[ExI] SpaceX launch

[Adrian Tymes]

Combining four emails...

On Fri, Dec 10, 2010 at 2:05 AM, Eugen Leitl <eugen at leitl.org> wrote:
> On Thu, Dec 09, 2010 at 02:28:45PM -0800, Adrian Tymes wrote:
>
>> Indeed.  I would very much like to see viable plasma thrusters that can
>> generate 1G+ of thrust.  The trick seems to be either generating or storing
>
> Electrip propulsion is low-thrust high-Isp (current VASIMR does
> 4 N, 6000 s specific impulse). The advantage of LEO-LLO plasma-driven
> transfers is not time (currently ~6 months) but low propellant usage,
> making maximal use of each kg to LEO. Unmanned payloads can also tolerate
> much higher deccelerations on landing.

Yeah, but the main problem right now is the high cost per kg to LEO.
There's only so much optimization you can do.  Further, it sets a
high minimum cost, which discourages experimentation: at launch
costs of $1000/kg (which is on the cheap side of today's capabilities),
even if you had a 100 g toy satellite, you couldn't launch it for $100
without a lot of coordination with other people (which drives the true
cost way above $100).

Launch to LEO requires much higher thrust than current electric
propulsion systems.

>> I wonder if there's a chemical reaction that can take its reactants up to
>> plasma phase, without need of external input?
>
> No, chemical bond energies are orders of magnitude below that
> required. You'd do much better using nuclear sources, like
> solar photovoltaics (with the added advantage that you're
> not carrying the power source) and nuclear reactors driving
> the electric propulsion unit.

So I have learned.

On Fri, Dec 10, 2010 at 3:27 AM, Eugen Leitl <eugen at leitl.org> wrote:
> On Thu, Dec 09, 2010 at 04:06:06PM -0800, Adrian Tymes wrote:
>> If one can superheat the fuel anyway, one might as well simply superheat it
>> directly to plasma and don't bother reacting it.  Unless one includes a
>> short-lived fusion reactor as part of the engine, of course.
>
> Your problem is mass. E.g. fusion-enhanced VASIMR would have to
> to produce enough additionals thrust to overcome the added
> mass.

This is indeed the main problem of high thrust electric engines:
scaling up the thrust without, well, massively scaling up the
mass.

> I personally think that tracked beam propulsion (in the weaker
> form, via e.g. a rectenna array feeding a VASIMR unit) will win
> by virtue of leaving (most of) the drive at home.

It's got possibilities, but:

1) Large (and expensive) ground installation needed

2) How does it provide thrust on the far side of the planet, in order
to stabilize the orbit?  Or is it deflected to provide a horizontal, or
even downward, thrust to shift from launch to orbit trajectory once
the proper altitude is reached?

>> Then again, that's "short-lived" as in "approximately 10 minutes", whereas with
>> modern experimental fusion reactors, 10 seconds is considered a very long
>> reaction.  (Which makes me wonder how they think they'll get it to commercial
>> practicality, which requires sustained power output for hours at least.  I also
>> can't help but wonder if engineering for extremely short reactions is part of
>> the reason why big fusion reactors have not produced progress
>> commensurate with their expense.)
>
> Speculation without knowledge is rarely useful.

Given the members of this list, it's possible that someone has the
knowledge to answer that.  ;)

2010/12/10 Dan <dan_ust at yahoo.com>:
> My comment which kicked this off was: "What I'm really waiting to see is
> private launcher to a private space station -- not a private launcher that's
> merely another government contractor." I'm not understanding what seems to
> me to be a rather sarcastic response. Yes, I know there are no private space
> stations for a SpaceX manned rocket to go to at this time. I was expressing
> my desire to see these.

Ah, I meant no offense.  Rather, I was saying that the private launchers go
for what milestones and triumphs they can get.  Don't forget to celebrate
today's progress, if it is progress, just because there are yet further
objectives as yet unreached.

On Fri, Dec 10, 2010 at 8:44 AM, spike <spike66 at att.net> wrote:
> On Thu, Dec 09, 2010 at 04:06:06PM -0800, Adrian Tymes wrote:
>> If one can superheat the fuel anyway, one might as well simply
>> superheat it directly to plasma and don't bother reacting it.  Unless
>> one includes a short-lived fusion reactor as part of the engine, of
> course. Adrian
>
> Of course if it is heated to plasma it wouldn't react anyway in the chemical
> sense.  I wrote it in a confusing way.  I shoulda said, react the chemicals
> to get them hot, then superheat the exhaust to plasma.

That makes sense.  Although, I still wonder if it might be more mass
efficient to completely separate the exhaust material from that which
generates the heat.