Endoatmospheric propulsion systems

[opensource]

If very high energy density – due to fusion reactors – becomes a reality, then how might propulsion systems here on earth change. I’ve been studying the thrust to weight and lift to drag ratios for various propulsion technologies which are in use. I’m curious what innovations and efficiencies the maintenance of a plasma and the creation of very hot fusion reactions could provide to high-thrust propulsion systems. I’ve found a lot of interesting papers on low thrust propulsion systems for space travel – which is why I’m raising this question specifically.

[annodomini2]

Plasma propulsion for in Earth Atmosphere (I’m not saying for other planets in the solar system) isn’t practical due to the ionising radiation.

For planetary bodies with little or no atmosphere, relative to their minimum orbital velocity such as the moon or Mars, rail launch could be used. (A fusion reactor could be one possible power source).

In Earth atmosphere, you could, in theory, use a reactor to produce heat and convert water to steam, unfortunately this has major weight and efficiency issues (it was considered with Fission reactors in the ’60s). Designs that are currently being developed, such as Skylon would make these look big and heavy.

What would be needed is a revolutionary technology that could produce an adequate force, using only electricity or some other form of energy as a power source and no mass ejection. As far as I am aware, no one has achieved this yet.

[opensource]

What we’re talking about I think – to be quite vague – is a technology that best converts between energy and matter. The more of the latter you have, the more effective the propulsion system will be in this sea of gas and gravitation. Sound right?

[Brian H]

I seem to recall a SSTO MHD spaceplane design powered by FF from some years ago. Anyone have a link?

[Ferret]

opensource wrote: What we’re talking about I think – to be quite vague – is a technology that best converts between energy and matter. The more of the latter you have, the more effective the propulsion system will be in this sea of gas and gravitation. Sound right?

Generally, a rocket engine is more effective in terms of fuel used when it puts more energy into less matter. Energy means kinetic energy. Thus, an efficient rocket engine has higher exhaust speeds for less matter exhausted. For atmospheric engines things change a bit, since air may be taken in and used as the exhaust. But basically you still want to use the least amount of fuel.

Now for FF atmospheric propulsion, one would probably have to send the FF ion jets into a heating chamber, where they heat the air, which is then expelled to the back. It is much like a turbojet or a ramjet engine, only you use the FF exhaust to heat the air instead of burning some petroleum fuel. In terms of fuel used, this is much more efficient than a jet engine, since you use nuclear energy and the fuel quantity is some 1 000 000 times smaller. It remains to be seen whether it is more efficient in terms of engine mass, too. You don’t want a huge, heavy engine to do the same work as a jet engine. If you wanted that, the solution were right around the corner: a thermal nuclear fission reactor powering an aircraft engine. Think about an aircraft carrier power source on an airplane.

[annodomini2]

Ferret wrote:

What we’re talking about I think – to be quite vague – is a technology that best converts between energy and matter. The more of the latter you have, the more effective the propulsion system will be in this sea of gas and gravitation. Sound right?

Generally, a rocket engine is more effective in terms of fuel used when it puts more energy into less matter. Energy means kinetic energy. Thus, an efficient rocket engine has higher exhaust speeds for less matter exhausted. For atmospheric engines things change a bit, since air may be taken in and used as the exhaust. But basically you still want to use the least amount of fuel.

Now for FF atmospheric propulsion, one would probably have to send the FF ion jets into a heating chamber, where they heat the air, which is then expelled to the back. It is much like a turbojet or a ramjet engine, only you use the FF exhaust to heat the air instead of burning some petroleum fuel. In terms of fuel used, this is much more efficient than a jet engine, since you use nuclear energy and the fuel quantity is some 1 000 000 times smaller. It remains to be seen whether it is more efficient in terms of engine mass, too. You don’t want a huge, heavy engine to do the same work as a jet engine. If you wanted that, the solution were right around the corner: a thermal nuclear fission reactor powering an aircraft engine. Think about an aircraft carrier power source on an airplane.

Nuclear propulsion has been tried:

http://en.wikipedia.org/wiki/Nuclear_aircraft

The main issue that it wasn’t continued was political, due to fear of a Fission reactor crashing in a populated area.

The main technical issue was weight, reactor + shielding being quite heavy.

Like most technical issues there are solutions to these problems, but there are obviously practical limits.

For FF, the main issue for space use in an orbital launch capacity is you are effectively trading weight. Yes you have less actual fuel, but the reactor itself has a large mass and your propulsive element still needs to be there.

Air with a turbine is only practical up to about Mach 2.5 at which point the air temperature entering the engine is so high that it literally melts the engine.

Ram systems can work to higher speeds, but they are not self starting, so you need multiple propulsion elements. Using an electric heat source would require the system to generate ridiculous temperatures as you need to maintain sufficient deltaT to generate thrust. See SR-71.

Above about Mach 5-10 you’ll need internal mass to throw out the back as the temperatures will well exceed the limits of the materials we have today.

Starlite was a possibility, but unfortunately Maurice Ward died last year and as far as is public, he took the recipe with him.

As you state more speed/energy can result in reduced mass, but there is a limit, in the sense that, above a certain temperature you’re effectively throwing a hugely powerful ion beam out the back of the engine. This would (at least) create the same political issues as with flying a Fission reactor, if not more.

[jamesr]

annodomini2 wrote:
Air with a turbine is only practical up to about Mach 2.5 at which point the air temperature entering the engine is so high that it literally melts the engine.

You can use a pre-cooler such as the SABRE design from Reaction Engines in the UK http://www.reactionengines.co.uk/

They are aiming to be able to have one engine that can seamlessly accelerate from 0 through mach 5, then transition to using a small onboard oxygen tank to morph into a rocket for the last bit to orbit, then all the way back.

(incidentally they are based on the same site at Culham where the JET and MAST tokamaks are)

[annodomini2]

jamesr wrote:

Air with a turbine is only practical up to about Mach 2.5 at which point the air temperature entering the engine is so high that it literally melts the engine.

You can use a pre-cooler such as the SABRE design from Reaction Engines in the UK http://www.reactionengines.co.uk/

They are aiming to be able to have one engine that can seamlessly accelerate from 0 through mach 5, thetransition to using a small onboard oxygen tank to morph into a rocket for the last bit to orbit, then all the way back.

(incidentally they are based on the same site at Culham where the JET and MAST tokamaks are)

I know, but this would intend to use purely air, no cryogenic hydrogen available to cool the inlet, generating that kind of cooling would require huge amounts of energy. If you’re carrying hydrogen as reaction mass why bother with an f2f freactor.

Dumping the heat may be a problem.

[vansig]

annodomini2 wrote:

Above about Mach 5-10 you’ll need internal mass to throw out the back as the temperatures will well exceed the limits of the materials we have today.

Starlite was a possibility, but unfortunately Maurice Ward died last year and as far as is public, he took the recipe with him.

As you state more speed/energy can result in reduced mass, but there is a limit, in the sense that, above a certain temperature you’re effectively throwing a hugely powerful ion beam out the back of the engine. This would (at least) create the same political issues as with flying a Fission reactor, if not more.

Actually,
my understanding is that Ward’s family holds the Starlite recipe, but that industry isn’t really that interested in dealing.
Starlite is an ablative heat shield, and adequate ablative heat shields already exist.

Apart from the NIMBY attitudes about anything “nucular”, the ion beam will have about as much environmental impact as a lightning strike. presently lightning is seen as good for the planet, as it replenishes the ozone layer and makes nitrates, which are good for growing plants.

[Joeviocoe]

vansig wrote:

Above about Mach 5-10 you’ll need internal mass to throw out the back as the temperatures will well exceed the limits of the materials we have today.

Starlite was a possibility, but unfortunately Maurice Ward died last year and as far as is public, he took the recipe with him.

As you state more speed/energy can result in reduced mass, but there is a limit, in the sense that, above a certain temperature you’re effectively throwing a hugely powerful ion beam out the back of the engine. This would (at least) create the same political issues as with flying a Fission reactor, if not more.

Actually,
my understanding is that Ward’s family holds the Starlite recipe, but that industry isn’t really that interested in dealing.
Starlite is an ablative heat shield, and adequate ablative heat shields already exist.

Apart from the NIMBY attitudes about anything “nucular”, the ion beam will have about as much environmental impact as a lightning strike. presently lightning is seen as good for the planet, as it replenishes the ozone layer and makes nitrates, which are good for growing plants.

What would be the impact of 200+ lightning strikes per second?

[zapkitty]

Joeviocoe wrote:
What would be the impact of 200+ lightning strikes per second?

FF beam pulse ≠ lightning.

200 hz lightning would be the equivalent of about 7,500,000 FF cores going full tilt (using LPP’s figure of 80 kJ gross ion beam output per pulse)…

Exactly what kind of ship were you trying to power with 7.5 million FF cores? 🙂

From another calculation I did elsewhere that would be enough FFs to keep about 17 and a half supercarriers airborne…

[Joeviocoe]

zapkitty wrote:

Exactly what kind of ship were you trying to power with 7.5 million FF cores? 🙂

Well, I can’t tell you that. But I CAN tell you this… ‘That it’s no moon…’

[annodomini2]

vansig wrote:

Above about Mach 5-10 you’ll need internal mass to throw out the back as the temperatures will well exceed the limits of the materials we have today.

Starlite was a possibility, but unfortunately Maurice Ward died last year and as far as is public, he took the recipe with him.

As you state more speed/energy can result in reduced mass, but there is a limit, in the sense that, above a certain temperature you’re effectively throwing a hugely powerful ion beam out the back of the engine. This would (at least) create the same political issues as with flying a Fission reactor, if not more.

Actually,
my understanding is that Ward’s family holds the Starlite recipe, but that industry isn’t really that interested in dealing.
Starlite is an ablative heat shield, and adequate ablative heat shields already exist.

Apart from the NIMBY attitudes about anything “nucular”, the ion beam will have about as much environmental impact as a lightning strike. presently lightning is seen as good for the planet, as it replenishes the ozone layer and makes nitrates, which are good for growing plants.

At what thrust level are you doing these numbers?

[vansig]

force from thrust will balance drag, so really what matters is the top speed. at high mach numbers, air heats up enormously due to shock.. it even ionizes. the solution, for space planes, is to fly higher.

skylon is designed to fly at 26 km altitude, where air is rarefied to around 3.5% of normal atmosphere

[annodomini2]

1. You didn’t answer my question.

2. With the higher altitude and lower air pressure you have resultant lower reaction mass available for propulsion.

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You hit 2 balancing points:

1. Where you are at an altitude that there is sufficient reaction mass to generate sufficient thrust to overcome drag and accelerate.

2. The input air temperature is actually low enough to provide sufficient delta T that your heating mechanism is actually imparting energy into the incoming air flow.

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