is always an
> orbit that can be plotted.

That's also true; but I think we are trying for less delta-v in
most cases. Delta-v costs according to a square law even with
mass drivers; you need the capacitors for storing enough
energy. And capacitors are pretty expensive in practice.

> So, with a stationary mass driver propelling payloads, but not the
> NEO itself, no two launches would have the same orbit. Granted the
> changes would be incremental over time because the delta-v and
> orbital inclinations will be changing.

It might make sense in some situations, you'd need to do
plots of the delta-v against time to find out what proportion
of the orbit the earth is in range. But in general I suspect
the launch windows are too small.

There's also big issues in trying to catch kilogram sized
payloads that are flying towards you at km/s speeds. Using
rocketry solves that problem with a piece of technology
called 'bags' and 'thrusters'.

With mass drivers they had a neat solution for that from the
moon, but I don't see that that would generalise.

> --- In spacesettlers@y..., Ian Woollard wrote:
>
>>I don't consider the mass launcher to be a good solution
>>for NEOs. The problem is they are very expensive- it
>>solves a non problem: 'What do we use for reaction
>>mass?'
>>
>>On the moon, sure, well, even there, it's arguable. It
>>works; but if we can get to sources of water- and there
>>has to be water out there, right? Then we get to have
>>solar steamers and they are MUCH cheaper to build, have
>>more thrust and are much more flexible than mass drivers.
>>
>
> A mass driver propelling payloads, and not the NEO, doesn't need
> thrust in the way that I think you're framing your answer. The
> payload will have the orbit and delta-v to reach earth-space.

Yes. It can be made to work. But won't be if there are other
ways to do the same thing that are cheaper. It looks like
this is the case in fact.

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

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