Might be useful to think of it as similar to a car with modern unibody construction instead of using a box steel frame and heavy body panels like a 1955 Buick. The sheetmetal in a unibody is so thin you could easily bend it, sometimes by hand, but welded up it can protect you in a car crash while still being very light - much lighter than that Buick.Make it out of lithium-aluminum and you're on the bleeding edge of light & strong.Next stop: someone builds a booster out of composites.
NC machining is easy, set it and forget it. Welding stringers and hoops takes more touch labor.
Quote from: Jim on 08/08/2011 01:27 amNC machining is easy, set it and forget it. Welding stringers and hoops takes more touch labor.Remember that the stir welding process is now automated.
Quote from: Dave G on 08/08/2011 12:26 pmQuote from: Jim on 08/08/2011 01:27 amNC machining is easy, set it and forget it. Welding stringers and hoops takes more touch labor.Remember that the stir welding process is now automated.Still have to put the hoops into place and clamp them
So how much lighter are optimised structures like those used on modern launch vehicles compared to a plain aluminium or steel cylinder of uniform thickness and sufficient strength / stiffness? Two times? Five times? More than that?
Quote from: Jim on 08/08/2011 02:06 amQuote from: docmordrid on 08/08/2011 01:31 amNext stop: someone builds a booster out of composites.Delta IV payload payload fairing, interstage, second stage intertank, first stage intertank and aft aeroshell. Composites for cryogens is some years away or not at all.Falcon 9 also has composite intertanks and fairings. I would even guess the aeroshell in the back, too. What's more, even Proton-M block III, have those parts made out of composites. I know that the Polish Stako company does 300Bar SCUBA tank out of composite (not aluminum lining with composite reinforcement, pure composite).Obviously a SCUBA tank and an RP-1 are orders of magnitude differente sizes. But they SCUBA tank had a proof pressure of 450bar, while the Falcon 9 has what, 3.3Bar? I mean, eventually we might see or not cryogenic composite tanks, but non cryo should be seriously studied.The other thing that I would study (and I guess was already but nothing came out of this) would be fibre reinforced cryogenic tanks. With some kevlar mesh around, should be able to support quite a bit of extra pressure, which should save on the wall thickness. It would most certainly require to be kept slightly pressurized for structural stability. But something like 1.2Bar of air should do it.
Quote from: docmordrid on 08/08/2011 01:31 amNext stop: someone builds a booster out of composites.Delta IV payload payload fairing, interstage, second stage intertank, first stage intertank and aft aeroshell. Composites for cryogens is some years away or not at all.
Next stop: someone builds a booster out of composites.
Minimum gauge becomes a significant issue for small diameter, thin composite pressure vessels. Leakage risk goes way up, along with cost. Pretty much nothing can beat aluminum for expendable tanks.
Quote from: HMXHMX on 08/09/2011 04:53 amMinimum gauge becomes a significant issue for small diameter, thin composite pressure vessels. Leakage risk goes way up, along with cost. Pretty much nothing can beat aluminum for expendable tanks.I don't quite follow what you wrote, I'm sorry. The first phrase is about small pressure vessels, and has nothing to do with expendable tanks, right?In that case, I've happily used Stako's plastic composite paintball tanks (1.1L), at a working pressure of 300Bar with no problem. Regrettable, that's not ideal for the two main uses of those kind of sized tanks: paintball and firefighters breathing air. The first simply don't have any training and some do stupid things, like putting oil in the feeding valves, at 300Bar air (that's equivalent to 60Bar or 870psi Oxygen). The second group, has to fight it's way through fire a debris. But in general for any other application, those tanks work great. You do have the buoyancy issue for SCUBA.I'm sure nothing beats Al for expendable tanks, for now. What I was saying was that it was well worth it to keep researching some mix of materials and techniques to beat it. But as we always find, if you had the launch rate to do a machine to build composite tanks more cheaply thank Al tanks, then you'd better research a fully reusable launch system, right?
Given enough production rate, it is cheaper to automate anything. This is more a question of whether it's worth making an automated stringer positioning and clamping device or not. My guess is that for less than twenty cores per year it isn't. And probably the number is closer to one hundred. A custom machine like that would cost how much? 500k?An operative should be able to do at least two segments per day. Let's assume that the falcon takes 30 segments, that's 15 days of work (for that specific task). Let's say that the working year is 240 days, that's 16 cores per year. Times five years (machine cost vs operative's wage) that's 80 cores. And I haven't added the financial cost. Nope, I don't see an economic reason to actually automate this task. Not with less than 20 cores per year.If SpaceX has any sort of serious financial officer, he'll keep it as manual as possible until the supposed 20 cores per year are realized. Again, this assumes that said operative is only doing that task. Since it takes some time for the welding machine to do it's work, he could work on two things at a time.
The four cases when you manufacture products.1. Low production rate, immature product.2. Low production rate, mature product.3. High production rate, immature product.4. High production rate, mature product.As far as significant labor savings and recovery of your tooling costs. Assembly automation only makes sense in case four.
Is part of the reason for the propulsive landing engines on the side to keep debris from bouncing into and wrecking the heatshield (whether on Mars or Earth)?
Quote from: go4mars on 08/25/2011 02:32 amIs part of the reason for the propulsive landing engines on the side to keep debris from bouncing into and wrecking the heatshield (whether on Mars or Earth)? That might be beneficial, but the IMHO there are two main reasons for it:1) For crew launch, they can begin firing immediately in the case of an abort, without waining for a separation event (like CST-100).2) For landing on Mars, it allows them to avoid a supersonic parachute. Terminal velocity on Mars is supersonic (because the air is so thin), and it is really hard to fire a retrorocket into a supersonic (incompressible) flow. Viking and Phoenix got around this with a supersonic parachute that slowed the capsule to high subsonic before the rockets were ignited. Dragon gets around this by putting the exhausts in the stagnant flow behind the shock, thus allowing the engines to ignite while the vehicle is supersonic. This is quite clever if it works, and removes an entire level of complexity from landing on Mars.
Quote from: simonbp on 08/25/2011 04:03 pmQuote from: go4mars on 08/25/2011 02:32 amIs part of the reason for the propulsive landing engines on the side to keep debris from bouncing into and wrecking the heatshield (whether on Mars or Earth)? That might be beneficial, but the IMHO there are two main reasons for it:1) For crew launch, they can begin firing immediately in the case of an abort, without waining for a separation event (like CST-100).2) For landing on Mars, it allows them to avoid a supersonic parachute. Terminal velocity on Mars is supersonic (because the air is so thin), and it is really hard to fire a retrorocket into a supersonic (incompressible) flow. Viking and Phoenix got around this with a supersonic parachute that slowed the capsule to high subsonic before the rockets were ignited. Dragon gets around this by putting the exhausts in the stagnant flow behind the shock, thus allowing the engines to ignite while the vehicle is supersonic. This is quite clever if it works, and removes an entire level of complexity from landing on Mars.Thank you very much. Fascinating response! There appears to be at least 3 significant advantages (with the main disadvantages being complexity and cosine losses).
An additional supersonic decelerator possibility is simply to use propulsion. While this appears straightforward, there is little experience firing larger thrusters directly into a high dynamic pressure supersonic flow. Flow stability, flow-control interaction and thermal protection are some of the design issues that surround use of this technology.