Using atmosphere as propellant

Discuss the technical details of an "open source" community-driven design of a polywell reactor.

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DeltaV
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Using atmosphere as propellant

Postby DeltaV » Fri Jun 11, 2010 7:31 am

I don't post over at NASASpaceFlight.com, since there is someone there already using my "deltaV" moniker (lower case D), but the following post by "kfsorensen" drew my interest:

Re: Role of NTR/BNTR/NEP in future architectures
« Reply #449 on: 06/10/2010 11:28 PM »

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Quote from: KelvinZero on 06/10/2010 10:42 PM Im not at all a fan of nuclear launch vehicles, but I have a question,

Why do nuclear and beamed power designs not exploit the atmosphere as propellant?

For a launch vehicle, the first thing you want to do is "get clear" of the atmosphere in your ascent trajectory, so using atmospheric constituents as propellant is counter-productive.

This is true for traditional chemical propulsion, where you have to get to orbit in a hurry before you run out of cryogenic or hydrocarbon propellants.

This is not true for a winged Polywell-powered flying machine using relatively small amounts of H and B11 to heat atmosphere as propellant. Propulsion via p-B11 fusion allows a more leisurely ascent and descent, with indefinite atmospheric cruise before and after orbit.

zapkitty
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Re: Using atmosphere as propellant

Postby zapkitty » Fri Jun 11, 2010 1:06 pm

DeltaV wrote:
This is true for traditional chemical propulsion, where you have to get to orbit in a hurry before you run out of cryogenic or hydrocarbon propellants.

This is not true for a winged Polywell-powered flying machine using relatively small amounts of H and B11 to heat atmosphere as propellant. Propulsion via p-B11 fusion allows a more leisurely ascent and descent, with indefinite atmospheric cruise before and after orbit.


Missing a needed caveat... when achieving orbit atmospheric augmentation is only useful just as long as you are able to accelerate while in atmosphere.

For accessing space added velocity accrues towards your goal faster than added height. Both can help, but once you are going as fast and as high as possible there's no further benefit to remaining in atmosphere.

Once you've attained your maximum atmospheric velocity then staying in atmosphere is waste of resources... an unneeded strain on your ship structure at the very least.

And air-aug itself needs to take into account max internal heating, which will depend on the air-aug method chosen...

Stoney3K
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Re: Using atmosphere as propellant

Postby Stoney3K » Fri Jun 11, 2010 1:42 pm

zapkitty wrote:
DeltaV wrote:
This is true for traditional chemical propulsion, where you have to get to orbit in a hurry before you run out of cryogenic or hydrocarbon propellants.

This is not true for a winged Polywell-powered flying machine using relatively small amounts of H and B11 to heat atmosphere as propellant. Propulsion via p-B11 fusion allows a more leisurely ascent and descent, with indefinite atmospheric cruise before and after orbit.


Missing a needed caveat... when achieving orbit atmospheric augmentation is only useful just as long as you are able to accelerate while in atmosphere.

For accessing space added velocity accrues towards your goal faster than added height. Both can help, but once you are going as fast and as high as possible there's no further benefit to remaining in atmosphere.

Once you've attained your maximum atmospheric velocity then staying in atmosphere is waste of resources... an unneeded strain on your ship structure at the very least.

And air-aug itself needs to take into account max internal heating, which will depend on the air-aug method chosen...


When you have near-indefinite amounts of sustained thrust, there is no reason you would need to gain velocity *and* gain height at the same time. Push downward with a steady 1G of acceleration and you'll stay afloat as long as you need it.

Once you're clear of atmo, accelerating towards your needed orbital velocity is a lot easier because the atmosphere isn't creating drag anymore.

Re-entry would be just as easy: Burn retrograde until you're no longer in orbit, and compensate the effect of gravity with downward thrust. You will then lose orbital velocity while staying in the same place/altitude, and the engines would give you a perfectly controlled descent into atmospheric flight or hover.

If the fuselage can take it, making a drag re-entry through atmo is, of course, more efficient because gravity is free. Engine thrust can then compensate for overspeed (and too much drag). Otherwise, this landing is going to get very interesting...
Because we can.

Nik
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Slightly OT: Skylon's Sabre engines.

Postby Nik » Fri Jun 11, 2010 3:09 pm

Reaction Engine's Sabre design is air-breathing to high Mach, transitioning from jet through combined jet/rocket to pure rocket. Their design should manage this using a remarkably efficient heat exchanger to chill air-flow.

This may integrate nicely with a polywell-driven electric jet system...

WizWom
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Postby WizWom » Fri Jun 11, 2010 6:19 pm

Any system using an atmospheric engine has to balance the usefulness of the airbreather against the mass of it. Is 3-5 kps worth the added mass?

One concept has been using water injection into an heated aristream in atmosphere and a heat-based rocket engine for added thrust outside atmosphere. Since high speed electric fans are light and effective, I put my vote on this concept as reaching SSTO first.
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DeltaV
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Re: Using atmosphere as propellant

Postby DeltaV » Fri Jun 11, 2010 6:46 pm

zapkitty wrote:Missing a needed caveat... when achieving orbit atmospheric augmentation is only useful just as long as you are able to accelerate while in atmosphere.

For accessing space added velocity accrues towards your goal faster than added height. Both can help, but once you are going as fast and as high as possible there's no further benefit to remaining in atmosphere.

Once you've attained your maximum atmospheric velocity then staying in atmosphere is waste of resources... an unneeded strain on your ship structure at the very least.

There is another factor involved for this hypothetical Polywell SSTO system, which normally doesn't arise with the LEO systems used to date (Shuttle/STS operation touches on it, though, w.r.t. transatlantic abort and reentry crossrange).

If you have the capability of indefinite atmospheric cruise using only air as propellant, both before and after orbit, then your operational flexibilty with respect to which departure and arrival air-spaceports can be used increases tremendously.

Orbital mechanics dictates specific boost and reentry loci, but the added atmospheric freedom compensates. Airline style operation becomes a real possibility. I would imagine that flight to/from the boost/reentry locations would be done at something less than the highest/fastest possible, in order to reduce structure thermal issues. The highest heat loads would only occur during boost and reentry.

Stoney3K
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Postby Stoney3K » Fri Jun 11, 2010 6:51 pm

WizWom wrote:One concept has been using water injection into an heated aristream in atmosphere and a heat-based rocket engine for added thrust outside atmosphere. Since high speed electric fans are light and effective, I put my vote on this concept as reaching SSTO first.


We already have very efficient gas turbines which can transition from air-breathing jet to scramjet, to any other working fluid, so there's no need to reinvent the wheel there.

Combine with high bypass turbofan systems for low speed, atmospheric flight, and mount the engine nacelles on a set of gimbals similar to a V-22 to control your direction of thrust.

Which leaves you with a set of operational modes:

Engines in turbofan mode (electric driven):
* Atmospheric, low speed/hover: Engines straight down, using RCS to maintain stabilty or control direction of hover
* Atmospheric, subsonic speed: Engines pointing in flight direction, taking advantage of lifting body fuselage. Augment lift with RCS or MLT if neccessary.

Engines in scramjet mode (fan blades feathered, turbine bypassed):

* Atmospheric, supersonic: Engines straight forward, steer with RCS

* Atmospheric, transition to space: Engines forward, fueled with auxilliary working fluid (possibly waste helium from reactor, or CO2 from life support), augment stability and on-board gravity using MLT

* Space: Craft in full horizontal mode with gimbals locked, engines in scramjet or rocket mode, propelled with aux. working fluid. Attain orbital velocity and fly transit/escape trajectories using MLT. Smaller maneuvers using main engines or RCS.

Joss Whedon got it right. ;)
Because we can.

DeltaV
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Postby DeltaV » Fri Jun 11, 2010 7:04 pm

WizWom wrote:Any system using an atmospheric engine has to balance the usefulness of the airbreather against the mass of it. Is 3-5 kps worth the added mass?

The system considered here would likely not involve extra mass beyond some variable geometry ducting and inlets (high-temp composites). Direct-conversion and REB mass (if that's the best heating method, and it probably is) would be needed anyway for beyond-atmosphere operation, along with propellant tankage.

WizWom wrote:One concept has been using water injection into an heated aristream in atmosphere and a heat-based rocket engine for added thrust outside atmosphere. Since high speed electric fans are light and effective, I put my vote on this concept as reaching SSTO first.

Efficient, superconducting, high power density electric fans would be great for VTVL, and electric turbines for low-medium speed forward flight. 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.

DeltaV
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Postby DeltaV » Fri Jun 11, 2010 7:11 pm

Stoney3K wrote:Combine with high bypass turbofan systems for low speed, atmospheric flight, and mount the engine nacelles on a set of gimbals similar to a V-22 to control your direction of thrust.

Gimballed thrust pods will probably melt at reentry. Everything has to be flush/sealed, especially the front, bottom and sides.

Stoney3K wrote:Augment lift with RCS or MLT if neccessary.

MLT eliminates all the other stuff.

WizWom
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Postby WizWom » Fri Jun 11, 2010 7:12 pm

Stoney3K wrote:
WizWom wrote:One concept has been using water injection into an heated aristream in atmosphere and a heat-based rocket engine for added thrust outside atmosphere. Since high speed electric fans are light and effective, I put my vote on this concept as reaching SSTO first.


We already have very efficient gas turbines which can transition from air-breathing jet to scramjet, to any other working fluid, so there's no need to reinvent the wheel there.


Sure there is. "gas turbines" use a fuel. I'm talking fuel-less electric/ augmented electric operation. Different bird entire.
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DeltaV
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Postby DeltaV » Fri Jun 11, 2010 7:21 pm

Someone wrote a letter to AWST suggesting the use of 20 GE-90 turbofans (~115K lb thrust each) in a flyback booster. 2.3M lb thrust total.

REB-heated versions would mostly eliminate the fuel mass.

Stoney3K
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Postby Stoney3K » Fri Jun 11, 2010 7:28 pm

DeltaV wrote:
Stoney3K wrote:Combine with high bypass turbofan systems for low speed, atmospheric flight, and mount the engine nacelles on a set of gimbals similar to a V-22 to control your direction of thrust.

Gimballed thrust pods will probably melt at reentry. Everything has to be flush/sealed, especially the front, bottom and sides.


That only holds if you're counting friction-based re-entry. Basically, if you can control your rate of descent and do not need to rely on atmospheric drag to slow you down, you can descend vertically on a dime with 2 KPH if you want.

Stoney3K wrote:Augment lift with RCS or MLT if neccessary.
MLT eliminates all the other stuff.


MLT might not be the most ideal way to go when you're in the atmosphere and in populated areas. Right now, it's too soon to tell, but MLT's must have their downsides as well. Maybe it's even just being plain power-hungry and therefore not practical in high drag environments.

Sure there is. "gas turbines" use a fuel. I'm talking fuel-less electric/ augmented electric operation. Different bird entire.


Gas turbines don't use fuel, they use a working fluid which is high pressure and temperature. Coincidentally, the most practical way to make such a fluid on Earth is to burn stuff ;)... but there are enough other methods of doing so.

If you use an electric fan only, it might be practical in the atmosphere, but not neccessarily outside. That would make it a dedicated system for atmospheric flight, and only useful when you have the power/weight budget to accomodate (e.g. capital sized sky-ships).

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)

DeltaV wrote: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.


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.
Because we can.

DeltaV
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Postby DeltaV » Fri Jun 11, 2010 7:55 pm

Stoney3K wrote:That only holds if you're counting friction-based re-entry. Basically, if you can control your rate of descent and do not need to rely on atmospheric drag to slow you down, you can descend vertically on a dime with 2 KPH if you want.

That requires propellant, if you don't have MLTs. Propellant has mass.

Stoney3K wrote:MLT might not be the most ideal way to go when you're in the atmosphere and in populated areas. Right now, it's too soon to tell, but MLT's must have their downsides as well. Maybe it's even just being plain power-hungry and therefore not practical in high drag environments.

Drag varies with the square of velocity. Ascend slower.

Stoney3K wrote: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)

The advantage is that, if you're using something like Polywell to provide the electricity, you're consuming minute amounts of H and B11 instead of gulping many gallons of kerosene (AKA jet fuel). Fuel mass becomes ignorable.

Stoney3K wrote: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 power hog is the REB (which likes high voltage) or whatever other electrical propulsion is used. The other stuff can be powered by a waste-heat turbine loop without having to down convert UHVDC.

Stoney3K
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Postby Stoney3K » Fri Jun 11, 2010 8:12 pm

DeltaV wrote:
Stoney3K wrote:That only holds if you're counting friction-based re-entry. Basically, if you can control your rate of descent and do not need to rely on atmospheric drag to slow you down, you can descend vertically on a dime with 2 KPH if you want.

That requires propellant, if you don't have MLTs. Propellant has mass.


It requires thrust which ever way you manage to create it. Whether you do it by chucking waste material out the window (RCS), suck in air, accelerate it, and throw it out the back (jet) or use MLT, that's beside the point.

If you're running off a Polywell, you've got a very abundant (almost limitless) supply of power, therefore a limitless supply of thrust. The reason we're using friction re-entries in these days is because spacecraft need to carry extra propellant for the re-entry as well.

There is enough excess mass that is wasted by a Polywell to use as a propellant anyway, as you can store the He ash in tanks and launch/ ionize it in RCS thrusters.

Stoney3K wrote:MLT might not be the most ideal way to go when you're in the atmosphere and in populated areas. Right now, it's too soon to tell, but MLT's must have their downsides as well. Maybe it's even just being plain power-hungry and therefore not practical in high drag environments.

Drag varies with the square of velocity. Ascend slower.


When you have the power, no reason to go slow. But when you're close to the ground, a Polywell and MLT's running at full blast for lift-off might pose a bit of a radiation hazard for the people standing around your ship. ;)

MLT's must have their downsides, as there's no free lunch in this business. Maybe they're slow to react on speed/direction changes, emit harmful radiation or consume too much power in certain situations.

Stoney3K wrote: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)

The advantage is that, if you're using something like Polywell to provide the electricity, you're consuming minute amounts of H and B11 instead of gulping many gallons of kerosene (AKA jet fuel). Fuel mass becomes ignorable.


Turbines run on more than jet fuel. A classic turbofan as you find on aircraft, with a combustion chamber, will run on Jet-A1, but there are enough other concepts that can make a turbine spin. Instead of a combustion chamber, you can use a plasma chamber to heat any working fluid you want, whether it be air into or out of the engine (during atmo), or another gas you happen to have on hand because your power source produces it as a byproduct.

If necessary, a Brayton cycle gas turbine can also run in a closed loop if waste of working fluid is a problem. Space Shuttle APU's work that way.

(As Uhura said: That thing's got to have a tail pipe?)
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Postby D Tibbets » Fri Jun 11, 2010 9:26 pm

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.

*Actual numbers would probably be closer to 1/10th of this.

Dan Tibbets
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