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Yeah, and having to go and recover the SRBs on Shuttle adds signifigant per-flight expense, which is the real killer, it all ought to be able to come back to the launch site. Recovery from several thousand miles away shouldn't be on the table, Nasa saves quite a bit on Shuttle having it return to KSC from orbit directly and just wheel it to the hanger.
As for payloads, I don't see any point bothering with an RLV that can't haul at least ten tons. Most satelites are bound for geosynch orbit and would need the extra mass for the upper stage engine, and I just don't think there is any substantial market for LEO satelites outside the military... Iridium failed miserably; I don't think any investors are going to want to try somthing like that again for a long, long time - there is just no market cheap launcher or not.
Its a chicken and egg... if you can't build a big enough vehicle to reach GEO, then you won't have the money to build a weaker one that can hardly reach LEO. This makes more sense to me... as much as I hate the bubble bursting, all this "Alt Space" stuff is nonsense until you can hit GEO.
And as far as any sort of spaceflight or serious longterm science work, anything under 10MT is silly; the extra complexity and hence cost of making large vehicles in smaller bites makes no sense at all, building ISS out of 20MT chunks has been a disaster, and trying to make anything out of little <5MT chunks will be even worse no matter how easy it is to launch the stuff.
For instance, the TransHab module, already much lighter than comparable Aluminum structures and most ISS parts, weighs in at 8MT - it isn't practical to make it in pieces, at least the modules must go up assembled. Building stuff in space is hard!
I think you underestimate the importance of keeping the weight down too; putting the 2nd stage under extra stress is not a trivial concern, rockets are already built with very small tollerances to keep the dry mass down, adding alot of extra supports and beefing up the stage will cut into your already tiny payload mass... which is too small for manned craft, most satelites, and for building anything in orbit.
There is just no market for small flights to LEO outside of military/gov't contracts, and there isn't going to be one for a long time, betting on making a rocket to go there is a losing proposition.
[i]"The power of accurate observation is often called cynicism by those that do not have it." - George Bernard Shaw[/i]
[i]The glass is at 50% of capacity[/i]
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I am really enjoying this exchange because it gets at some very important, fundamental issues. So allow me to continue the discussion.
1. Regarding the stress on the second stage, I think a straight-up system adds a bit of delta-v, compared to a two-stage system that launches diagonally, *but it involves a lot less delta-v than single stage to orbit.* That's not trivial, either; in fact, two stages greatly improves your mass margin and simplifies things like thermal protection. That's the comparison I'm offering. If SSTO is almost technologically achievable right now, a vertical launch system is not a problem.
2. Regarding whether 2.25 tonnes is useful, that depends on whether a second launch puts up a geosynchronous injection stage, or fuel to refuel an existing stage, or an ion tug. I think the current geosynchronous satellites *by themselves* currently mass about 5 tonnes. But that design fact is a function of the launch vehicles being used. When it costs $20,000 a pound (or whatever) to put something in geosynchronous orbit, you build big things that will function a long time and maximize output. If you can put things in geosynchronous orbit for $1,000 per pound using this system, people will redesign their satellites for it and save a bundle. Furthermore, replacements can be launched much more cheaply. Even an Iridium-like system could be launched for a tenth as much. That will generate some powerful economic forces shaping the nature of the satellites launched.
If a satellite can be launched into a low orbit for $1000 per kilogram ($500 per pound), and two additional launches can put up the fuel or the stages to get it to GEO, and if the crew on board the mini shuttle can complete the fuel transfer or the satellite-to-stage docking themselves in a twenty-four hour period while they are in orbit, won't it happen? It'd be a quarter as expensive as EELV.
Regarding on-orbit assembly of ISS and on-orbit assembly in general, I am under the impression that the high launch costs themselves cause higher manufacturing and assembly costs. The ISS was supposed to be a vibration free environment in order to do microgravity experiments, so fans had to be designed not to vibrate. Everything had to be designed to last as long as possible and be as redundant as possible. But if the ability to fly up replacement parts at $500 per pound every week exists, you design everything differently.
Regarding the transhab, where did you get the mass of 8 tonnes? I was under the impression the entire thing massed at 16 tonnes, but I may be wrong. Is the 8 tonnes a total? If so, one could launch a 2.25 tonne shell, 2.25 tonnes of life support equipment, 2.25 tonnes of interior walls and furniture, etc., one could still get a transhab-like thing into orbit. They had planned to convert a used third stage into Skylab originally, after all. If nothing else, one could do the same thing with one of the Rocket Company's shuttles!
One final comment. After reading the chapter about using the DH shuttle to go to the moon, I wrote the author and expressed skepticism that a moon expedition could be mounted for $300 million. He wrote back and said that one hundred million dollars goes a long way with undersea exploration. I think that's right. Living under the sea is pretty difficult; comparable to Mars, I think, maybe to low earth orbit as well. Yet low earth orbit equipment still costs ten or more times as much. A cheap way to low earth orbit will drive other economic forces and make staying in low earth orbit cheaper as well.
-- RobS
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It's also worth remembering that cheap low mass launches are useful for bringing things like fuel up to LEO. Of course, you run into the problem of launch failures with large numbers of launches but H2/O2 up to LEO for cheap is fuel is cheap fuel in LEO regardless of how its done. If you recall, I did some rough numbers for how much you can increase the mass to Mars with LEO refuelling and it's a very substantial increase.
A good HLV is always going to be necessary but cheap LEO access will always be useful even if the cargo loads are small.
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Just saying, SRBs are complex beasts, and they won't ever be cheap enough for a weekly-flight RLV with sufficent performance and reliability.
I don't even think bringing up small doses of fuel would be worthwhile for such a vehicle, considering the probability of vehicle failure for a large number of rocket flights to move really substantial masses of fuel.
I maintain that its silly to build anything in space out of such tiny pieces. The 8MT figure for TransHab probably just for the vessel & structure, and not the equipment/accomodations inside. Anyway, I doubt that you could even lift the single-lobe inflatable hull with only 2.25MT, and even if you did, you are going to attach the thing to the rest of the module in orbit? Iiii don't think you want to go and sew/weld/glue somthing you are going to trust with your life in space. 10MT is the smallest mass that I can even concieve of being useful, you just aren't going to be building anything in space...
Sure you could get the pieces up there, but it would be suisidal to try and construct them in orbit! You simply can't cut large structures down to such small packages, its far safer and easier to just launch them from the ground. Making people go out on spacewalks alot in general is somthing to be avoided, they really are quite dangerous overall, and i'm pleasently surprised nobody has died yet assembling the ISS.
Sending up satelites in pieces (actual vehicle + GTO injection stage) will also be quite a bit harder than you think, that injection stage or the satelite will have to have remote or automated docking capability, which is no easy feat even today. The stage would be too expensive, and too prone to failure... There isn't going to be a manned mini-shuttle for satelite launches, too expensive and too dangerous.
Spaceflight is much different than deep sea ocean travel... first and foremost, you don't have to push your entire vehicle straight up several hundred miles at hypersonic velocities riding a controlled explosion...
Too much Alt Space optimisim.
[i]"The power of accurate observation is often called cynicism by those that do not have it." - George Bernard Shaw[/i]
[i]The glass is at 50% of capacity[/i]
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OK, let's do a quick set of theoretical numbers here:
Let's say that we can build a small mass SSTO that puts 5 MT into LEO every cycle. Let's assume that launch costs are $500,000 per cycle and that the vehicle costs $100 million. LEts also assume that we can achieve the same 97% reliability that we currently get with launch vehicles.
Let's assume that we need 100 MT of fuel to LEO to gas up a Mars expedition. That's $20 million in launch costs for 100 MT. The total failure probability per launch vehicle is about 46%. So let's assume that we lose one launch vehicle in this operation. That adds up to $120 million per 100 MT or $1200/kg. Already, that cost is well below any competing launch system.
If we can get up to 10 MT per launch, the price per kg doesn't drop too much since the launcher costs goes up and you only get a logarithmic increase in total launcher reliability with half as many flights. Larger SSTO payloads per liftoff are nice but there is nothing wrong with small payloads. Fuel is fuel no matter what size container it's in. Don't forget that SSTOs with current technology are lucky to get 2% payload mass ratios. Getting 10 MT to orbit requires a big SSTO with commensurately large initial R&D costs.
Building an SSTO is not easy. The DC-X everybody was so nuts about hovered a couple times - big deal. Actually getting something like that to LEO and back a few times is much more difficult. If we're going to build an SSTO and not end up with a Shuttle-like boondoggle, we have to start small. A 5 MT to LEO cycler is a good starting point. 5 MT of supplies/crew/food to LEO is invaluable to a space program. For Mars Direct, every kg of supplemental stuff lofted to LEO boosts your final Mars payload by about 300-400 g. That's nothing to sneeze at. We still need dumb disposable HLVs and trying to muddy the whole SSTO waters with trying to close the gap betweeen them is going to do nothing but wreck the SSTO program.
I'm still in favor of an SRB option on these vehicles. SRBs are incredibly reliable as long as they're kept simple. Unfortunately, I was unable to find out the cost of commercial SRBs but they can't be too much. The Delta 2 is $55 million a launch and uses 9 Castor IV SRBs. The military has built literally tens of thousands of SRBs for missiles with high reliability and high thrust and low cost. Making a cheap SRB isn't tough. It's when you start trying to make an SRB really powerful and/or reusable that the cost starts going out of control.
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I suppose the weak link in this argument is the 97% reliability assumption. If the space shuttle had been designed differently--correct O-ring designs from the beginning and the shuttle sitting on top of the fuel tank instead of next to it--it would have a 100% success rate after 113 launches. We can be sure those mistakes won't be repeated, especially on a reusable vehicle. The shuttle main engines have fired something like 339 times (113 x 3) without serious problems. If one assumes a 99% launch success, then your 100 million dollar vehicle will get 500 tonnes into orbit before failure, adding $500/kg to the launch costs.
-- RobS
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I want to get back to the issue of small payloads, though. I understand people have an emotional reaction against the idea, but I remain to be convinced it won't work. For example, what about this:
1. Launch a 2.5 tonne manuevering unit with fuel, to maintain the future station's location and orientation.
2. Launch solar panels to power the station (maybe these go up earlier)
3. Launch a 2.5 tonne universal docking unit, cube-like, with at least three, preferably four docks attached to it.
4. Launch a 2.5 tonne unit with an arm. Dock it to the docking uinit.
5. Launch a 2.5 tonne habitat shell; nothing inside it. It will probably be smaller than the transhab, but it might still be bigger than the existing modules making up ISS. Use the arm to dock it to the docking unit.
6. Launch 2.5 tonnes of life support equipment and internal walls for the habitat. Dock the shuttle to one of the other ports, transfer everything into the habitat, pressurize it, set it up in a shirtsleeve environment.
7, 8: More stuff to set up module A.
9. Launch another docking unit and dock it to the other end of module A.
10-12. Launch module B and the stuff for its interior. Attach it to either docking unit.
13. Launch a 2.5-tonne structural element that will run "above" or below module A and rigidly connect docking units 1 and 2 together. This gives the station backbone. Maybe this is needed; maybe not. ISS has this, launched in 16-tonne sections.
14. Etc.
No one in their right mind would do this unless (1) a small shuttle was very reliable and (2) it was very cheap. IF those conditions are met, this system is basically what makes ISS work, but in smaller steps. If launches are cheap enough, this system could be made cheaper as well.
-- RobS
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No no, 2.5MT still doesn't get you anywhere... the aluminum pressure vessel docking module will weigh more then that, and that you just can't cut down any smaller... it has to be big enough to at least get people in and out of. A Soyuz orbital module sized thing with extra docking adapters is too small and it already weighs the limit, or the hab/lab/etc of any reasonable size (1/3rd of TransHab by weight) would weigh more than the limit, and the list goes on... 2.5MT is not enough to get you anywhere, even if it is reuseable. You just aren't going to go up there with two halves of a single pressurized body of any sort and connect them together.
And you just aren't going to do much other kind of construction either... it isn't practical to send up a logistics or lab module "empty," because its too hard to install all that equipment by hand in zero-G with the absolute minimum of tools that are practical to send up, its just too hard. Astronauts aren't going to be installing coolant pipes, wiring, or that sort of thing. Its far, far easier to just build the thing on the ground, and use a bigger rocket.
On a more general note, this sort of super-duper-small lego brick building system is not efficent enough. It just isn't. You will require far too many pieces to acheive a reasonable volume, and the cost of subdividing the target volume increases alot with each additional subdivision if for no other reason than the engineering difficulty; not to mention all the extra construction for bulkheads/adapters and the fuel for the space tug. The mass of all the additional docking adapters will also really add up, since they have to be built pretty strong, which will really kill the advantages of a superlight RLV.
Then you have to consider the volume too. TransHab doesn't compress down to a little block that will cram neatly in a 1M payload faring, and things like the docking block have to be big enough for people and stuff to move around in, so the superlight RLV will require a payload faring a few meters wide at the least... I think this will be hard to engineer into such a small rocket.
This is a terrible idea on many fronts, safety (due to complexity), difficulty of construction, and high cost of the station per-performance... This isn't even enough for a good tiny Salyut style space station, 10MT is the SMALLEST practical launcher to build anything with.
[i]"The power of accurate observation is often called cynicism by those that do not have it." - George Bernard Shaw[/i]
[i]The glass is at 50% of capacity[/i]
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Beryllium would make it lighter.
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I have to side with GCNRevenger on building structures with a low mass booster. If your parts get too small, the end product is mostly composed of interconnects which is very inefficient. Plus, in space assembly has proven to be far harder and less routine than was originally envisioned. Until our robot capabilities get much better, it's just not too practical.
However, I'm still maintaining that booster mass is irrelevant for liquids and gasses. Even if you're lofting 100 kg a flight, as long as your cost per kg and reliability are high, you're gold. Let's assume for a minute that the Bull type launchers are practical and reliable. (there's no evidence that they wouldn't be from the experience so far with such technology) Let's also assume $500/kg per launch for a 100 kg to LEO payload and a reusable shell. You've got some difficulties with the automated docking with so many payload but as long as you design your fuel depot to be able to shrug off minor collisions and ensure that a complete failure in the shell guidance systems leads to a miss, I don't see why this isn't a valid way to get mass to orbit. Likewise, small SSTOs of a few MT are within our reach - why not utilize that capability? SSTOs will not be capable of lofting structural elements anytime soon so why put that constraint on them?
Let the big dumb soosters do what they're good at - lofting big, assembled structures and let the cheap systems do what they're good at.
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GCNRevenger, you have convinced me. I agree, 2 point something is too small, and even ten tonnes would be dicey. But if someone else has additional information, I will be glad to listen.
-- RobS
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anyone know if people are still trying to peddle finishing the x-33 and venturestar concept. a while back i read that they were trying to get the US military to foot the bill...
the way i see it, is that the only way for a cheaper access to space is mass-production. put it on an assembly line and build a lot of them. however is anyone crazy enough to spend all the money to do it? i am a firm believer in the "if you build it, they will come" concept (not sure how realistic it is, but i can still dream)
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The x-33 concept doesn't have a chance of making it to orbit for the forseeable future. The required structural efficiency is nearly an order of magnitude too low to be accomplished by today's technology.
The Air Force currectly owns all of the x-33s hardware. IMO the most valuable portion of this design was the aerospike engine. I could see this being used as a kick stage for future scramjet technology.
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Or if we aren't afraid of building a large vehicle, make it a two-stage spaceplane with airbreathing/LOX-augmented lower capable of SR-71 Blackbird-like performance and a pure rocket upper with linear aerospike engines for orbit.
[i]"The power of accurate observation is often called cynicism by those that do not have it." - George Bernard Shaw[/i]
[i]The glass is at 50% of capacity[/i]
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