Calling our technical experts - Any chance this thing works?

Bill White · 2004-04-30 14:03:00

JP Aerospace has a novel idea for Earth to LEO - - airships to orbit:

John revealed for the first time that the primary long term goal of this diverse array of projects is to develop an airship that will go all the way to orbit. The 6000ft long V ship would start from a high altitude "Dark Sky Station" at 200k ft. Using electric propulsion, the vehicle would gradually gain speed and take advantage of the slight lift provided by the residual atmosphere even at such high altitudes. Simulations show such a vehicle could obtain orbital velocity in about 5 days.

http://www.hobbyspace.com/AAdmin/archiv … ml#JP]Link to quote

http://www.hobbyspace.com/AAdmin/archiv … Additional link

Does this thing have a snowball's chance of working?

John Creighton · 2004-04-30 14:30:57

Interesting, this could be good for people.

GCNRevenger · 2004-04-30 14:37:10

Mmmmm doubtful, ion engines don't put out that kind of thrust without getting pretty heavy. Also, the thrust provided by the ion engines must be greater than the drag of the thin air that the vehicle supposedly climbs on.

Would probably have lousy payload mass too.

Bill White · 2004-04-30 14:50:24

What if the thing were inflated with hydrogen? Don't worry about payload ratios, hydrogen is the payload.

We would need an on-orbit compressor to change gaseous H2 to LH2, correct?

Edit: After the H2 is pumped from the balloon, roll the thing up and dump it back to Earth as well for re-filling.

~Eternal~ · 2004-04-30 15:58:28

What if the thing were inflated with hydrogen? Don't worry about payload ratios, hydrogen is the payload.
We would need an on-orbit compressor to change gaseous H2 to LH2, correct?
Edit: After the H2 is pumped from the balloon, roll the thing up and dump it back to Earth as well for re-filling.

Dook · 2004-04-30 16:21:23

LOL, nice picture.

Euler · 2004-04-30 16:59:22

I don’t see how they will be able to get enough power for the ion engine. Ion engines work best at high isp, which means they need a lot of power to produce a little thrust. I think that a solar (or nuclear) thermal engine would have a better chance of working, though even that seems doubtful.

GCNRevenger · 2004-04-30 17:59:26

With the huge area the vehicle would have, electrical supply shouldn't be a problem, the problem is getting enough thrust to overcome the drag without the engines getting too heavy, and if the thing could carry a useful payload or not.

Hydrogen gas isn't that dangerous. You can stick a lit match into a tank of it... and the match will go out. The Hindenberg went up because they painted it with an alumium and phosphorous based paint... solid rocket fuel for the less chemically inclined.

Euler · 2004-04-30 18:17:19

With the huge area the vehicle would have, electrical supply shouldn't be a problem, the problem is getting enough thrust to overcome the drag without the engines getting too heavy, and if the thing could carry a useful payload or not.

Solar panels are heavy. Probably much heavier than the thruster itself, even without the having any support structure mass. Also, they can only generate power when the vehicle is not in the Earth's shadow.

ERRORIST · 2004-04-30 19:21:20

You can reflect light from a mirror on to it when it is in the shade of the Earth.

GCNRevenger · 2004-04-30 20:34:09

Correction, silicon solar pannels are heavy. Polymeric ones are not nessesarrily. In fact, it may be possible to paint or spray them on. Less efficent yes, but two such arrays 6000ft long and several hundred wide would add up.

Though the weight of the ion engines is still a killer.

John Creighton · 2004-04-30 21:25:03

How about combining the power from the solar cells, with fuel cells. If the airship is full of hydrogen this should be possible right?

I am kind of worried of what might happen to the ship at night time. Might it start to fall?

RobertDyck · 2004-04-30 21:51:54

Space rated silicon solar panels provide 184 W/m^2 for panels > 2.5 m^2, and with a 6 mil doped ceria coverslide (anti-static glass coating) it masses 1.33 kg/m^2. Triple junction GaInP2/GaAs/Ge solar panels provide 302 W/m^2 for panels > 2.5 m^2, and with the same coverslide mass 2.06 kg/m^2. That means silicon provides 138.3 W/kg while TJ provides 146.6 W/kg. This appears to be not much gain, but the mass doesn't include a substrate or support (backing, hinges, wires). Improved triple junction cells provide 330 W/m^2, but mass 2.36 kg/m^2 for only 139.8 W/kg. TJ cells are 5.5 mil thick, ITJ cells are 7.5 mil thick. You can get 7.5 mil thick TJ cells but they don't provide any more power and mass as much as ITJ cells.

Do you really expect polymeric solar panels to provide more power per kilogram? If so prove it, let's see the numbers.

Mark Friedenbach · 2004-04-30 22:02:00

How about combining the power from the solar cells, with fuel cells. If the airship is full of hydrogen this should be possible right?
I am kind of worried of what might happen to the ship at night time. Might it start to fall?

Better yet, get the power straight from the fuel cell. It'll have to vent hydrogen as it gains altitude, might as well put it to good use. As for H2 vs He, hydrogen is much cheaper (by a significant amount, wish I could find numbers)

John Creighton · 2004-04-30 22:33:52

I wonder how big one of these things could be made? Obviously, if the fuel is hydrogen that presents so real dangers. But the larger the ship the faster the exhaust velocity can be (i.e more efficiency) and the more fuel the ship has, in terms of hydrogen for fuel cells or by solar panels? Plus the more payload and fuel that can reach orbit the better.

BTW where would the best place to launch a really big ship be? The ocean? The parries? A desert?

Euler · 2004-04-30 22:51:19

Better yet, get the power straight from the fuel cell. It'll have to vent hydrogen as it gains altitude, might as well put it to good use.

You also need oxygen to get power from a fuel cell. If you have hydrogen and oxygen, then it would be more efficient to simply burn it in a conventional chemical rocket engine than it would be to convert it to electricity to power an electric engine.

Obviously, if the fuel is hydrogen that presents so real dangers.

I wouldn't worry about this too much. Hydrogen is only flamable if there is also oxygen.

But the larger the ship the faster the exhaust velocity can be (i.e more efficiency)

Isp should be pretty independent of ship size.

John Creighton · 2004-04-30 23:04:55

You also need oxygen to get power from a fuel cell. If you have hydrogen and oxygen, then it would be more efficient to simply burn it in a conventional chemical rocket engine than it would be to convert it to electricity to power an electric engine.

The conventional rocket does not have the advantage of buoyant lift. Moreover, the conventional rocket has a much lower exhaust velocity then electric propulsion. I’m thinking fuel efficiency, not energy efficiency.

Isp should be pretty independent of ship size.

Well, If you are limited to the magnitude of the electric field then the longer the distance you have to accelerate the particles the greater their velocity. Hence, the greater the ISP.

Mark Friedenbach · 2004-04-30 23:08:36

You also need oxygen to get power from a fuel cell. If you have hydrogen and oxygen, then it would be more efficient to simply burn it in a conventional chemical rocket engine than it would be to convert it to electricity to power an electric engine.

Well, I assume they'd use atmospheric oxygen, at least for most of the way up. The rate of venting would be real small, too small I'm sure to get continuous thrust from a traditional rocket. But maybe a pulse detonation engine could be designed, to accelerate in bursts when enough H2 and O2 has been accumulated.

I wouldn't worry about this too much. Hydrogen is only flamable if there is also oxygen.

Which there would be in the atmosphere for most of the trip. There could be a real danger from accumulating static charge during the trip.

Euler · 2004-04-30 23:14:45

The conventional rocket does not have the advantage of buoyant lift.

A conventional rocket engine can use the buoyant lift equally as well as a fuel cell electric engine.

Moreover, the conventional rocket has a much lower exhaust velocity then electric propulsion. I’m thinking fuel efficiency, not energy efficiency.

If fuel cells don't beat chemical rockets in energy efficiency, they won't have better fuel efficiency either because you still have to use up fuel to get your energy.

Well, If you are limited to the magnitude of the electric field then the longer the distance you have to accelerate the particles the greater their velocity. Hence, the greater the ISP.

Ion engine exhaust velocity is generally limited by the drop in grid voltage, rather than distance. Also, higher isp is not always better because it is inherently less energy efficient.

John Creighton · 2004-05-01 12:00:08

If fuel cells don't beat chemical rockets in energy efficiency, they won't have better fuel efficiency either because you still have to use up fuel to get your energy.

Is the fuel efficiency of fuel cells really that bad? I know conventional rockets the exhaust is hot, so a lot of thermal energy is lost in heat, and sense the exhaust hasn’t fully expanded by the time it leaves the rocket obviously there is a lot of energy lost do to that. Maybe a low thrust chemical engine could be developed that is a lot more efficient then current rockets.

John Creighton · 2004-05-01 12:24:25

Ion engine exhaust velocity is generally limited by the drop in grid voltage, rather than distance. Also, higher isp is not always better because it is inherently less energy efficient.

Why can't multiple plates be used. How about having a series of plates and use a voltage rectifier to create the voltage difference between plates.

Euler · 2004-05-01 14:27:53

Is the fuel efficiency of fuel cells really that bad? I know conventional rockets the exhaust is hot, so a lot of thermal energy is lost in heat

Remember that the temperature of a gas is the average kinetic energy of molecules in the gas (well, technically it is more complicated than that, but that is the general idea). This means that the higher the temperature of the exhaust is, the higher the exhaust velocity is. So in reality, the heat in the exhaust is not wasted, it is actually what is producing the thrust.

The H2/O2 reaction has an "ideal" isp of 528 seconds. The most efficient engines right now have a vacuum isp of about 465 seconds. This means that the efficiency is (465/528)^2= 78%.

Why can't multiple plates be used. How about having a series of plates and use a voltage rectifier to create the voltage difference between plates.

They use multiple plate in particle accelerators to get particles up to extremely high speeds. However, it adds complexity and reduces efficiency compared with a normal ion engine. It is also not necessary to get the isp values that we want the ion engine to have. For a given thrust, power usage is directly proportional to isp. That is a law of physics, not something that you can get around with technology. For a typical ion engine spacecraft, power production is a very large portion of the vehicle's mass. Eventually, you come to a point where it is more efficient to lower isp to reduce power usage than it is raise isp to reduce propellant consumption.

deagleninja · 2004-05-01 14:35:47

LOL what a strange coincidence! I recently wrote Robert Zubrin to ask if such a thing were possible and he said no.

John Creighton · 2004-05-01 14:42:58

Remember that the temperature of a gas is the average kinetic energy of molecules in the gas (well, technically it is more complicated than that, but that is the general idea). This means that the higher the temperature of the exhaust is, the higher the exhaust velocity is. So in reality, the heat in the exhaust is not wasted, it is actually what is producing the thrust.

That is a pour definition of temperature. IIRC temperature is proportional to the entropy (ie. The amount of disorder in the system. If all the molecules were travelling at the speed in the same direction the temperature would be close to zero. The heat is not what produces the thrust. Rather it is the pressure, or equivalently the momentum exchange. When the gas expands quickly entropy remains constant but the pressure and temperature change. Recall the ideal gas law:
PV=nRT

Euler · 2004-05-01 15:01:55

the temperature of a gas is the average kinetic energy of molecules in the gas

That was the definition that I was taught in chemisty class. I did say it was not exact, but it works out well enough in most cases. It should get the same results as the pressure model, as long it is used correctly.

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Re: Calling our technical experts - Any chance this thing works?

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