Calling our technical experts - Any chance this thing works?

John Creighton · 2004-05-01 15:20:33

Well, it works for an ideal gas. But in terms of rockets what reference frame do we measure the average kinnectic energy from. I say from a reference that has the same average velocity as the gas. Anyway, I think the exact definition of temperature is:
DS/dU
Where S is the entropy
And U is the internal energy

Entropy is a measure of the disorder in the system. When all states are equally likely entropy is equal to the natural logarithm of the number of states. The definition of temperature is derived from statistics, and two systems are considered of equal temperature if when placed in contact know energy flows between the two systems. It can be shown that this is the case if temperature is defined by the above definition.

John Creighton · 2004-05-01 15:44:32

For a given thrust, power usage is directly proportional to isp.

how close are current ion engines to this ideal?

Euler · 2004-05-01 16:16:42

Ion engines actually have higher efficiencies at higher with higher isp (though they still have less thrust/power at higher isp). They are about 60% efficient at 3000s, and I think they are about 80% efficient at 8000s. For isp values less than 3000s, Hall effect drives are better, with efficiency of about 50-60%. For really low isp (under 500), resistojets can get efficiencies of 60-90%.

GCNRevenger · 2004-05-01 20:23:00

http://www.fwn.rug.nl/solar/general%20m … ltaics.htm

Only hundreds of nanometers thick solar cells... Thats where you get the power from. Fuel cells, though efficent, are quite heavy and the oxygen would be hard to collect at that low a pressure. Also at highest altitudes, atmospheric gasses will begin to fraction, so its mostly hydrogen and helium.

bolbuyk · 2004-05-03 07:18:07

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 panes > 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.

A bit off-topic, but does this only be the case for a distance of 1 AU from the sun? Is a farther distance (eg Mars, 1.52 AU) give lower power/mass ratio´s? Can you simply calculatee this by dividing by the quadrature of the distance in AU?

ERRORIST · 2004-05-04 07:55:22

The moon.

RobertDyck · 2004-05-04 12:27:00

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.
A bit off-topic, but does this only be the case for a distance of 1 AU from the sun? Is a farther distance (eg Mars, 1.52 AU) give lower power/mass ratio´s? Can you simply calculatee this by dividing by the quadrature of the distance in AU?

Yes, light intensity is proportional to the inverse square of the distance from the sun. Solar panels have a fixed percentage conversion rate for light of a given spectrum. Light at the surface of the Earth has a different spectrum than space because our atmosphere absorbs some. If light intensity gets too low the panels will not operate properly, but you have to get much farther than Mars to reach that.

So let's see, the Mean distance from the Sun to Mars is 1.5237 times as much as the Earth so power produced should be 1/(1.5237^2) = 43.07%. To make it more complicated, as a spacecraft travels from Earth to Mars it gets farther away from the Sun so light gradually goes from 100% to 43.07%. And the figures are quoted as beginning of life, radiation degrades solar cells so electricity produced slowly goes down. TJ cells have 25.1% light conversion efficiency at Beginning Of Life, 21% End Of Life.

Interesting, the web site for Spectrolab lists new Ultra Triple Junction (UTJ) cells, but not panels. UTJ cells provide 28% BOL and 24% EOL. It says UTJ cells have a 140µm Ge wafer thickness, but doesn't give total cell thickness or mass; how very vague.

Bill White · 2004-05-22 21:20:36

http://msnbc.msn.com/id/5025388/]Update on the Ascender

C M Edwards · 2004-05-24 10:33:55

I finally got curious enough about this scheme to crunch some numbers. I think JP Aerospace's Airship To Orbit plan will work.

First, you should know that, while I don't know about an airship 1.6km long, it should be possible to carry this off with vessels of smaller size (closer to the size of the largest balloons flown to date, using similar materials). The vehicles to do this can be built.

Second, drag is not a limiting factor. Don't get me wrong: it's a factor, but it's just a parameter, not a show stopper. The airships in question are so large that atmospheric drag quickly slows them to some terminal velocity no matter how much thrust they get. At 61km altitude, atmospheric drag is very small, even on a 1.6km airship, but so is the thrust of an ion engine. Shortly after starting its ion drive, the airship can expect to reach a terminal velocity of only a few meters per second. However, the ship also gets a small amount of additional dynamic lift, which starts it ascending (also at some upward terminal velocity). This ascent lifts the airship to atmospheric regions where the air density is lower, and thus the air drag is correspondingly less and the terminal velocity is correspondingly higher. The vehicle goes faster as it rises. It still feels drag and still has a terminal velocity; the terminal velocity just keeps increasing as it goes higher.

What's more fascinating, the vehicle will never quite lose all of its dynamic lift.

Drag and dynamic lift both vary linearly with air density and vary as the square of relative air velocity. They'll stay in roughly the same proportion. The air density varies exponentially with altitude, eventually diminishing to near zero. But the terminal velocity increases exponentially with altitude, just on a different curve. So the effect of diminishing air density never quite completely cancels out the effect of increased velocity on the dynamic lift.

Similarly, because the air drag keeps decreasing as the ship climbs, the air drag never quite balances the engine thrust. Outside of a certain range of parameters, the rate of increase for terminal velocity can eventually diminish below useable levels. (An airship can only stay airborne for so long, and the gradual velocity increase must be sufficient to accomplish the ascent to orbit in that time, or it's all for nothing.) However, the acceleration merely falls below useful levels; it never goes to zero.

The airship continues accelerating _and_ climbing the entire time its engine produces constant thrust.

Constant thrust is important. A jet engine using ambient atmosphere for thrust would suffer an exponential loss of thrust as the atmosphere thinned out. It would reach the point where it couldn't produce any useful thrust long before it had reached orbital speed. Any engine capable of flying an airship to orbit must have its own fuel supply.

Higher thrust is obviously better than lower thrust. An ion engine producing 10N thrust could bring a twenty ton airship to orbital speeds in a year. That's really pushing the envelope as far as reliability and flight time. I think engines capable of at least 100N would be necessary for something like this, but that demands a power supply, fuel and rocket engine that a vehicle of this type might not be able to carry, and would still require a month or more of ascent time. (The 5 days advertised by JP Aerospace is unrealistic.)

Also, it's important to note that the orbital velocity eventually attained is itself a terminal velocity. The vehicle is so large that it still feels substantial drag at that speed and altitude. If more thrust isn't used to ascend to a higher orbit, or the airship's envelope isn't cast off to reduce drag, air drag will eventually slow the vehicle back down and cause it to re-enter the atmosphere. The re-entry shouldn't be violent though. If the envelope holds together in the plasma of the upper atmosphere, it may be possible for an airship to rondevous with a smaller orbiting satellite and go right back down the way it came up.

It's slower than advertised, but it will still work.

Airships can fly to orbit. Who knew!

Bill White · 2004-05-24 11:09:54

Question:

The plan seems to use helium.

Safety would suggest multiple cells, pehaps hundreds of cells to prevent any one leak from being catastrophic. Why not use a combination of hydrogen and helium?

Hydrogen in an interior "bag" encapsulated in a helium outer bag. Or 90% helium and 10% hydrogen in the outer bag.

The helium will reduce fire risk and the hydrogen will be lighter.

C M Edwards · 2004-05-24 13:25:54

A helium:hydrogen ratio less than 10:1 does not reduce fire risk at any altitude. Even then, it only reduces risk for upper altitudes. Hydrogen will burn in air at 1 millibar partial pressure.

It's freaky.

RobS · 2004-05-24 15:42:46

The key to make this work is a power source, probably a laser or some other form of beamed power from the surface (or maybe from orbit) On the othre hand, if solar cells can be made light enough. there certainly will be plenty of surface area for them on the airship!

-- RobS

SBird · 2004-05-24 19:49:51

One big question is whether we have enough helium to fill such an airship. The total worldwide helium production capacity is estimated at 29 million cubic meters. These mile long airships would eat up a significant portion of our entire helium reserves which is functionally a non-renewable resource.

In other news, I was just reading some reports of lead selenide nanocrystal solar cells that have the capacity to boost solar cell efficiency up to 60%.

Mundaka · 2004-05-25 00:57:55

Rxke · 2004-05-25 01:56:42

and He is getting more expensive by the day. People in the industry are getting worried, it has so many uses...

C M Edwards · 2004-05-25 13:22:42

There really wouldn't be that much helium in such an airship. Sure, it's huge, but most of that volume is filled with gas at lower pressure than on the surface of Mars.

Hydrogen is not unusable for this application, and although it is flammable, would not be explosive or even burn particularly hot at the pressures involved.

Bill White · 2004-05-25 13:53:02

There really wouldn't be that much helium in such an airship. Sure, it's huge, but most of that volume is filled with gas at lower pressure than on the surface of Mars.
Hydrogen is not unusable for this application, and although it is flammable, would not be explosive or even burn particularly hot at the pressures involved.

Maybe a dumb question. Would you want multiple chambers, some of them closed off and "rolled up empty" for lack of a better term and only open those chambers as you gain altitude?

High altitude balloons - IIRC - are designed to expand and expand until they pop to allow the gas density to diminsh as altitude increases.

Perhaps an inner tube enclosed by an outer tube which remains closed off by a valve until a given altitude is reached.

Mark Friedenbach · 2004-05-25 14:05:20

they don't pop, but they have vents on the bottom to relieve excess pressure.

C M Edwards · 2004-05-26 09:28:25

Such an airship would have to consist of multiple cells, not just for structural strength but for control. Varying gas levels and pressures in individual cells can be used to control lift.

Technically, the airship would have to be a superpressure balloon, although it need only retain enough pressure to hold its shape - a few pascals of air pressure pressure, no more.

C M Edwards · 2004-05-27 08:20:30

I know of no reason that JP Aerospace's ATO vehicle can't be launched starting from a lower altitude than 61km. There's no reason this vehicle has to launch from the mesosphere. Upper atmosphere weather becomes amenable to ultra-large airships almost as soon as you get into the stratosphere. The vehicle just has to start at a stable altitude where it won't get frozen. (Atmospheric temperature reach their minimum close to the jet streams at the bottom of the stratosphere, then increase as you go up.) There are still wind currents in the stratosphere, but the ATO's lift is only determined by relative velocity, not windspeed. The ATO should have enough dynamic lift to ascend to the mesosphere under power whether it has to fight headwinds or not, then put on orbital velocity from there.

Starting from the stratosphere would not serve a military function, since it's still in range of the enemy's missiles. However, it would make an ATO into one heck of a heavy lift launch vehicle.

At 61 km, a vehicle as large as that proposed by JP Aerospace could only weigh a few dozen tons, including payload, fuel, and all its structure. That's all the atmosphere is dense enough to float at that altitude without dynamic lift. Dynamic lift is available all the way from the stratosphere, though. The same size vehicle, allowed to start from 25km, could weigh in at more than 1000 tons. And it would still fly.

If you want to launch battlestar galactica, this is the way to do it.

It takes more fuel to start from the stratosphere, and more life support supplies for the longer mission time. However, I think we might be able to find it somewhere in that 1000 tons.

Balloons have already been flown to 30km carrying more than 200 tons. Just starting from that conservative figure for vehicle mass, I think we could get 100 tons payload to orbit using this method. Using electric propulsion, (those acre-sized solar arrays on the ATO don't have to be abandoned in space), that's sufficient to launch a Mars Direct style mission.

We should definitely look into this. If it works, it would be more efficient and cheaper than anything flying today, using well understood technologies. It promises Zenit-scale launch costs for Energia-scale payloads. And the same type of vehicle will work at Mars, too, for both the ascent and descent. Our astronauts could soft-land in a blimp.

This could be the launch technology that allows us to colonize Mars!

Euler · 2004-05-27 21:02:24

I am still not convinced that this would work. As the airship rises due to dynamic lift, it would eventually reach a point where it is so high that the buoyancy from the balloon would be negligible, and the weight would be almost entirely supported by dynamic lift. That means that in order to keep accelerating and gaining altitude, the drag/lift ratio has to be smaller than the trust to weight ratio. Using ion engines, you can expect the craft to have a T/W ratio of 10^-4 or less. I don’t think even very efficient airplanes have drag/lift ratios that are that low, so I find the idea that an airship can do so highly dubious.

C M Edwards · 2004-05-28 11:46:29

The airship, before launch, floats at some equilibrium altitude. To compute it's thrust to weight ratio, it's important to note that the parameter of interest is the net weight, not just the displacement. (Airplanes don't float, and thus have no equilibrium altitude.) Before launch, the net weight is zero, and the thrust to weight ratio is effectively infinity. It's not hard to get smaller than that, and any little change in lift or weight will raise the airship (ion engine thrust, thermals, a dropped screw, etc.).

The drag is very small as well -- nearly equal to the thrust, just a tiny amount less. So the lift doesn't need to be stupendous, either. The climb rate for a vehicle of this type would start out at about 2 inches per minute. All that's required is that the tiny amount of lift developed stays greater than the net weight.

The net weight does increase as the vehicle climbs above its equilibrium altitude, but it does not jump from zero to the full displacement at the moment motion starts. The lift increases with altitude as well (as a result of greater speed), and does not decrease as long as the speed does not decrease and the lifting body holds its shape. Acceleration does decrease as the net weight increases, but never quite goes to zero. In fact, the ascent speeds up again during the last leg of the flight because the net weight begins decreasing back to zero again as the vehicle enters the microgravity conditions of near orbital velocity.

Theoretically, an airplane could do the same thing, if it could withstand the correspondingly higher initial velocities it needs to reach the same altitude. An airship can start with zero velocity, and needn't worry about how many times the speed of sound it's travelling until it's actually ready to light its engines.

The thrust:weight and lift:drag ratios can be favorably maintained all the way up. This thing can fly.

Now, who's interested in a 1000T to orbit heavy lift vehicle?

Bill White · 2004-05-28 12:06:41

Now, who's interested in a 1000T to orbit heavy lift vehicle?

How much money are we talking about for a proof of concept vessel? Not 1000T but maybe just 10kg to LEO?

A balloon and an ion engine and power for the engine and batteries to keep the engine running at nighttime, right?

Are lightweight solar panels and lightweight batteries the current show-stopper?

= = =

Edit: See this http://www.abovetopsecret.com/forum/thr … discussion page: one co,mment suggests an Ascender might cost 1/40th of a Global Hawk.

= = =

2nd edit: See this http://minerals.er.usgs.gov/minerals/pu … 3.pdf]link concernign US helium supplies.

An Ascender needs 5.2 million cubic feet (IIRC based on web articles) and it appears in FY2002 228 million cubic feet of helium was transferred.

Dark Sky station will need more than 5.2 million cubic feet, much more.

Rxke · 2004-05-28 15:20:01

Some posts from one of the people behind the project on Slashdot http://slashdot.org/~Alfred%20Differ]here

Interesting is he says ion-engines would only be their third choice, but it's apparently classified?

Bill White · 2004-05-28 15:39:34

I enjoyed this http://slashdot.org/comments.pl?sid=108 … 27]comment.

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

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

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

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

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

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

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

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

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

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

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