Subject: Space-tech Digest #141 Contents: Inflatable space structures (7 msgs) Cryogenic cooling (1 msg) P2-E booster discussion (6 msgs) Beanstalk mathematics question (1 msg) ------------------------------------------------------------ From: walter@ee.ubc.ca (Walter Wohlmuth) Subject: inflatable space structures To: space-tech@cs.cmu.edu Date: Sun, 6 Dec 92 11:08:42 PST I just read an article in the Dec. 1992 edition of IEEE potentials regarding inflatable space structures. There seemed to have been a lot of research into vechiles of this type in the early days of the space program and also a bit of research into inflatable antennas currently. Surprisingly, i haven't found any info on a space structure that is inflated in space as opposed to on earth or in a near earth orbit. So here are some of my ideas and thoughts. There appears to be many advantages to exploring this area more fully in the near future: 1. The structure can take on the shape of a dumbbell, the outer portions could be inflatable sections which the inner would supply gases to the inflated sections. The inner section would be exposed to the vacuum of space maintaining it at a low temp. A control system could regulate the amount of gas injected into the inflated sections which would take into account of radiative heating by the sun. An exhaust port which expounds the gases could be used for movement in space. Potential for probes to Venus & Mars. 2. The outer shell of the inflated sections could use diamond like films and mylar. These materials are able to withstand high temperatures, potential for probes into the sun. 3. The inflated sections can be expanded to cover a cross-section of possibly a few thousand meters. Can be used to destroy meteors or comets which pose a threat to the earth. The multilayered skin would absorb most of the fragments. The costs of this structure are not too great in comparison to present probes and vechiles. Size and weight requirements are low. Due to the small amount of gas needed to pressurize such a vechile, solar radiation can be used to some extent as a power source. Probs.: microcracking of the skin upon inflation, quality and uniformity of the skin, fragile. These problems may be significant, but shouldn't be insurmountable. Any comments, including flames, are welcomed. Walter Anthony Wohlmuth walter@ee.ubc.ca Tri-University meson Facility Vancouver, Canada microelectronics lab ------------------------------ From: henry@zoo.toronto.edu Date: Sun, 6 Dec 92 18:17:55 EST Subject: Re: inflatable space structures To: space-tech@cs.cmu.edu > ... The inner section would be exposed to the > vacuum of space maintaining it at a low temp... Just exposure to vacuum won't keep the temperature low; it has to have a direct view of black sky (and no direct view of the Sun) to stay cold. >3. The inflated sections can be expanded to cover a cross-section of > possibly a few thousand meters. Can be used to destroy meteors > or comets which pose a threat to the earth... To destroy them by collision you need substantial mass, I'm afraid. A collision with a balloon will vaporize a small amount of the leading surface and leave the remainder of the incoming object pretty much untouched, unless it's so fragile that this causes it to break up. >Probs.: microcracking of the skin upon inflation, quality and uniformity > of the skin, fragile. Puncturing by micrometeorites strikes me as the big problem, actually. Objects with large surface areas don't survive unpunctured for very long. Henry Spencer at U of Toronto Zoology henry@zoo.toronto.edu utzoo!henry ------------------------------ To: space-tech@cs.cmu.edu Subject: Inflatable structures From: can2can@ziggys.cts.com (Tim Edwards) Date: Mon, 07 Dec 92 07:23:48 PST Just a thought, I remember reading about how they made the body of the 'looks like a Cord' car. A material called Royalite, by Uniroyal. There was a leathery plastic on a roll, that was heated and vacuformed over a mold. The result cured into a foam center rigid skin for the car. Can we come up with a 'skin' that will react upon inflation to thicken into a self supporting structure without requiring continued presurization? can2can@ziggys.cts.com - BBS (619)262-6384 Ziggy's Den Of Iniquity ------------------------------ Date: Wed, 23 Dec 92 11:08 PST From: trost@cse.ogi.edu (Bill Trost) Subject: Re: inflatable space structures to: space-tech@cs.cmu.edu henry@zoo.toronto.edu writes: ... Puncturing by micrometeorites strikes me as the big problem, actually. Objects with large surface areas don't survive unpunctured for very long. With regards to that particular problem: There are some folks out in Tillamook, Oregon, building their own blimps in an old WWII(?) airship hangar. Blimps face roughly the same problem -- you want to keep the helium in, and you want to keep the birds, rocks, mooring tower, etc., from puncturing the blimp. They discovered that the easiest way to do this was to make a two-layer blimp. The outer layer is some strong, lightweight, puncture-resistant substance, fashion-designed to your favorite color. The inner layer is a clear, thin, non-permeable bag that keeps the gas in. The big win from this is that the blimp itself is some amount lighter than it would be if there were a single, all-purpose bag for the helium. Also, the outer layer can be changed with relative ease; the advantage of this on the ground mostly being to change colors, but I can see space structures wanting to be able to switch to a new cover if the old one gets beaten up. That, and I suspect it's easier to mount "stuff" on the outside of your structure if it's a different material than what's keeping the air in your structure. Actually, with a space structure, you want three layers -- another layer on the inside to keep the occupants from tearing holes in their structure. While this needn't be as "strong" as the outer layer (people move much slower than meteors), it might be rather difficult to keep in place. The outer layer is held out by the gas-bag layer; the inner layer will have to be held to the gas-bag layer with duct tape or spring rods or something. Bill Trost Space is nature's way of keeping everything from happening all in one spot. ------------------------------ To: space-tech@cs.cmu.edu Subject: Inflatable structure From: can2can@ziggys.cts.com (Tim Edwards) Date: Thu, 24 Dec 92 07:54:24 PST There is/was a 'plastic' by UniRoyal called Royalite that was used to make bodies for plastic body cars. The flexible material cures and the center layer foams to give a rigid structure from a material that started looking like naugahyde. Is there a way to inflate the structure that would have a rigid result? then add membrane and internal scuff-coat. can2can@ziggys.cts.com - BBS (619)262-6384 Ziggy's Den Of Iniquity ------------------------------ Date: Mon, 04 Jan 93 13:21:58 +0000 From: Dominic Herity Subject: Re: inflatable space structures To: Bill Trost Cc: space-tech@cs.cmu.edu, dherity@cs.tcd.ie > Blimps face roughly the same problem -- you want to keep the > helium in, and you want to keep the birds, rocks, mooring tower, etc., > from puncturing the blimp. They discovered that the easiest way to do > this was to make a two-layer blimp. An important difference is that birds, rocks, etc are unlikely to puncture both layers. A micrometeorite, however, will puncture both layers ... unless it is vapourised by the first, of course. Is this the idea ? ------------------------------ Date: Mon, 4 Jan 93 08:56 PST From: trost@cloud.rain.com (Bill Trost) To: Dominic Herity Cc: space-tech@cs.cmu.edu Subject: Re: inflatable space structures I write: They discovered that the easiest way to do this was to make a two-layer blimp. Dominic Herity inquires: An important difference is that birds, rocks, etc are unlikely to puncture both layers. A micrometeorite, however, will puncture both layers ... unless it is vapourised by the first, of course. Is this the idea? Yes, exactly. The advantage of this arrangement is that each layer can be optimized for a single property, instead of trying to design one all-purpose material. And, since each layer has been designed for a single specific purpose, you'll probably need far less of it then you would if you used a single layer, saving on weight. ------------------------------ Date: Wed, 9 Dec 1992 11:44-EST From: Donald.Lindsay@GANDALF.CS.CMU.EDU To: space-tech@cs.cmu.edu Subject: Re: refrigerators Summary: Cryogenic cooling info I poked around and came up with: "Development and preflight qualification testing of a range of cryogenic coolers for applications from 20 K to 80 K", British Aerospace Space Systems Ltd, in Cryogenics 1992 Vol 32 #10 p.850. I was impressed by the graph showing how much vibration they'd managed to eliminate. (and other articles no doubt, that's the most recent such) Look for proceedings of the Conference (and also Symposium) "on Low Temperature Electronics", several have been held. or contact Sunpower, Inc., 6 Byard Street, Athens, Ohio (614)594-2221, FAX (614)593-7531 This company designs and builds Stirling engines, both for power (solar or external-combustion), and for refrigeration. They have a tiny little sealed, mass-balanced refrigerator. If you want to cool electronics, they've sold all the rights to an unnamed large corporation, sorry. ------------------------------ Date: Sat, 12 Dec 92 19:15 PST To: space-tech@cs.cmu.edu Subject: P2-E on LOX, Isp realities From: Bruce_Dunn@mindlink.bc.ca (Bruce Dunn) The following material is possibly a repost. Two postings that I made to space-tech were never mailed back to me, and I am going to assume that at least some people (maybe all people) never got them. Apologies to those who may have seen these before. ------------------------------ From: Bruce_Dunn@mindlink.bc.ca (Bruce Dunn) Subject : Why LOX Doesn't Help P2-E LOX is cheap, non-toxic, and gives high Isp in an engine. Otherwise, it does not have any virtues. It is not particular dense (1.14, vs 1.44 for peroxide) and is cryogenic. Watch what happens when I try to use it in the P2-E design: Reference Vehicle: 2 stage minimum P2-E vehicle, with a lower stage of 900 tons total mass, and an upper stage of 100 tons mass. Fuel for lower state is peroxide/propane at 7.5 to 1 mass ratio, giving a bulk density of 1.23. Pressurization is by 8.6 tons of helium at 298 K. Mass fraction of first stage is 0.8749. Payload to LEO is 12.6 tons. Trial Vehicle One: Use LOX in place of peroxide, at LOX/propane ratio of 2.5. Keep stage at same total mass, and assume, against all available evidence, that the same steel can be used without derating for cold-induced brittleness (or that a perfect insulation with a zero volume and zero mass is used between the LOX and the steel). Assume helium pressurization is by gas at 200 K (cooled by LOX), needing 17.0 tons of helium (larger volume, colder gas). Bulk density of propellants drops to 0.795, and mass fraction of stage drops to 0.835. Making appropriate correction for specific impulse, payload to LEO decreases to 11.8 tons from 12.6 tons. This hardly encourages using the cryogenic propellant even if a perfect insulator existed. In practice, with an insulator or a aluminum/insulator/steel composite tank, the steel on the outside would be cold and would have to be tempered to a higher toughness (and lower tensile strength) before it could be used. This would increase the tank mass and further reduce the payload, even before figuring in the volume and mass occupied by the insulator. Trial Vehicle Two: Redesign the vehicle to use a cluster of 3 aluminum LOX tanks and 3 aluminum propane tanks (for tank material commonality) each with a diameter of approximately 2 meters (volume ratio of propellants is near 1 to 1). Wall thickness is about 22 mm, which is probably within the range of reasonably easy weldability. Assuming a reasonable yield strength for aluminum (400 Mpa), the mass fraction decreases to 0.814 and the payload decreases to 10.4 tons. So, there are no payload increases to be had by using LOX. Assuming peroxide at $2000 per metric ton (George Herbert), LOX at $100 per ton (a guess, better figures would be appreciated) and liquid helium at $20,000 per ton (as per some recent figures from people using large quantities for cooling superconducting magnets) we get for a single P2-E stage approximately: 694 tons of peroxide, costing 1,388,000 8.6 tons of helium, costing 172,000 or 523 tons of LOX, costing 52,300 17 tons of helium, costing 340,000 All of these costs are insignificant relative to the other costs of the vehicle and launch services. Some of the apparent cost savings of LOX will disappear anyway when it is remembered that we have not yet accounted for LOX lost by evaporation during storage and during tank cool down. More importantly, the "savings" when using LOX eat into the payload. ------------------------------ From: Bruce_Dunn@mindlink.bc.ca (Bruce Dunn) Subject : Theoretical vs. Real Isp Paul Deitz writes: > By "theoretical Isp" do you mean the Isp as computed by a program > like CEC71 (i.e., shifting and equilibrium flow Isp's)? Exactly. It is possible using somewhat tedious hand calculations or an appropriate computer program (which I don't have by the way) to come up with a theoretical specific impulse (or exhaust velocity, which amounts to the same thing). The value depends on the heat available from the chemical reaction (which together with the specific heat of the products determines chamber temperature), the molecular weight of the exhaust gases, the expansion ratio of the nozzle, and the ambient pressure (eg. sea level). Most methods give results for a given propellant that vary by no more than a fraction of a percent, as long as the same assumptions are made. Typical results in books on propellant chemistry are listed for a hypothetical engine with a chamber pressure of exactly 1000 psi, and which expands the gases until their pressure drops to exactly 14.7 psi (1 atmosphere). Typical values for common propellants are: LOX/H2 391 LOX/RP-1 301 LOX/NH3 295 (used in the X-15 if I remember rightly) N2O4/N2H4 291 H2O2/RP-1 278 Depending on expansion ratio and ambient pressure, the predicted result varies widely. For example LOX/RP-1 which is listed as 301 at sea level with the optimum expansion ratio, has a vacuum specific impulse which varies with the expansion ratio: Expansion Ratio Vacuum Isp for LOX/RP-1 10 320 20 340 50 356 The actual Isp in real engines never reaches the theoretical Isp. In an engine such as the SSME, the ratio of real Isp to theoretical Isp is supposedly about 97 or 98%. However, the F1 engine seems to be a dog. According to theory, this LOX/RP-1 engine, with an expansion ratio of 16, should have a vacuum Isp of approximately 336. However it delivers only 305. One source of the loss is that about 2% of the propellant is diverted to the gas generator in order to power the turbopumps, and generates almost no thrust (this would account for an Isp loss of about 6). Other losses can come from incomplete combustion, gas friction, and exhaust gas divergence (gas which doesn't go straight back from the nozzle). The Soviet RD-170 engine burning the same propellant produces the remarkable vacuum specific impulse of 335. Assuming this figure is correct (and the Soviets haven't done something like conveniently ignoring the propellant used to power the pumps), this would imply both a very efficient design (low losses) and probably also a high expansion ratio. The rocket engine manufacturers probably have empirical methods of estimating losses and thus extrapolating from theoretical Isp to real Isp. These are not in any of the textbooks on propulsion that I have read. I faced this problem when trying to estimate the Isp for the P2-E. The theoretical value in a vacuum for propane and peroxide with a 12 to 1 expansion is somewhere around 310 or so (estimated by extrapolation from predicted values for other combinations, rather than calculated directly). However, the engine won't give 100 % of its theoretical Isp, so the problem becomes how much to derate it. It is simply wishful thinking to use a theoretical Isp figure in the design of a real vehicle. My final figure, which is based on the rather mediocre efficiency of the F1 and RS-27 engines, is an Isp of 290. This is more of an informed guess than a real calculation however. I would love to have a rocket engineer firstly calculate the theoretical Isp of my propellant combination at various ambient pressures, and then tell me how much to allow for losses. Bruce Dunn Vancouver, Canada Bruce_Dunn@mindlink.bc.ca Date: Sun, 13 Dec 92 17:20 PST ------------------------------ To: space-tech@cs.cmu.edu Subject: Helium From: Nick_Janow@mindlink.bc.ca (Nick Janow) Bruce, your argument against LOX makes sense. I hoped that LOX might allow pressurization without helium, by heating the O2 directly, but you pointed out the flaw in that too. However, you have expressed a desire to avoid wasting a limited resource (helium). Have you considered separating the peroxide from the helium using a membrane or membrane/sliding piston in the tanks? This would allow the helium to be hotter without decomposing the peroxide. Hopefully, this would reduce the amount of helium enough to allow for recovery equipment. Depending on the quality of the seals and the time to recover the booster, the tanks could retain a significant fraction of the helium. If this system actually is feasible and worthwhile, it could be put off until after the rest of the system proves itself as a non-recoverable booster. It might be too complex (ie. number of things to fail) to be worthwhile, but it might be worth looking into. Nick_Janow@mindlink.bc.ca ------------------------------ Date: Sun, 13 Dec 92 20:33:56 -0600 From: pgf@srl03.cacs.usl.edu (Phil G. Fraering) To: Nick_Janow@mindlink.bc.ca, space-tech@cs.cmu.edu Subject: Re: Helium I'm suprised noone's yet suggested a recovery mechanism where the excess helium in the tank is bled off into a large balloon of some sort... ;-) ------------------------------ Date: Sun, 13 Dec 92 19:51 PST To: space-tech@cs.cmu.edu Subject: Helium From: Bruce_Dunn@mindlink.bc.ca (Bruce Dunn) If a P2-E the booster is expended, it will be coming down without parachutes and will probably hit the water so hard that any hope of tank integrity is a dream. If the booster however is recovered by parachute, then the helium could no doubt be salvaged (it is after all worth tens if not hundreds of thousands of dollars). This would simply involve closing the tank valves after the propellants are exhausted. The simplest way to salvage the helium, as suggested by Phil Fraering, is probably to use it to fill a blimp which could be towed back to port. Alternately, if it is considered safe enough to handle a booster while under pressure, the pressurized booster could be towed into port where the helium could be removed. As far as putting a membrane or piston over the peroxide to allow higher helium temperatures (Nick Janow), it might be possible but adds another complication. I like the idea better of filling the ullage space of the tank with a gas like nitrogen or argon, which when the hot helium enters will compress to a dense insulating gas layer between the helium and the peroxide. I am continuing to tune up the P2-E conceptual design. I have started assuming helium pressurization by gas at an average temperature of 50 C, rather than 20 to 25 C. This is still cooler than the inside of your car which has been parked in the sun for a couple of hours, and I think that it won't bother the peroxide. I have also found that for the minimum P2-E launcher (1 P2-E stage, 1 upper stage), the P2-E stage does not need to be fully pressurized for the whole burn. What appears to work well is to provide only a "half load" of helium, which can keep up the full tank pressure only during the first 50% of the propellant. Following this, the tanks are allowed to blow down, gradually lowering the pressure (and thus engine thrust) until it is half the initial value at the end of the burn. This automatically limits the acceleration of the vehicle, which otherwise is excessively high near the end of the burn. -- Bruce Dunn Vancouver, Canada Bruce_Dunn@mindlink.bc.ca ------------------------------ From: ssi!lfa@uunet.UU.NET (Louis F. Adornato) Subject: Beanstalk mathematics To: uunet!cs.cmu.edu!space-tech@uunet.UU.NET Date: Thu, 7 Jan 93 12:29:51 CST X-Mailer: ELM [version 2.3 PL11] Could someone on the space-tech list please hold forth at length on the mathematics involved in a beanstalk/skyhook system (as in Clarke's "Fountains of Paradise"). What I'm looking for is a determination of how long we can presently make a tether capable of lifting two tons (about the payload of a shuttle orbiter). Thanks in advance. Lou Adornato uunet!ssi!lfa | The secretary (and the rest of the company) Supercomputer Systems, Inc | have disavowed any knowledge of my actions. Eau Claire, WI | ** Space IS our future! ** ------------------------------ End of Space-tech Digest #141 *******************