Solar Thermal Ground to Orbit - Solar Thermal Tech to launch.

Tom Jolly · 2002-05-05 11:13:34

I'm an advocate of Solar Thermal technology to get to orbit. Not that it will necessarily work, but I believe it might be engineerable. Very briefly, the idea is that you use solar energy to heat up a tube-array that's being fed atmospheric gas. The solar collector is made using a big reflective mylar balloon. The design I've been toying with is a long tube-shaped hot-air balloon, parabolic reflective mylar on the bottom, clear mylar on the top. The thermal transfer tube (that you are heating with the reflected solar) runs down the middle of the balloon, and has an intake scoop on one end and a thrust nozzle on the other. It floats, since technically it IS a hot air balloon.

The negatives are; its top speed is limited by the scoop drag (kinetic) versus the thrust (est. max temp at 2000 deg. C, considering material limits) to about 1.3 kps. This isn't bad, but it isn't orbital velocity. One obvious partial solution is to take a tank of water along and feed it into the thermal tube. I haven't run the mass/thrust calcs on it yet, so I don't know what that will do for me. You also can only launch in the daytime, and have to keep your parabolic reflector pointed at the sun. With a tiltable half-tube array, this actually isn't that difficult.

The positives are; it's incredibly cheap; I'm working on the prototype in my garage. You don't use any fuel for the first 1.3kps, and by then you are on the high edge of the atmosphere. The fuel, if you take any water along, is fairly easy to find (if you have a hose). Thanks to the lift/drag ratio, (assuming you have some lifting surfaces) you don't have to sweat burning up on reentry, especially since you're using atmosphere as propellant and can land pretty much anywhere you want.

I may be nuts to pursue this, but it irks me to look at all the X-Prize entries and see that ALL of them are chemical rockets. I'd like to see a design that breaks the chemical paradigm.

Any comments are welcome; I've got a web page that goes into more detail at My Webpage

Any comments or suggestions are quite welcome. Improvements on the design concept are welcome, too.

Tom Jolly

Phobos · 2002-05-05 13:08:26

Have you actually built any models to test your hypothesis? It's a creative concept, and if it does work, would be not only cheap but environmentally friendly. If you get the chance you should post some drawings on your website.

Canth · 2002-05-05 13:30:14

There are some big problems with your design. Two of them are that your balloon is going to poduce less lift and thrust as you get higher and without lots of extra fuel will produce no thrust and regardless of fuel no lift. You have to make quite a high speed to acheive the hights your talking about and your spacecraft will produce thrust only for the first few miles in addition your ballon's mass to drag ratio will be enormus. Starting higher (as a baloon lets you) decreases the possible thrust of an air breathing system by allowing it less time in thicker air unless the lift is high and allows it to gain a lot of speed from it. In short it will need a lot of kinks worked out and a lot of redisign (I can think of a lot of other problems) but some vaugly similar design may one day fly, although I doubt to space but at least in the atmosphere. If you think you can make it work, persue it.

Tom Jolly · 2002-05-05 16:18:53

No, I haven't built any models yet. Done some tube brazing, and done some experimenting with methods to seal mylar seams, and have some bits on order, but no model yet. It takes a lot of time.

To answer the other questions; Less lift and thrust; this isn't exactly true; the lift at the elevation I expect to reach ceases to be aerodynamic lift and becomes kinetic-gas lift. The atmospheric gas throughput will remain fairly constant throughout; at 1 KPS, you can scoop gas that's a tiny fraction of the pressure that you need at sea-level. If you need 10 m/s at sea level to get lift to stay up, then you can deal with gas 1/100th as thick to stay up at 1KPS, somewhere around 30 kilometers. Drag, of course, increases as a function of the square of the velocity, so that's your limiting factor, but your air-scoop covers the front of the tube design, and surface drag isn't quite the concern at these elevations. Yes, there are a lot of kinks to work out, but I think that's mostly because no one has worked on something like this before.

Josh Cryer · 2002-05-05 19:12:14

How does the thermal transfer tube work?

Sorry for my utter ignorance on the subject.

Shaun Barrett · 2002-05-05 20:03:39

Hi Tom!
I love your idea, though I guess it will be some time before all the potential glitches get ironed out!
At the risk of distracting you from your work with questions when you should be toiling in your garage, I am curious about something: Since this is to be, essentially, a spacecraft, and since we're hoping it will actually achieve orbit one day, you must presumably be planning to incorporate a pressurised passenger cell(? ). And you will be needing some kind of thruster arrangement for changing orientation in space, and the usual computer guidance and communication equipment.
What concerns me is the weight of all this stuff at launch. How big will the "hot air balloon" have to be to achieve lift-off at the beginning of the mission? It seems a worrying likelihood that your tubular craft will have to be huge and will therefore suffer from an excessively large cross-sectional area; thus leading to high air-resistance and reduced velocity.
I don't know. I'm probably wrong. Just a thought.
In any event, I wish you well and look forward to news of your progress.

Tom Jolly · 2002-05-05 22:05:39

Josh; the thermal transfer tube is just a black tube that gets really hot when you zap it with your parabolic reflector, which runs the length of the tube. Air collected by your scoop passes through the center of the tube and gets hot, then gets ejected out your nozzle. The trick, of course, is to make sure the overall kinetic energy of the gas you're spewing is higher than the kinetic energy of the gas hitting your scoop.

Shaun; I'm speculating on a small pod hanging below the balloon-tube, and maybe a tank or two of water; possibly not more than a thousand pounds with a man inside. You don't actually need much; consider the size of the Gemini capsule, then take away the heat shielding requirements, and add in the much-smaller radio technology we use now. I'm still tossing around some ideas for manueverability in space; you have all this solar energy and water, putting on attitude control nozzles shouldn't be a big deal. I'm also questioning how much needs to be computer controlled and how the design could support seat-of-the-pants flying. It's not like you need a particular entry or exit trajectory with this beast. Also, added weight means more heat due to the lift/drag ratio, since I'd be riding lower in the atmosphere, so it's imperative to keep the payload mass as small as possible.

I figure the thing would have to be nearly the size of a Goodyear Blimp with respect to volume, but much narrower and longer, so as to reduce the nose/scoop area. Buoyancy is not as critical, though, since I expect some lifting surfaces in the design, eventually. It just may have to go a few meters per sec to fly. Your points are well taken, thanks for the input.

Canth · 2002-05-06 16:03:36

You would need some sort of heavy duty heat resistant plasic. Even with such a large volume and low weight it is going to be going hellishly fast when it starts reaching thicker parts of the atmosphere. I haven't done the math due to lazieness so I might be wrong, but before you assume your craft won't burn up and will generate enough thrust I would run the numbers with real atmospheric densities and stuff. There are some really heat resistent plasics out there and you probobly won't generate enough heat to really get destructive. Another option would be to have a giant ultra light plastic parachute to help you slow down before you hit the thicker air.

Tom Jolly · 2002-05-06 22:04:23

The whole idea is that if you are running into atmosphere with enough kinetic energy to heat you up, then that same kinetic energy will be providing you lift and raise you up to a more tenuous part of the atmosphere where your plastic isn't melting. Since I haven't run numbers on this, I can't give you any kind of realistic figures, but if your lift/drag = 1.0, and the balloon-rocket needed, say, only 1KW for lift, then it will never have to dissipate more than 1 KW. It's just a matter of figuring out the max heat I can take, then deriving my minimum lift-to-drag from there.
Or vice-versa!

C M Edwards · 2002-05-07 14:17:39

Hello Tom.

I've visited your site, and I have good news and bad news about your idea.

The good news is that your description of your thermal engine violates conservation of energy. You give too much significance to the kinetic energy of your working fluid (the intake air) and neglect the fact that the energy from your heat source (the sun) is added to it. What that boils down to is, no matter how hot the intake temperature is, the operating temperature will be hotter because of the solar heat. The temperature gradient inside the engine will always point in the same direction, regardless of how fast it moves through the air. The limiting factor will be pressure, not temperature. The engine would just have to keep the air flowing from the high pressure zone (everything in front of it) to the low pressure zone (everything in back of it). This is good news, because it means that you could squeeze better performance out of the engine itself than you seem to be expecting.

The bad news is, it still wouldn't work. Drag gets the balroc below the cloud deck and thin air is waiting to finish the job in the stratosphere.

Basically, the math works out this way: The engine thrust is approximately linear with respect to velocity -- the faster you go, the more thrust (in theory) you can get. But the aerodynamic drag increases approximately as the square of the velocity, so it starts out less that the thrust but quickly increases to equal it, slowing the balroc to some terminal velocity. Now, you can go higher to thinner air, where there is less drag for the same amount of speed. But the air density falls off exponentially, which means that the density where the balroc's thrust is cut in half comes before the density where the drag is decreased to the same force.

The balroc would have a terminal velocity, probably much closer to 13m/s than 1300m/s, and going higher would reduce, not improve, its performance.

However, some interesting notes:

Blimps can be made of sheets of aluminum, steel, and other refractory substances. The US Navy's ZT series used this technology. So one can do a lot better than the 300K or so available with mylar/polyesters.

A big metal blimp with transparent panels would make one heck of a good solar collector.

The potential energy of position at just 20kmhigh is equal to the kinetic energy of 600m/s, and balloons will float at that altitude with large, rocket-sized, payloads. In addition, depending on design, being above that much atmosphere could save as much as another 600m/s worth of energy that would otherwise be expended overcoming drag. 20km high is definitely the place to launch from if you can reach it, especially if your vehicle has a lot of drag, like say, a big metal blimp.

Hmm...

CME

Bill White · 2002-05-07 15:18:26

Even if you cannot reach LEO, or go nearly as fast as one might hope, might this idea be useful as a fuel efficient way to accomplish more mundane flight?

Could you fill the "insides" with helium and use the solar powered thruster for forward propulsion?

As for construction, if you stick with Mylar, perhaps a sailmaker could sew a prototype for you, and perhaps help you acquire various weights of Mylar cloth. Recreational sailing has gone very high tech in the past few decades and Mylar is now well below the cutting edge in sail making technology.

A sailmaker should also have a CAD system for designing a number of irregular shaped pieces to be sewn together to form the proper curvatures - a simple ellipse would seem to be be child's play - and laser cutting tools to cut the Mylar fabric into all the right sized pieces.

During a slow spell (off season) an enthused sailmaker might do all this at cost, or below cost.

If you are determined to do it yourself, perhaps buy or borrow a book on modern sailmaking to help you cut and sew the mylar sheets for your prototype and go aluminum with the full sized version.

Final note: "Sew" is an old fashioned term since sophisticated expoxy is often used instead of thread - however - given the potential high temperatures, old fashioned sewing may not be a bad idea either.

C M Edwards · 2002-05-08 12:09:03

It's possible to get a ballpark estimate for how large a solar collector would need to be to provide power to a rocket/jet/spacecraft. The craft needs to accelerate at a certain rate, and it takes power to accelerate. The solar collector needs to provide more power than that.

To accelerate at 1G requires P=F^2/(2m) power, where P is power output, F is net force (specifically, the change in momentum), and m is the rocket mass. That translates to 48W/kg to accelerate at this rate.

The solar radiation constant is about 2kW/m^2. So, a 1m^2 collector should provide enough power to allow 41kg to hover in 1G, assuming 100% efficiency. 100% efficiency will never happen with a solar thermal rocket. Ever. 10% overall efficiency for a solar rocket is generous, though reasonable, but would allow only 4kg to accelerate at 1G. Increasing that to 2G's decreases the allowable mass to just 2kg/m^2.

A blimp-collector with a collector cross-section of 500m^2 (20m diameter, >50m length, with the collector being half the lift cell) could conceivably be used to heft a gross "lift-off" weight of 1000kg.

How much could that get to orbit? Well, a solar thermal rocket running on pure hydrogen can reach exhaust velocities as high as 3000m/s at just 60C in near vacuum. This means that a solar thermal rocket is capable of reasonable performance _even if entirely constructed of coke bottle grade polyethylene_. Using kevlar can raise the operating temperature as high as 200C, allowing exhaust velocities of 4000m/s. (For comparison, the space shuttle main engines operate between 4000m/s and 4500m/s using exotic ceramics.) Using actual metal in its construction increases still further the temperature at which its engine can operate.

To reach orbit with such a craft, it would be necessary to attain an operating temperature where the exhaust velocity of hydrogen was at least equal to orbital velocity. That happens at 1600C, which can be withstood by titanium, stainless steel, etc. Getting hydrogen that hot is problematic, though, and heating enough of it to power a rocket is even more so.

Hmm...

CME

PS: A useful formula for estimating exhaust velocity in vacuum is v = 250 * SQRT( T / W ) +/-10%, where v is the estimated exhaust velocity in m/s, T is the operating temperature in degrees kelvin, and W is the mean molecular weight of the exhaust. Note that the error bar is fairly large for this calculation, because pressure, specific heats, etc. are neglected. Note also that the estimate I gave for performance are on the conservative end of +/-10%. In practice, actual performance can be pushed to higher levels.

Canth · 2002-05-08 21:03:53

You could concievably design an aerodynamic ultra thin solar collecter made of mylar which at certain times of day could focus light on a solar thermal jet engine (either tubo pump or pulse). This engine could then go into ramjet mode when enough speed was attained and rocket mode (using a carried fuel such as water) upon leaving the usable atmosphere behind. Such a structure would be hard to balance and need to be very aerodynamic but as far as I can tell it should work. I am not sure of its merits as a reusable vehicle but it could possibly be made cheaply enough to not need to be reusable. It would be hard to make it an X-prize contender.

C M Edwards · 2002-05-09 07:00:26

I would expect that building a solar collector into the lift cell would demand a fixed volume for the system. That limits the altitude to which the rocket alone can rise. It also creates a need to shield the engine from the sun when you don't want it to run.

If it's light enough, it can be floated to around 30km or so by a separate balloon "booster", which can do double duty as a shroud for the collector.

I've found another difficulty, though. The specific heat of hydrogen is fairly high, meaning that a rather large collector would be required to heat it to even modest operating temperatures.

CME

Tom Jolly · 2002-05-09 22:30:44

Addressing issues in somewhat reverse order;
Canth;
I thought about the ramjet/scramjet option, but it does require some fairly high air pressure which would translate to your outer frame, which means the balloon would have to be really tough stuff, that is, metal. Plus, keep in mind the whole object here is to do this really cheaply. Plus, a ramjet implies that you want to combust something. The rocket mode, (with water) however, is quite reasonable, and I?d certainly do that to go beyond the limits set by the thrust/drag equation.

Bill White; Yes, professional balloon makers or sailmakers are certainly an option. Cameron Balloons in England might be handy to use, but then, the Mylar I?m dealing with isn?t fabric, it?s just a very flat 2 mil reflective sheet. I wouldn?t be able to get any decent reflectivity off fabric Mylar (unless you know of some type that?s mirror-smooth that I haven?t heard of...certainly a possibility). Is there such a thing?

CM Edwards;
First off, your Solar Incidence is a bit high; in space (near Earth) the solar is only 1400W/m^2, and on Earth it?s closer to 800W/m^2. Not that this helps me...I would prefer your 2000.

You state that 1G acceleration (required to support a vertically lifting object) requires 48W/kg. I have some issues with the formula for it. If you check the units (kg, m, s, and such), they don?t work out. P = F^2/(2*m). The actual formula is derived from Nm = Joules = Ws, which gives you Newton meters divided by seconds for your total Watts, treating the ?meters? as though there is no gravity present, which for 1 second gives you 9.8N*9.8m / 1s = 96.04W per kilogram. Of course, this too is worse for me.

I wouldn?t launch vertically anyway, so the idea of hovering and using up 100W per kg is almost irrelevant until I?m very, very high up. If we pretend that my balloon is big enough to give me positive buoyancy, then any thrust at all (treating the balloon like a jet plane) is going to propel me forward, and any forward motion will provide me lift, assuming I have some lift surfaces. A flat surface with aspect ratio 2, angle of attack 6 degrees, will give me a lift to drag ratio of 6, which is pretty darned nice. Thus, if I have two lift surfaces of X drag, and a scoop area of 4X, I could still pull an L/D of 1/1. (these L/D numbers are pulled from an old aerodynamics book). Incidently, I compared sea-level aerodynamic drag to kinetic drag for super-tenuous gas, and found out that aerodynamic drag is about 3 times higher, presumably due to the higher boundary-layer interaction.

The very low estimate of blimp speed needs to be bumped up quite a bit; Sanyo has a blimp that does 65 mph, or roughly 30 meters per second at sea level. Of course, it has two 180 horsepower engines. What fraction of that is delivered to the propellers is anyone?s guess. In an atmosphere 1/10 as thick, you should certainly be able to go a heck of a lot faster. While the density is 1/10, the higher velocity means you?re encountering more gas, so the drag is higher both due to the mass encountered AND the velocity squared. Based on this, the new max velocity, V2 is equal to V1 time the cube root of 10, or, 64 meters per sec. Likewise, at 1/1000th the desity, the limit due to drag (assuming a 30m/s limit at sea level) would be 300m/s.

Anyway, my blimp concept isn?t really blimp-shaped, so it won?t share blimp drag characteristics. I?d like a long, thin, continuous diameter tube, with a scoop about the same size as the tube diameter. At low elevation, I?ll accelerate until thrust equals drag. I would select the L/D >1, by a bit. So, I set the maximum drag so it always gives me more lift than the weight of the balloon rocket. This implies that my thrust will have to be equal to L + D to give me any lift at extremely high elevations (hadn?t considered this much before now...this will obviously slow me down a lot). At low elevations, I?m getting help from the fact that the balloon is acting like a balloon. As long as thrust is > (L + D - Buoyancy), then I?ll keep rising, and as long as I?m rising, the air gets thinner, D drops, and my tube throughput drops, bringing me back down again. However, I always intended to operate the scoop in ?overflow mode?, that is, back pressure limits the input rate and throughput. Even as the air thins out, my throughput will remain fairly constant. Up to a point. This, too, lowers my top speed somewhat, since my mass in doesn?t equal mass out.

Since you forced me to rationlize a few of the concepts I?m considering, I can see that I will be running slower, but I still have the option of dumping water into the tube and going into rocket mode at a high elevation and bypassing the entire drag limit problem, since I?d just go straight up (more or less). Whether I could get enough delta-v to go orbital, I?ll leave to the next set of calculations. I just want to get this built so I can see if it works *at all*. Thanks anyway for clearing up my thinking process for me, though.

I know I haven't resolved all the questions presented...

C M Edwards · 2002-05-10 07:28:51

Hello Tom.

I apologize for the error in the Power formula. The actual formula is P = F^2/2m', where m' is actually the rocket's reaction mass (kg/s), not its mass (kg). The idea is that the power of the exhaust is equal to the power added to the rocket, but I did not keep track of which was which.

I guess that means that I, too, have violated conservation of energy.

CME

C M Edwards · 2002-05-10 07:29:34

Hello Tom.

I apologize for the error in the Power formula. The actual formula is P = F^2/2m', where m' is actually the rocket's reaction mass (kg/s), not its mass (kg). The idea is that the power of the exhaust is equal to the power added to the rocket, but I did not keep track of which was which.

I guess that means that I, too, have violated conservation of energy.

CME

C.COMMARMOND (FR) · 2002-08-27 06:08:31

Note: my english is bad...

Hi,

your project seems very interresting. I think you are creating a new way to fly, but if i don't think you will go in space this way, i think that it could be a good first stage for a rocket.

I'll try to explain my view of the thing:
If you don't do a tube but a kind of 'flat tube' where the shape is like a plane wing, you will get a vertical force with the speed to lift it, if the lower part is black to heat the inner gas (increase pressure so maintain the shape with less gas, release the 'in excess gas' and make the all thing lighter: 1 liter air = 2,7 grams) , the side are mirrors (to concentrate light to a central pipe) and the top is clear, you could create a pipe in the middle of the wing to guide the light to your rocket(thruster?) (like in optical fibers) and then concentrate a lot of power on a small surface and heat something to exhaust (it is good to concentrate all the light on a small surface to get a high temperature and get a high exhaust speed because of e=mv^2 ). I think CME could get more than me from this design. To fly, this wing should make long s trajectory so that the mirrors always send the light to the central pipe and then to the rocket(thruster?) and there, you can put water to create thrust. If you can get a small compressor on board, what about condensing water from the air which goes through the wing?

Sorry that my english is too poor to express what i think, i hope this can help?

Continue on your work and good luck...

Christian COMMARMOND

Tom Jolly · 2002-10-17 16:06:58

Hi, Christian. I like the idea of a flying wing with the thermal tube running down the center of the wing, perhaps with forward-sweeping wings with the thrusters at the tips. I think such a design would be very stable aerodynamically. The downside of such a design would be higher drag, but the + side would be an outrageously high lift/drag ratio. So, this design variation might work pretty nicely. I'll have to run the numbers on this, too, to see if it will work better than my existing design.

Terraformer · 2012-05-24 08:42:43

We had a lot more on this before the Great Crash...

According to Atomic Rockets, Solar Moth would get 900s, 4000N @ 100kg. If we use Ammonia, we could perhaps get higher thrust for lower Isp, and using an oxygen after burner, we could increase the thrust further, though again at the expense of Isp.

One of it's main attractions for me is the possibility of using a storable propellent such as Ammonia while still achieving high Isp, as well as obiviating the need for a heat shield. I think we need to take a good look at the varius components of rockets and see how we can minimise their masses or even do away with them entirely.

RGClark · 2012-05-27 01:52:15

Terraformer wrote:

We had a lot more on this before the Great Crash...
According to Atomic Rockets, Solar Moth would get 900s, 4000N @ 100kg. If we use Ammonia, we could perhaps get higher thrust for lower Isp, and using an oxygen after burner, we could increase the thrust further, though again at the expense of Isp.
One of it's main attractions for me is the possibility of using a storable propellent such as Ammonia while still achieving high Isp, as well as obiviating the need for a heat shield. I think we need to take a good look at the varius components of rockets and see how we can minimise their masses or even do away with them entirely.

Do you have a link for that, Terraformer?
BTW, per your sig file, Sierra Nevada is reviving the HL-20 via their Dream Chaser spacecraft.

Bob Clark

GW Johnson · 2012-05-27 09:56:46

I suppose some sort of solar-thermal airplane could be built. NASA and others have already flown solar-photovoltaic aircraft, including that big one at about 70,000 feet. But, these have all been very slow, and very lightweight/flimsy things. None has been capable of significant payload.

That's not to say it couldn't get better over decades: compare a WW1 biplane observation plane with today's jet transports. It did take about a century of concerted effort to grow that capability.

If I was to do a solar-thermal engine for a slow craft, I'd be looking at solar-heated piston and turbine engines, all for driving airscrew propellers. Even after a century, the propeller is best suited for slow flight. The solar PV designs all do that, too.

As for ramjet, that's very high speed flight. Even the pitot-inlet stovepipe engines had to be moving just about transonic to do much good, and they'd peak out about Mach 2 or so. Below about 300 mph, engine Isp was lower than a plain old solid rocket (180-250 sec). You could get thrust > drag, but it just wasn't worth it.

The better ramjets are supersonic inlet types, with minimum takeover Mach numbers in the 1.5 to 2 range. That takes a big booster just to get moving that fast. These types of ramjet engines can peak out in the Mach 4 to 6 range. Airframes capable of these speeds are quite different from the slow fliers that solar power currently seems able to push.

GW

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