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#1 2014-02-04 17:18:57
- eliaslor
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- Registered: 2014-02-01
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Solar Thermal for Ground Launch
Hi, I read a Wikipedia article that suggests that there is technology being produced that will allow small rocket ships to be launched from land using solar power and an ammonia propellant. Does anyone here have an opinion on the likelihood of this technology being produced and used in the near future (the next 100 years)? Any help would be appreciated. Here is the excerpt from Wikipedia:
Solar thermal rockets have been proposed [7][full citation needed] as a system for launching a small personal spacecraft into orbit. The design is based on a high altitude airship which uses its envelope to focus sunlight onto a tube. The propellant, which would likely be ammonia, is then fed through to produce thrust. Possible design flaws include whether the engine could produce enough thrust to overcome drag, and whether the skin of the airship wouldn't fail at hypersonic velocities. This has many similarities to the orbital airship proposed by JP Aerospace.
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#2 2014-02-05 03:43:47
- Terraformer
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#3 2014-02-05 12:18:25
- GW Johnson
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Re: Solar Thermal for Ground Launch
Frontal drag will always be an insoluble problem for supersonic/hypersonic blimp ideas, no matter what kind of rocket you try to push it with.
GW
GW Johnson
McGregor, Texas
"There is nothing as expensive as a dead crew, especially one dead from a bad management decision"
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#4 2014-03-09 15:44:32
Re: Solar Thermal for Ground Launch
Frontal drag will always be an insoluble problem for supersonic/hypersonic blimp ideas, no matter what kind of rocket you try to push it with.
GW
Drag will be a problem for solar thermal launch from the ground. But I don't rule it entirely. For instance it doesn't have to be a blimp. It could be like, say, a large supersonic aircraft such as the XB-70 where the focusing lenses or mirrors cover the entire wing.
Bob Clark
Old Space rule of acquisition (with a nod to Star Trek - the Next Generation):
“Anything worth doing is worth doing for a billion dollars.”
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#5 2014-03-09 17:42:43
Re: Solar Thermal for Ground Launch
Hi Eliaslor! Welcome to NewMars!
It's certainly a very attractive idea, and solar thermal propulsion is not without it's uses. However, there are two significant issues with Solar Thermal for ground launch, as I see it.
The first of these is power usage. Let's say we're using a lifting body design (Highly non-trivial!) on a 9.5 km/s trajectory to LEO. Let's say your craft has a Lift to Drag ratio of 4 (not optimized for any particular speed, this is a very good one). Let's say you want 10 tonnes of payload and your structural mass fraction is equal to 15% of the fuel mass. Let's say you get an Isp of 875 s.
The mass ratio will be 3.0. Gross Liftoff Weight will be 45 tonnes, and fuel mass will be 30 tonnes. If the Thrust-to-weight ratio of the rocket is .3 [This is very much a lowball value, even given the optimistic lift to drag ratio I assigned to this craft], it will be necessary to produce 126 kN of thrust. At 875 s, this necessitates the use of 14.7 kg/s of fuel. Assuming an engine efficiency of 70% (High), this will necessitate an engine power of 154 GW.
Assuming an insolation of 1000 W/m^2 (very high for Earth's surface) this will necessitate 154 square kilometers of concentrating mirrors. This is about three times larger than the area of Manhattan, and almost as large as all of the city of Toronto.
And this is only for 10 tonnes of payload. Still think it's a good idea?
Pointing is another issue. Parabolic concentrators need the sun to be directly over head at all times in order to function properly. This is pretty hard to do when you're moving at a speed of multiple kilometers per second.
So, this one's a no. But stick around and I'm sure you'll be coming up with good ideas left and right 
-Josh
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#6 2014-03-09 20:45:18
- SpaceNut
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Re: Solar Thermal for Ground Launch
Ammonia propellant exhaust dissociates into hydrogen and nitrogen sounds like elctrolysis... but the combustion of ammonia in air is very difficult in the absence of a catalyst (such as platinum gauze), as the temperature of the flame is usually lower than the ignition temperature of the ammonia-air mixture. The flammable range of ammonia in air is 16–25%. The combustion of ammonia to nitrogen and water is exothermic: 4 NH3 + 3 O2 → 2 N2 + 6 H2O (g) (ΔH°r = −1267.20 kJ/mol)
http://www.astronautix.com/props/loxmonia.htm
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#7 2014-03-10 16:20:50
- GW Johnson
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Re: Solar Thermal for Ground Launch
Solar, whether thermal or PV, tends to be a "low density" and rather "slow" energy source, here on Earth. It's worse yet further out from the sun. Dunno, might be better at Mercury. 10 times higher light flux there.
What about using solar as a way to derive and store the captured energy in a different form? A form that can be used very rapidly. One example is solar electrolysis of water into LOX and LH2. Liquify them, requiring more energy yet, which in principle could also be solar-derived. Then burn those propellants "suddenly" in a rocket engine, to go where you want to go. That way your vehicle need not carry around all the gear required to make its energy source.
GW
GW Johnson
McGregor, Texas
"There is nothing as expensive as a dead crew, especially one dead from a bad management decision"
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