New Mars Forums

Response to CM;
You said;

?The thrust provided by a rocket engine is F = m? v.e, where m? is the rocket?s reaction mass (in kg/s) and v.e is its exhaust velocity.  So, to increase the thrust of a rocket engine, you can either increase its reaction mass or increase its exhaust velocity.  However, the kinetic power of the rocket exhaust is P = ? m? v.e^2.  This means P/F = ? v.e.  So, if a rocket has twice the ISP of another rocket engine using the same reaction mass, that rocket has half the thrust or less.  Thus, an  advanced propulsion system, such as NERVA, that expects to operate at double the ISP of conventional chemical systems but still produce the same thrust must either double its rate of propellant use or double the rate at which it can put thermal power into the propellant.?

It seems to me, just using F=m?v.e, that if you double the Isp with the same mass-flow rate, you double the thrust, since F is thrust, not P/F. The interpretation of P/F = 1/2 v.e can be better understood as P= 1/2 Fv.e, which according to my Sutton means the POWER TRANSMITTED TO THE VEHICLE (Sutton actually uses P = Fv.e, dropping the 1/2, which we argued about extensively in class years ago). The Power transmitted to the vehicle is actually a function of the square of the exhaust velocity, which makes sense, since you?re increasing the kinetic energy of the vehicle with respect to Earth.

Response to Canth; While orbital speed is somewhere in the neighborhood of 7.5 kps, total delta-V for a launch is about 9.1 kps to get there. This is due to two factors; atmospheric drag,
and the fact you have to lift your vehicle vertically out of the atmosphere, which doesn't contribute at all to your orbital vector. Thanks to mass ratio, you're using far more fuel-mass in that initial 2 miles than you use in the next 4 miles (too lazy to do calcs right now), since you are lifting all that propellant. I'm guessing that 300 mph at 2 miles altitude contributes quite a lot more than 1%, probably closer to 10%.
Tom

Oops. I see Poppa already suggested the first option. Well, I'm with him, then.
Tom

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...

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.

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.

Gravity Probe-B is supposed to launch this year to test the gravito-magnetic effect. I work with Lockheed on the Payload Transportation System, so we get to haul GPB around from building to building for various tests. Pretty small satellite, but the technology on that thing is just awesome. I think they also call the gravito-magnetic effect the "Lense-Thirring frame-dragging effect", but I might be confusing two different GR predictions.
Tom

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