Using atmosphere as propellant

[WizWom]

Stoney3K, the rest of us are working in the realm of "Real, proven" - not "might possibly be, if we wish hard."

If you have a massless thruster, then, of course you can go to the moon on a whim and never have to play with reaction mass.

Just like if I have a Unicorn to ride, I never have to worry about getting girls.

[kunkmiester]

Gas turbines don't use fuel, they use a working fluid which is high pressure and temperature.

As I understand it, when an engineer says "gas turbine" he means a turbine engine burning regular fuel, but rather than provide thrust, the power is pulled out via extra turbines on the back end to turn the shaft, providing mechanical power, rather than thrust.

[93143]

An electric fan could be used to compress a working fluid, to subsequently heat it and generate thrust, but in that case, I see no advantages over using an electrically powered version of a turbofan engine. (with that, I mean, replace the fuel injectors with a plasma chamber)

In a Brayton cycle jet engine, high temperature means lousy thrust efficiency. Low temperature means lousy thermal efficiency, and corresponding low thrust efficiency.

Turbofans are an attempt to get around this, by using a high thermal-efficiency, low thrust-efficiency turbojet to power a fan.

But this is a Polywell-powered vehicle - why do we need a heat engine to do what we could do with a superconducting electric motor?

Ducted fans all the way - don't even bother heating anything. (At least, not until you start going fast enough that you need to switch over to ram/scram mode...) ...this is assuming you can run the motors on HVDC; the mass of the stepdown equipment changes the trade balance quite a bit...

The problem is that they require low voltage, which requires extra mass to down-convert Polywell's ~1.5MV to a usable range. Maybe this can be done lightly with high-frequency converters, but I haven't been able to get any of the EEs on this forum to provide mass estimates.

This:

The P-B11's ~2.4 MeV helium ion powered direct energy power conversion systems currently run between a calculated 0.10-to-0.30 kg/kWe specific mass with overall conversion efficiency in the 70-to-80% range, dependent on whether you need moderate voltage dc or low frequency ac power distribution. If all you need is high frequency RF in the form of a high power HF or VHF such as VASIMR uses, then ~0.10 kg/kWe has all ready been demonstrated at the MWe level by the Japanese.

Unless you plan to use EPS couplings or run a ship-wide superconducting HVDC power distribution grid, you're going to need to convert that 1500kV into something more useful anyway. I mean, you're going to need a few hundred Volts of three-phase AC to run most essential and auxilliary ship systems, such as computers, life support, and sensors/navigation, so there's no reason why you couldn't run a set of fans off a 3ph bus as well.

The difference is in how big the down-converter needs to be. The non-propulsive systems are never going to consume a significant amount of power relative to the engines, and thus a down-converter big enough to provide them with power will be utterly insignificant next to one that can step down the entire reactor output.

Decellerating from oprbit requires either atmospheric drag/ friction or ejection of a gas. The helium produced from a Polywell could provide very efficient thrust, but the power would be tiny. If you accelerate or eject the helium through nozzels, even at 100,000 ISP,you would only have perhaps 10^ 23helium nuclei per second* (~10 GW). That would be ~ 1 mole or 4 grams of helium per second. 4 grams * 100,000 ISP= ~400 Kg of thrust. Not enough to make a significant difference on a re-entry thermal load, and way to little to counteract gravity after you decellerated more than a tiny amount below orbital velocity. Once into the dense atmosphere (and subsonic?) electric fans might provide enough thrust to hover.
So you need a stored working fluid to provide de-orbiting thrust if you want to avoid the problems of atmospheric friction. If a mixed fusion product can provide enough thrust at say 5,000 ISP, then you may be able to budget for the on board fuel (working fluid) to avoid re-entry heating.

10 GW is (as you noted afterwards) ~2e22 alphas per second. That's 0.000143 kg/s, at a specific impulse of 1.2 milllion seconds, for 1691 N (172 kgf) of thrust. At 8.32 kg/s of diluent (which makes the original helium stream kinda superfluous), the specific impulse is 5000 seconds and the thrust is 408 kN, or about 41,600 kg of thrust. (This is all assuming 100% conversion of fusion power to jet power...)

Still not good enough for a vehicle that's likely to weigh over 1000 mT at the very least...

[DeltaV]

The P-B11's ~2.4 MeV helium ion powered direct energy power conversion systems currently run between a calculated 0.10-to-0.30 kg/kWe specific mass with overall conversion efficiency in the 70-to-80% range, dependent on whether you need moderate voltage dc or low frequency ac power distribution. If all you need is high frequency RF in the form of a high power HF or VHF such as VASIMR uses, then ~0.10 kg/kWe has all ready been demonstrated at the MWe level by the Japanese.

Let's use the upper end of that range. Ignoring all losses for a very rough estimate, (6x10^6 kW)*(0.30 kg/kW) = 1,800,000 kg or 3,960,000 lb. Just for direct + down conversion. No radiation shielding, aerostructure, propellant, etc. Electric fans will need lighter converters or will have to use electrostatic motors.

Hmmm... let the fans be the electrostatic motor armatures? Naah... corona losses to atmosphere.
Unless... you shoot alphas at, or intermittently connect direct converter output to, the outer rim of a ring-bounded fan, said outer rim being in a vacuum or SF6...

[Stoney3K]

If you have a massless thruster, then, of course you can go to the moon on a whim and never have to play with reaction mass.

I was just trying to avoid using MLT's and other reactionless devices.

When you're in the atmosphere, you've got reaction mass for free (namely, the atmosphere). The trick is to find a system that can give enough additional energy to the intake air and vector it to propel your ship in the direction you want.

Once you're clear of the atmosphere (to the point where the atmosphere is not massive enough to use as a propellant), you're going to need other means to push your ship in the right direction. Fortunately, in space, you need less reaction mass to make that work, since there is no atmospheric drag you need to overcome.

If you were to build ducted fan engines only, they would enlarge your ship's footprint (and cross-section), which has its uses, but not necessarily for a spacecraft which spends most of its time outside of the atmosphere. You can combine ducted fans with any other means of propulsion, but you still need a clear air path.

Ducted fans all the way - don't even bother heating anything. (At least, not until you start going fast enough that you need to switch over to ram/scram mode...) ...this is assuming you can run the motors on HVDC; the mass of the stepdown equipment changes the trade balance quite a bit...

That would also be an option and the ultimate choice would probably depend on the weight of the step-down gear and the feasability of SC electric motors.

If the stepdown equipment proves too heavy, you can always install a heat engine, which gives you the generators for driving all other ship systems for free.

[WizWom]

In a Brayton cycle jet engine, high temperature means lousy thrust efficiency. Low temperature means lousy thermal efficiency, and corresponding low thrust efficiency.

Um... NO. Adding heat on the high side lets more drop happen with the same ambient. That's why you heat an airstream to increase jet exhaust speed. The added kinetic energy means the gas must have higher pressure, which means it will push out the back faster.

I think you need to go back and explore the actual dynamics of a jet.

Turbofans are an attempt to get around this, by using a high thermal-efficiency, low thrust-efficiency turbojet to power a fan.

But this is a Polywell-powered vehicle - why do we need a heat engine to do what we could do with a superconducting electric motor?

Ducted fans all the way - don't even bother heating anything. (At least, not until you start going fast enough that you need to switch over to ram/scram mode...) ...this is assuming you can run the motors on HVDC; the mass of the stepdown equipment changes the trade balance quite a bit...

A jet is a ducted fan designed for high speed. That's all it is. If you could change the length depending on airspeed, that would be cool. But ducted fans have horrid performance above about mach 0.3; we want to hit mach 15 at least in atmosphere. Jets are an added expense in mass, to allow takeoff without a tow.

[Stoney3K]

A jet is a ducted fan designed for high speed. That's all it is. If you could change the length depending on airspeed, that would be cool. But ducted fans have horrid performance above about mach 0.3; we want to hit mach 15 at least in atmosphere. Jets are an added expense in mass, to allow takeoff without a tow.

You can use turbofan operation up to mach 0.7-0.9, after wich you could feather the fan blades and switch to ramjet or scramjet operation.

Once the amount of intake air becomes too little to sustain thrust, shut the intake doors at the front and continue with on-board propellants. At that point, you'd probably be at the edge of space and you'd need only small amounts of propellant to speed up and gain velocity for orbit.

Remember that the ship, if powered by Polywell, has a virtually unlimited capability to keep the engines running and sustain thrust. Which allows you to move freely in both the atmosphere and in/out of orbit.

[DeltaV]

The P-B11's ~2.4 MeV helium ion powered direct energy power conversion systems currently run between a calculated 0.10-to-0.30 kg/kWe specific mass with overall conversion efficiency in the 70-to-80% range, dependent on whether you need moderate voltage dc or low frequency ac power distribution. If all you need is high frequency RF in the form of a high power HF or VHF such as VASIMR uses, then ~0.10 kg/kWe has all ready been demonstrated at the MWe level by the Japanese.

Let's use the upper end of that range. Ignoring all losses for a very rough estimate, (6x10^6 kW)*(0.30 kg/kW) = 1,800,000 kg or 3,960,000 lb. Just for direct + down conversion. No radiation shielding, aerostructure, propellant, etc. Electric fans will need lighter converters or will have to use electrostatic motors.

Actually, using all 6GW for fans would be overkill. For a SWAG efficiency of 0.75, that corresponds to 54 GE90 115 Klb thrust turbofans (83.2 MW each) giving a total thrust of about 6.2 Mlb.

[93143]

I just noticed this...

In a Brayton cycle jet engine, high temperature means lousy thrust efficiency. Low temperature means lousy thermal efficiency, and corresponding low thrust efficiency.

Um... NO. Adding heat on the high side lets more drop happen with the same ambient. That's why you heat an airstream to increase jet exhaust speed. The added kinetic energy means the gas must have higher pressure, which means it will push out the back faster.

I think you need to go back and explore the actual dynamics of a jet.

I think you need to read more carefully.

Thrust efficiency is N/W. Thermal efficiency is nondimensional. Do not confuse the two.

Remember, thrust is proportional to exhaust velocity, but power is proportional to exhaust velocity squared. Higher exhaust velocity = lower thrust efficiency.

And if you're just using a turbojet, the only way to decrease the exhaust velocity is to decrease the hot-side temperature and/or the pressure ratio. Which reduces thermal efficiency, and drags the thrust efficiency down with it.

This is why turbofans were developed.

Turbofans are an attempt to get around this, by using a high thermal-efficiency, low thrust-efficiency turbojet to power a fan.

But this is a Polywell-powered vehicle - why do we need a heat engine to do what we could do with a superconducting electric motor?

Ducted fans all the way - don't even bother heating anything. (At least, not until you start going fast enough that you need to switch over to ram/scram mode...) ...this is assuming you can run the motors on HVDC; the mass of the stepdown equipment changes the trade balance quite a bit...

A jet is a ducted fan designed for high speed. That's all it is. If you could change the length depending on airspeed, that would be cool. But ducted fans have horrid performance above about mach 0.3; we want to hit mach 15 at least in atmosphere. Jets are an added expense in mass, to allow takeoff without a tow.

I said ducted fan, not cowled propeller. The duct can be any shape you like. (In this case it would be quite long and almost certainly variable-geometry, since a good chunk of it at least is also used for hypersonic propulsion).

The main point I was trying to get across is that Brayton cycles suck, so if we can use electric fans without incurring the mass-budget cost of stepdown equipment for main propulsive power, we should.

On the other hand, if a substantial fraction of full power is not required for non-hypersonic speeds (and it likely isn't, even for VTOL), there may be no strong reason (depending on how waste heat is handled) not to just take the efficiency hit and pour on more power... fusion fuel is cheap...

[DeltaV]

I think using "electrostatic" fan motors without needing heavy UHVDC down-conversion is possible...

Maybe driven by direct converter UHVDC output, or maybe by alpha beam impingement on rotors or stators, with no direct conversion HW needed (which would require magnetic focusing coils inside the vacuum chamber and evacuated magnetic conduits for the alpha beams to get them to the fan locations). The motors would probably need to be enclosed in vacuum or SF6, with rotary seals (or would the power-to-spare make up for any corona/arc losses for open motors?). An alternative would be one or a few motors inside the vacuum chamber sending power out via rotary-sealed driveshafts, but vacuum purity would be difficult to maintain with all the wear, tear, and arcs (yes, arcs can occur in a vacuum).

Which is more feasible: routing 1-2 MV DC to multiple fan locations without arcing (motors in series would reduce the voltage drop across each, and the arcing likelihood), or routing alpha beams magnetically (beam splitters would be needed for a many-fans design)?

Of course, a more ideal situation would be to have available lightweight down-conversion HW, which would allow the use of existing, high power density, low voltage motors such as http://www.synchrony.com/products/high- ... ators.aspx or their superconducting offspring. Then you are only routing low voltage cables. But the mass needed for down-conversion seems excessive at present.

[93143]

The trouble with routing alpha beams around is that they are highly non-neutral (in direct proportion to reactor power), so they spread much faster than thermally.

[DeltaV]

The trouble with routing alpha beams around is that they are highly non-neutral (in direct proportion to reactor power), so they spread much faster than thermally.

So constraining them magnetically for any significant distance would likely impose an unacceptable mass penalty, due to the number of magnets required.

I wonder if an electrostatic beam guide would work. Evacuated, nonconducting outer tube, smaller metallic tube inside, electrically isolated. The inner tube gets a positive charge from errant alphas and acts as an electrostatic lens for the remainder moving along its axis.

[93143]

Gauss' Law...

[DeltaV]

Not enough coffee.
OK, concatenated Einzel lenses, electrostatic quadrupoles or something similar:

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

The Transport of Charged Particle Beams
http://escholarship.org/uc/item/16c261qt (click Download PDF)

2-MV electrostatic quadrupole injector for heavy-ion fusion
http://www.escholarship.org/uc/item/3hv89087 (click Download PDF)

http://accelconf.web.cern.ch/AccelConf/ ... PLT039.PDF

http://www.pelletron.com/escomp.htm (lower voltages)

Focusing of He+ beams using a compact electrostatic quadrupole lens system

[93143]

That's better.

Still, they're talking about ~1 A beams in the 2-10 MV range at most (Guharay et al. are talking about 2 mA at 10-15 kV, and the National Electrostatics lenses are only rated to handle up to 150 kV beams, probably at small currents). A 6 GW Polywell with 14 cusps generates ~300 A per cusp. Also, while the average energy of the beam is 2.9 MeV, it isn't monoenergetic; the fast excited beryllium the first alpha kicks off can fission in any axis it likes... this isn't necessarily a showstopper, but it means you have to deal with relatively slow-moving ions, which requires shorter lens spacing and thoroughly sinks the assumption (already invalid even for the high-band alphas) that beam energy >> beam electrostatic potential...

I suspect this system would be quite heavy, if not completely unfeasible.


Also, I swear I had this idea (quadrupole focusing of ion beams) several years ago, and concluded - without doing the math - that it would never work...