#91
|
|||
|
|||
The 100/10/1 Rule.
Henry Spencer wrote:
In article , Pat Flannery wrote: ...keeping the ET's changing center of mass within the limited gimbal range of the SSMEs absolutely dictated putting the LOX tank as far away from the SSMEs as humanly possible. I thought the gimbaling would probably be the main problem... the thing is already fairly unstable without getting it completely unstable by sticking the LH2 on top and having it constantly trying to go out of control. As I understand it, aerodynamic stability did not end up being an overwhelming concern, because the SRBs are such a large part of the stack's mass for most of the atmospheric flight. (Their nozzle gimbals also handle most of the control then.) By the time they depart, the air is thinning out fast and the importance of the aerodynamics is dwindling. The main issue was reducing movement of the center of mass and keeping it within SSME gimbaling range, so that the off-center SSMEs can always have their thrust vector pointed toward it. The SRBs and the ET LOX tank totally dominate the stack's center of mass for most of the flight, so their centers *had* to be in roughly the same direction, as seen from the SSMEs. Was there any particular reason the LOX ended up on top in the Atlas? Gimbaling limits again? Gimbal range wasn't an issue with the engines directly under the tanks. Reducing aerodynamic instability, to go easier on the control system, might have been. I don't think I've ever seen a discussion of exactly why Atlas has the LOX on top. I note that both Jupiter and Thor had it on the bottom. Designers with different priorities, probably. The Atlas's balloon tanks minimize the structural-mass penalty of putting the heavy LOX high up, so the more conventional structures might be expected to encourage putting it on the bottom. Max Hunter, who was chief engineer for Thor, in later years was big on saving structural mass by putting the LOX right above the engines, so that may have been the issue for Thor. My consideration is the structural efficiency gained from having GEM-60s span the engine thrust structure to the oxygen intertank segment, especially with GEM-60 thrust vector control. The idea of a large lightweight pressurized hydrogen tank at the top of the stack, with no heavy payload attached to it, other than a nose cone reentry vehicle, is intuitively appealing for high g levels at the top of the flight path. How much trouble that presents atmospheric flight is another matter. Certainly a hydrogen powered vehicle isn't moving too fast down there. The center of pressure for a simple symmetric SSTO rocket is pretty much dead center, so I'll write a program to more accurately simulate it all. On the other hand, simply upgrading to large shuttle style SRBs makes the attachment points come out ok again, with the oxygen up on top. -- Get A Free Orbiter Space Flight Simulator : http://orbit.medphys.ucl.ac.uk/orbit.html |
#92
|
|||
|
|||
The 100/10/1 Rule.
"Pat Flannery" wrote in message ... Without staging, one of the main arguments for a seacoast launch site vanishes, No, you still want it near a coast, so it doesn't accidentally fall on someone important. That's why the spaceport in NM is fine- the worst that would happen is it falls on Texas. |
#93
|
|||
|
|||
The 100/10/1 Rule.
Pat Flannery wrote:
: : :Fred J. McCall wrote: : : Actually, no. You still have all the usual downrange range safety : issues. This is why launches over water are preferred. If something : goes wrong, you're less likely to hit something you don't own. : : :I say Polar launches take off from North Dakota and fly over Canada, :then if something goes wrong it won't fall on something _we_ own. :-( Not only that, but we could claim that they shot it down and invade. It's all about oil, you know... BLAME CANADA!!! [:-), for the humour impaired.] -- "Adrenaline is like exercise, but without the excessive gym fees." -- Professor Walsh, "Buffy the Vampire Slayer" |
#94
|
|||
|
|||
fun with expendable SSTOs (was The 100/10/1 Rule.)
In article ,
Pat Flannery wrote: We've read up on your "Brown Bess" booster concept; if you were going to make an unmanned expendable SSTO, how would you go about it, and what propellant combo would you use? First, as Richard observed, I'd drop the "unmanned". If it's reliable enough for expensive cargo, it's reliable enough for people. Like, for example, me. :-) (There are people who suggest building relatively unreliable rockets to be used for bulk cargo -- water, fuel, etc. -- only. I don't think this actually works out well. You still need moderately good reliability, say, 80-90%, if only to avoid being fined for littering :-). I don't see a significant cost or complexity advantage to be had from the difference between that and the 98-99% of conventional expendables. If you can dependably get 80-90%, it should cost very little extra to hit 98-99%.) (Getting to 99.9% is harder, as witness the fact that no existing expendable has definitely achieved it -- there are a few uncertain cases where moderate production runs simply had no failures -- with the *possible* exception of the Soviet-era Soyuz launcher. It should be feasible, given careful design, a high flight rate, and automated production. Even 99.99% is probably not out of reach for expendables, if you sweat hard on things like systematic process improvement. Beyond that is strictly reusable territory.) The real major dividing line is reusable vs. expendable. Here, by definition, we're talking expendable. After that is the big question of whether whoever's paying for it has constraints to impose: use existing engines, no Russian subsystems, a minimum payload size, etc. They also might have opportunities to offer, e.g. use of shuttle-ET production facilities. Many of these things can severely constrain the design. Assume none of this. The major subsystem question is engines: buy or build? Buying means you don't have to get into the engine-development business, which saves a lot of trouble and may look less risky to potential investors. There are some downsides: (a) it's a lost dimension of competitive advantage, (b) the choice of existing engines is somewhat limited and can severely constrain design choices (in particular, ruling out many unconventional approaches), and (c) buying engines tends to be expensive and to involve a lot of hassles. I'd favor build, if only to relax design constraints. The major specs issue is, how much payload to what orbit? Orbital inclination affects delta-V requirement by determining how much help you get from Earth's spin. The big question for orbital altitude is whether the orbit is low enough for a direct-ascent trajectory -- continuous burn all the way up, like Gemini or Apollo -- or requires a Hohmann ascent like the shuttle, injecting into an elliptical orbit and then doing a final insertion burn at apogee. Hohmann ascent would always be more efficient if the atmosphere didn't get in the way. In real life, direct ascent usually incurs little penalty up to 300-400km, but gets rapidly worse thereafter. The nice thing about direct ascent is no engine restart. And even with Hohmann ascent, you pay a price for higher orbits. Absent outside constraints (e.g. cargo delivery to ISS), I'd favor direct ascent to 250-300km, high enough to last a little while and give the payload time to maneuver higher or be picked up by a tug. As for how much payload... depends on whether there's a specific mission constraint. If not, I would favor relatively small payloads, giving a small launcher and frequent flights, and relying on orbital infrastructure (assembly base, tug, fuel depot) to assemble larger systems. Smallness actually is not that important -- launcher cost scales much more strongly with complexity, thinness of margins, and closeness to the leading edge of technology than with sheer size -- but frequent flights are beneficial in many ways. *How* small depends on how much inconvenience you're willing to accept. There are cutoff points where inconvenience rises sharply because you can no longer launch particular objects in one piece, plus a general slow rise in inconvenience as orbital assembly operations multiply. If you want at least the option of launching people, that obviously sets a minimum size. For serious orbital operations, I see a high payoff for being able to launch a two-man ferry spacecraft, sort of a stripped-down Gemini, in one piece: it lets you have one pilot and one passenger, so the passenger doesn't need exhaustive training in emergency procedures for the ferry. Gemini weighed a bit under 4t, with early-1960s technology and greater capabilities than the ferry really needs. An aggressive modern design could come in quite a bit lighter. For serious orbital operations, the other thing that it would be nice to launch in one piece is a minimal habitation module. Perhaps inflatable... but with an expendable SSTO, a "wet workshop" approach using the spent stages is also very attractive. Say: Launch #1 carries a life-support module with consumables, integrated with the spent stage and with a docking hatch at the top. Launch #2 carries a multi-hatch docking node integrated with the spent stage, and a tug; the tug maneuvers it to mate with #1, and sticks around to supply attitude control and reboost. A ferry docks with one of the ports on the node, and you're in the space-station business. (Actually berthing would be better than docking, but that's a detail.) How much does each load have to weigh? That would need more study, but it's interesting to note that the Apollo-Soyuz Docking Module was about 2t. Could this sort of scenario be done with payloads of 2t or less? Probably, but it might get pretty tight. 5t should be lots. Let's be mildly aggressive and set the payload at 3t. I'd want to look into infrastructure issues -- size of manufacturing machinery, size of facilities, etc. -- and if it didn't look like a slightly bigger launcher would cross any boundaries that made things significantly harder or more expensive, make it bigger just on general principles. Materials etc. cost very little; the infrastructure issues are the main things that make a launcher cost more just because it's bigger, and they mostly rise in sudden jumps, not in a steady slope. And far more people have regretted making a launcher a bit too small than have ever regretted making it slightly too big. Anyway, let's cut to the chase -- this has already taken rather longer than I meant to spend on it :-) -- and look at the launcher. This is based on some past thought but without rigorous calculation for this particular design problem. Shape: a plain cylinder with a cone on top, or possibly a two-slope cone like the nosecone for Apollo 5 (which has lower drag and more usable volume) -- simple to make, simple to analyze. More the proportions of say, a Jupiter than a Delta -- the shorter, fatter shape has a bit more drag but is a lot stiffer and less prone to bending problems. Very light tanks, probably pressure-stiffened like the old Atlas. Likewise for the nose -- that was done on Atlas for SCORE and some other flights. (Here the nose stays on until reaching orbit, after which it hinges up and over to expose the payload, staying on the rocket so it goes back down when the rocket deorbits itself.) Either aluminum alloy or composite -- that would need more investigation. Composites are stronger and lighter, but more hassle to make, and there might be minimum-gauge issues with such light sheets, and composite LOX tanks are still iffy. Pressurization in the tanks is just enough for structural purposes, i.e. not very much. Boost pumps at the bottom of the tanks, or possibly the bottom of the feed lines, add enough pressure to prevent cavitation in the main pumps. (This approach is out of fashion but it has been done successfully in the past; it avoids having to make the tanks stronger and heavier to permit higher pressures.) One interesting option is to make the boost pumps jet pumps, recirculating a bit of the output from the main pumps to the jets in the boost pumps. (That too has been done.) The oxidizer is LOX -- cheap and dense. The fuel is probably propane -- slightly better performance than kerosene, less tendency to leave oily residues and otherwise misbehave, and it's still liquid and quite dense at LOX temperatures. Finally, for engines, I'm partial to the idea of an aerospike with a ring of small individual chambers. The small chambers help keep the scale of most engine-development facilities down. The aerospike provides altitude compensation and also permits a light, compact nozzle with a very high expansion ratio in vacuum. Expander or gas-generator cycle, preferably the former if enough heat can be had. (It's been done with propane.) Post-separation attitude control with propane cold-gas thrusters, and deorbit by dumping residual propellants through the engines. -- spsystems.net is temporarily off the air; | Henry Spencer mail to henry at zoo.utoronto.ca instead. | |
#95
|
|||
|
|||
The 100/10/1 Rule.
In article ,
Pat Flannery wrote: ...Without staging, one of the main arguments for a seacoast launch site vanishes, and you now have all sorts of options open to you as to where you want your launchpad at. You'd probably still have to launch over water for development launches, although once reliability is established, an inland launch site becomes a possibility. You could make it a manned launcher, but NASA would try to get their hands on it, and it would end up being dragged down to KSC. NASA really doesn't have any say in the matter any more. It's the FAA that you have to keep happy. In fact, rather than snapping it up, NASA will do its best to ignore it and/or cast doubt on the whole idea -- after all, if it succeeded, it would make them look incompetent. -- spsystems.net is temporarily off the air; | Henry Spencer mail to henry at zoo.utoronto.ca instead. | |
#96
|
|||
|
|||
fun with expendable SSTOs (was The 100/10/1 Rule.)
On 11 Mar, 06:57, (Henry Spencer) wrote:
In article , Pat Flannery wrote: We've read up on your "Brown Bess" booster concept; if you were going to make an unmanned expendable SSTO, how would you go about it, and what propellant combo would you use? First, as Richard observed, I'd drop the "unmanned". If it's reliable enough for expensive cargo, it's reliable enough for people. Like, for example, me. :-) (There are people who suggest building relatively unreliable rockets to be used for bulk cargo -- water, fuel, etc. -- only. I don't think this actually works out well. You still need moderately good reliability, say, 80-90%, if only to avoid being fined for littering :-). I don't see a significant cost or complexity advantage to be had from the difference between that and the 98-99% of conventional expendables. If you can dependably get 80-90%, it should cost very little extra to hit 98-99%.) A slight modification: Allow untried rockets for fuel, water etc. For example, Atlas and Delta could compete for the valuable cargos (~6 launches). Those two plus Falcon could compete for the fuels (18 cargos). Next time round, all three can compete on both contracts. Another new entrant could compete on the fuel launches. I would actually say, rather than fuel, launch "Earth Departure Stages" with LOx / Kerosene propellant. These would bolt / dock together to launch more expensive cargoes. Then NASA doesn't need to do orbital propellant transfer, which they believe to be impossible. |
#97
|
|||
|
|||
The 100/10/1 Rule.
In article ,
Jorge R. Frank wrote: So a dense-propellant SSTO doesn't really need less delta-V to reach orbit, but you use a smaller delta-V term when modelling one using the rocket equation. Depends on whether you think of delta-V as the actual change in velocity, or as the change the vehicle could achieve in ideal conditions -- that is, a measure of vehicle performance or required vehicle performance. The latter is often the more useful view. Even in a fairly idealized situation, not all of the vehicle-performance delta-V actually shows up as final orbital velocity, because some of it has to go to achieve the necessary altitude. So I would phrase it a bit differently. The dense-propellant SSTO has to achieve the same orbital velocity, but its gravity losses are lower (as are its drag losses, although that's less important), so the total delta-V the vehicle must deliver (equal to the velocity it could achieve in drag-free gravity-free space) is lower. -- spsystems.net is temporarily off the air; | Henry Spencer mail to henry at zoo.utoronto.ca instead. | |
#98
|
|||
|
|||
The 100/10/1 Rule.
Rand Simberg wrote: And at worst, it kills a moose, My sister was once bitten by a SSTO vehicle. ;-) or a polar bear (whose numbers are apparently increasing). http://www.telegraph.co.uk/news/main...9/wpolar09.xml Per the headline, I blame global warming. What will make this interesting is that if the polar ice all goes away, the polar bear may re-evolve brown coloration over a period of many generations, as its white coloration will be a net deficit to camouflaging it in its environment. Pat |
#99
|
|||
|
|||
fun with expendable SSTOs (was The 100/10/1 Rule.)
Henry Spencer wrote: And far more people have regretted making a launcher a bit too small than have ever regretted making it slightly too big. That's certainly true, in fact if you make it 3 tons, in fairly short order you will find most of your commercial payloads will weigh 3 tons, as your customers size their payloads to lift capability. Next, they will add a second stage. :-D The oxidizer is LOX -- cheap and dense. The fuel is probably propane -- slightly better performance than kerosene, less tendency to leave oily residues and otherwise misbehave, and it's still liquid and quite dense at LOX temperatures. Hank Hill fully agrees with this fuel choice. :-) There was some speculation that the "Aurora" (or whatever the thing was if it existed) might have used propane due to its greater density than LH2 to cut down propellant tank size. Considering that I've seen propane cylinders for refueling lighters that have very thin aluminum walls, is it even necessary to chill it? Pat |
#100
|
|||
|
|||
The 100/10/1 Rule.
Pat Flannery wrote:
: : :Rand Simberg wrote: : And at worst, it kills a moose, : :My sister was once bitten by a SSTO vehicle. ;-) I hear SSTO vehicle bites are nasti... |
Thread Tools | |
Display Modes | |
|
|
Similar Threads | ||||
Thread | Thread Starter | Forum | Replies | Last Post |
The 100/10/1 Rule. | kT | Space Shuttle | 156 | March 28th 07 03:25 AM |
Going Forth to Rule the World | Warhol | Misc | 0 | May 22nd 06 05:19 PM |
Is this like some kind of rule? | Rich | Amateur Astronomy | 7 | January 16th 06 12:59 PM |
Republicans Rule | Mark | Misc | 5 | May 28th 04 12:56 PM |
Does Religion Rule ? | G=EMC^2 Glazier | Misc | 2 | March 4th 04 11:34 AM |