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Serious propulsion



 
 
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  #1  
Old January 31st 05, 12:16 AM
Allen Thomson
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Default Serious propulsion

Probably such things have been hashed out before, but allow me to
solicit the opinion of the readership on this question that has
come up in some off-line discussions:

Supposing you wanted a reactive propulsion system -- say a
nuclear rocket, electrical thruster, mass driver or other
throw-mass-out-the-back widget that had continuous thrust =
1 kN, ISP = 10,000 sec (= 100 km/sec Ve) and a total run
time = 1 megasecond or greater, preferably getting toward
10 Ms.

If it needs a reactor or other power supply, plumbing,
radiators to get rid of excess heat, etc., ignore that and
concentrate on the thrust-producing part. It's ok to have a
system composed of shorter lifetime parts if that helps
(e.g., banks of 100 N thrusters with individual lifetimes
of 100,000 seconds that get shed as they burn out).

Does current technology or anything that can be reasonably
foreseen in the next 20 years support anything like that? If
so, what might it be?

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  #2  
Old January 31st 05, 04:17 PM
David Summers
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10 Ms = 116 days: That is definately achievable for ion engines,
though the power source (dutifully ignored) will be impressive.
Remember, you have to take all the energy with you - interstellar space
is remarkably energy-free.

However, your big problem is propellant. For 1 kN @ 10,000 sec Isp,
you are using about 10 g/s. That means you have 100,000 kg of
propellant in orbit (expensive, probably impossible with the best
propellant, Xenon), and you are only pushing it at 1kN - so you aren't
accelerating very fast.

Where do you want to go?

-David

  #3  
Old January 31st 05, 11:50 PM
Allen Thomson
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David Summers wrote:

Where do you want to go?


Inside the solar system outwards of Earth, say low Mars orbit at
a minimum, but also major asteroids, Europa orbit, Titan orbit,
etc. as you think you can do it.

Massive-as-possible unmanned cargo payloads on Hohmannish
trajectories and fast-as-possible passenger vehicles are
the idea.

  #4  
Old February 1st 05, 12:23 AM
Allen Thomson
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However, your big problem is propellant. For 1 kN @ 10,000 sec Isp,
you are using about 10 g/s. That means you have 100,000 kg of
propellant in orbit (expensive,


100 tonnes in LEO is indeed expensive, but doable with existing
technology, even if we don't revive Saturn 5 or Energiya.


probably impossible with the best propellant, Xenon),


This assumes, I think, an electrical thruster.

Maybe that's the right solution; maybe a mass driver slinging
sand is. That's part of the question.

and you are only pushing it at 1kN - so you aren't
accelerating very fast.


That's ok. Actually, that's where the run-time requirement
came from, though, admittedly, it kind of begs the question/
assumes something about the solution. Something that can
provide one to a few cm/s^2 would be fine.

  #5  
Old February 2nd 05, 09:49 AM
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The Orbiter part of the Space Shuttle is around 100 tonnes.

  #6  
Old February 3rd 05, 04:42 PM
David Summers
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Well, you have lots of options in that area - solar sails, ion drives,
even direct LOX/LH propulsion will work. Mass slingers have some very
hard problems to overcome - and in the end would almost certainly at
best have the same thrust and Isp as ion drives. The areas you
mention still receive a lot of energy from the sun, so ion drives (or
solar sails if you can afford the risk) are probably the best bet. The
main problem with ion drives for this is that the best propellant,
Xenon, is rather rare - there may not be enough available in the
forseeable future for this mission.

-David

  #7  
Old February 3rd 05, 05:03 PM
Henry Spencer
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In article .com,
Allen Thomson wrote:
Supposing you wanted a reactive propulsion system -- say a
nuclear rocket, electrical thruster, mass driver or other
throw-mass-out-the-back widget that had continuous thrust =
1 kN, ISP = 10,000 sec (= 100 km/sec Ve) and a total run
time = 1 megasecond or greater...


Mmm... Even at 100% efficiency, this requires 50MW of continuous power,
which will not be easily had. (Jet power is 0.5*thrust*exhaustvelocity.)

That kind of exhaust velocity is almost certainly impractical with a mass
driver or anything similar.

Most electric thrusters are out too. Ion thrusters can get up into that
range easily enough, although they are more usually optimized for lower
Isp; you will need a whole bunch of them to get 1kN. The power source and
thruster hardware will be quite heavy.

There is no way that solid-core nuclear can do 100km/s. Even gas-core
probably tops out around 50km/s. 100km/s or more should be feasible with
systems that don't try to separate fission fuel and propellant -- NSWR or
imploded-pellet fission, for example -- but operating costs will be high
and the exhaust generally rather dirty.

Fusion or antimatter rockets can deliver the performance, but fusion is
beyond what we can build now, and antimatter is beyond what we can fuel
now (the hardware technology is rather easier than fusion [!!], but
large-scale antimatter production would require massive infrastructure).

Does current technology or anything that can be reasonably
foreseen in the next 20 years support anything like that? If
so, what might it be?


Current technology can stack up an array of ion thrusters, but powering
them is problematic. That is much too big for current-technology solar;
the problem is not the solar cells but the structural dynamics of enormous
lightweight solar arrays. 100MW space reactors are not off-the-shelf
items either. Solving either in the next 20 years is conceivable, but not
a small project, and probably will not happen without specific need.

100km/s dirty-fission or antimatter systems are probably buildable without
major new technology, but again, it would be a big project and is unlikely
to happen within 20 years unless someone makes a major effort to do it.
--
"Think outside the box -- the box isn't our friend." | Henry Spencer
-- George Herbert |
  #8  
Old February 4th 05, 06:37 AM
John Schilling
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"Allen Thomson" writes:

Probably such things have been hashed out before, but allow me to
solicit the opinion of the readership on this question that has
come up in some off-line discussions:


Supposing you wanted a reactive propulsion system -- say a
nuclear rocket, electrical thruster, mass driver or other
throw-mass-out-the-back widget that had continuous thrust =
1 kN, ISP = 10,000 sec (= 100 km/sec Ve) and a total run
time = 1 megasecond or greater, preferably getting toward
10 Ms.


If it needs a reactor or other power supply, plumbing,
radiators to get rid of excess heat, etc., ignore that and
concentrate on the thrust-producing part.



Oh, in that case, no problem. Ion thrusters can do that quite
easily, just a matter of scaling. Possibly some sorts of plasma
thrusters as well, but if you talk to whomever winds up owning
Boeing's ion thruster shop after the current round of mergers
and selloffs, convince them you are serious, they can make you
up a cluster of high-power ion thrusters that deliver the sort
of performance you are looking for, without even stretching the
state of the art.

The hundred or so megawatts of electric power it will take to
run the thing, most of us would consider a showstopper, but you've
told us to ignore that part of the problem. And yes, the ion
thruster array will be heavy (kiloton) and expensive (gigabuck),
but unless you're invoking magic the power supply will be heavier
and more expensive, so no worries there. If it really matters,
I suspect the second gigabuck could shrink the mass down to a
hundred tons, but anything beyond that is going to be *really*
expensive.


--
*John Schilling * "Anything worth doing, *
*Member:AIAA,NRA,ACLU,SAS,LP * is worth doing for money" *
*Chief Scientist & General Partner * -13th Rule of Acquisition *
*White Elephant Research, LLC * "There is no substitute *
* for success" *
*661-718-0955 or 661-275-6795 * -58th Rule of Acquisition *

  #10  
Old February 5th 05, 12:07 AM
Allen Thomson
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John Schilling wrote:
"Allen Thomson" writes:


continuous thrust = 1 kN, ISP = 10,000 sec
(= 100 km/sec Ve) and a total run time = 1 megasecond
or greater, preferably getting toward 10 Ms.


they can make you up a cluster of high-power ion thrusters
that deliver the sort of performance you are looking for,
without even stretching the state of the art.


Could they really? That's excellent news.

And yes, the ion thruster array will be heavy (kiloton)


Apparently you have a specific design, or design in mind.
What, in general, are you thinking about?

and expensive (gigabuck),


A pittance. I'd be happy to fund out of beer money.(*)

but unless you're invoking magic the power supply will be
heavier and more expensive, so no worries there. If it
really matters, I suspect the second gigabuck could shrink
the mass down to a hundred tons,



Ok, so we're up to a few gigabucks for a 1000 tonne thruster and
a somewhat smaller power supply.

I didn't think this thing would be possible at all, but you've
given me hope. Please continue.

but anything beyond that is going to be *really* expensive.


Like a hundred gigabucks?


(*) Well, actually, my beer funds are a little low at the
moment, but NASA should be happy to pick up the difference.

 




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