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tracer
2009-Sep-15, 06:57 PM
Speculative scientists have gone to great lengths to devise theoretical high-exhaust-velocity propulsion systems for spacecraft -- nuclear fission plasma, the Orion H-bomb pulse rocket, controlled fusion, the Bussard ramscoop and related RAIR designs, you name it.

But hardly any attention, so far as I can tell, has been paid to the SMALL reaction thrusters that all these spacecraft are going to need for low delta-V maneuvers.


Modern spacecraft use chemical rockets for these thrusters, often powered by hypergolic propellants (chemicals that spontaneously combust on contact with one another). This allows the thrusters to be QUICK and RESPONSIVE: at an instant's notice, they jump from 0 thrust to max thrust and quickly switch back off again, over and over, sometimes in rapid succession.

But their need for chemical propellants makes them HORRIBLY inefficient when compared with the theoretical designs for high-Isp main engines. If our Space Cadets on their fusion torchship want to go traipsing around the solar system for weeks at a time without a fuel stop, they're going to have to carry quite a bit of weight in maneuvering-thruster propellant for all the little rotations and docking maneuvers and whatnot.


Are there any practical designs out there for responsive, low-delta-V-per-use maneuvering thrusters that don't use such inefficient chemical propulsion? Electric arc-jets or quick-acting ion engines, or something?

Larry Jacks
2009-Sep-15, 07:54 PM
For attitude control, you often don't need thrusters at all (e.g. Hubble). The technology goes by different names (momentum control wheels is one name) but the concept is simple and has been used in space for decades. To cause rotation in one direction, you change the rotation of the momentum control wheel (action/reaction). This is generally good for low rates of rotation. For higher rotational rates, you're back to needing thrusters. All of the electric propulsion schemes out there have very low thrust so they're insufficient for things like docking. You need more thrust to achieve high rotational rates than electric engines can provide.

sanman
2009-Sep-15, 09:24 PM
Pulsed Inductive Thrusters are electrical thrusters which are supposed to be capable of providing significantly high thrust. Do a Google search on them, if you like. They are capable of multi-megawatt power coupling, and can produce upto tens or possibly even hundreds of newtons in thrust. Because they work through induction, they won't suffer any electrode erosion.

Likewise, VASIMR is supposed to be able to variably trade between high efficiency and high thrust, and could be used for space travel as well as station-keeping.

Needless to say, both of these technologies would probably require nothing less than a nuclear powerplant in order to produce significant thrust.

tracer
2009-Sep-15, 09:50 PM
For attitude control, you often don't need thrusters at all (e.g. Hubble).

True, you can use gyros and such for rotation, but for low delta-V translation maneuvers, you have to have thrusters.

I'm kinda liking the Pulsed Inductive Thrusters that sanman pointed to. This article (http://trs-new.jpl.nasa.gov/dspace/bitstream/2014/38357/1/05-1846.pdf) puts it at about 50% electric energy efficiency, with an Isp of 2000-9000 seconds, which is 5-25 times the Isp of chemical rocket thrusters.

Murphy
2009-Sep-16, 01:55 AM
Just an idea, but how about not using separate thrusters at all, but diverting the thrust from the main engine into smaller channels in different parts of the ship?

In a similar way to how the Harrier Jump Jet diverts the thrust from the main jet engine into downward facing tubes, thus providing upward force for hovering. I believe it's called Thrust Vectoring.

I'm not exactly sure how you would arrange such as system of a nuclear rocket, but I'm sure it could be done.

cjl
2009-Sep-16, 02:20 AM
Just an idea, but how about not using separate thrusters at all, but diverting the thrust from the main engine into smaller channels in different parts of the ship?

In a similar way to how the Harrier Jump Jet diverts the thrust from the main jet engine into downward facing tubes, thus providing upward force for hovering. I believe it's called Thrust Vectoring.

I'm not exactly sure how you would arrange such as system of a nuclear rocket, but I'm sure it could be done.
During most of a spacecraft's lifetime, the main engines aren't firing. This makes it somewhat difficult to divert the thrust...

JonClarke
2009-Sep-16, 10:32 PM
Just an idea, but how about not using separate thrusters at all, but diverting the thrust from the main engine into smaller channels in different parts of the ship?

In a similar way to how the Harrier Jump Jet diverts the thrust from the main jet engine into downward facing tubes, thus providing upward force for hovering. I believe it's called Thrust Vectoring.

I'm not exactly sure how you would arrange such as system of a nuclear rocket, but I'm sure it could be done.

Not an impossible idea. Reactors remain hot after they are shut down, to keep them cool small amounts of propellant are run through them, This can be used to run a turbine to provide electrical power (google Breyton cycle). Given this requires plumbing it is not impossible to divert some of this hot gas into thrusters.

mugaliens
2009-Sep-17, 08:57 AM
If our Space Cadets on their fusion torchship want to go traipsing around the solar system for weeks at a time without a fuel stop, they're going to have to carry quite a bit of weight in maneuvering-thruster propellant for all the little rotations and docking maneuvers and whatnot.

Not really. With inter-craft navigation using technology similar to GPS and computer control to finesse the approaches, tiny thrusters are all one needs.

The problem is that you still need decent thrusters, as rolling something like the shuttle using ion propulsion would take a couple of hours!

IsaacKuo
2009-Sep-17, 12:04 PM
it is not impossible to divert some of this hot gas into thrusters.
True, but will it be more mass efficient than chemical thrusters? In order to even compete with chemical thrusters, the diverted gas will need to be very hot and it needs to not lose this heat to the plumbing of your diversion system. Considering the losses through the plumbing and the mass of the plumbing itself, it may be worse than traditional chemical thrusters all around.

A better use for your solar panels or power reactor may be to slowly convert water into hydrogen and oxygen. As long as you only store a small amount of hydrogen and oxygen at a time, you can use simple compressed gas storage tanks instead of troublesome cryogenics. This lets you use relatively high Isp hydrogen-oxygen chemical maneuvering thrusters rather than lower Isp thrusters which use propellant suitable for long term storage.

eburacum45
2009-Sep-17, 08:01 PM
In theory you can use tethers for fine adjustments and orbital adjustments. Send out a tether in one direction and cut it loose; you'll move in the opposite direction. Or use it to generate electical power, and thereby cause drag. Alternately you could pump electricity into a suitably designed tether and that would create thrust under certain circumstances:
http://www.vectorsite.net/tarokt_5.html

...a conductive tether could also be used to provide propulsion for orbital adjustment. It is another simple fact of physics that running a current through a conductor creates a magnetic field. If the satellite sends current generated by its solar arrays through the tether in one direction, the magnetic field produced by the tether will oppose the Earth's magnetic field, causing magnetic "drag" and degrading the satellites's orbit. If the satellite sends the current through the tether in the other direction, it will work with the Earth's magnetic field, and the satellite will rise.

JonClarke
2009-Sep-17, 09:48 PM
True, but will it be more mass efficient than chemical thrusters? In order to even compete with chemical thrusters, the diverted gas will need to be very hot and it needs to not lose this heat to the plumbing of your diversion system. Considering the losses through the plumbing and the mass of the plumbing itself, it may be worse than traditional chemical thrusters all around.

A better use for your solar panels or power reactor may be to slowly convert water into hydrogen and oxygen. As long as you only store a small amount of hydrogen and oxygen at a time, you can use simple compressed gas storage tanks instead of troublesome cryogenics. This lets you use relatively high Isp hydrogen-oxygen chemical maneuvering thrusters rather than lower Isp thrusters which use propellant suitable for long term storage.

I never said it was more efficient, only possible :)

Siguy
2009-Sep-17, 09:59 PM
What about MPD thrusters? Are those suitable for reaction control? I know they use a lot of electricity, so they can only be pulsed, but they produce a lot of thrust with high specific impulse. They're also very simple, and can even work in an atmosphere.