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mlorrey
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« Reply #480 on: 12/24/2008 06:33 AM » |
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Thats debatable, and not that simple. I doubt the 15 second figure. Prior experience people go by for example is the plasma damage X-15 suffered on one flight when a test scramjet cause some serious damage simply from the shock wave plasma at hypersonic speed.
The X-15 was an inconel airframe was it not? Not titanium...
It was both: "The wing presented a difficult design problem, to account for uneven heating from leading edge to trailing edge and between lower and upper surfaces. At high angles of attack, inconsistent heating typically subjects the wing's lower surface to temperatures 400° F higher than those of the upper surface. The result of higher heating at the leading edge and lower surface is that these two surfaces try to expand faster than the rest of the wing. Thus, the wing structure had to be designed to allow for this expansion without deforming to a large extent, while, at the same time, carrying rather large airloads. A balance was achieved by allowing some expansion of skin to alleviate a part of the thermally induced stresses, and by the use of titanium internal structure, which has a higher elasticity than Inconel X. The internal structure provides enough restraint between attach points to give the hot wing surfaces a tufted-pillow appearance as they try to expand. Corrugations in the internal structure allow it to flex enough to keep skin stress within tolerable limits. " Inconel was primarily the skin/TPS/leading edges. An all-inconel airframe would have been too heavy to fly, esp with those little wings, wing loading would have been huge, which would have increased thermal and structural stresses. Shuttle wing structure: http://spaceflight.nasa.gov/shuttle/reference/shutref/structure/wing.html
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jongoff
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« Reply #482 on: 12/26/2008 03:01 AM » |
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Drag losses are much worst with small vehicles. Depends on how you do it. As it is, drag losses are roundoff errors on typical launch vehicles (I think I was hearing ~200m/s for a typical EELV). There are options like air launch or other things that can help deal with that. Or you can just bite the bullet and deal with it. Admittedly if you get *too* small, drag losses get truly heinous, but I'm not talking about a 5kg to orbit vehicle, a 1000kg to orbit vehicle is still a respectable sized craft, but small enough to keep the development cost cheap enough to be feasible. A small RLV is even more difficult. But it's easier on the one dimension that matters most--the financial one. Raising hundreds of millions or billions of dollars for a bigger RLV is a lot harder than raising a smaller sum for a vehicle that is a lot smaller. There still are technological challenges with small RLVs (especially if you try to go too small), but if you don't overcome the financial challenge, you don't get to even try on the technical side. And quite frankly, I have higher confidence in being able to solve the technical issues than the financial ones. ~Jon
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kkattula
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« Reply #483 on: 12/26/2008 03:26 AM » |
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Also it should be a lot easier to raise the money for a big RLV, once a small RLV has demostrated regular operations and retired most of the technical risk.
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clongton
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« Reply #484 on: 12/26/2008 01:18 PM » |
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Also it should be a lot easier to raise the money for a big RLV, once a small RLV has demonstrated regular operations and retired most of the technical risk. In an oblique sort of way that was one of the unstated goals of the X-Plane program, specifically the X-15 and its intended successors. I remember one of the pilots (I don't remember which one) gave a talk at my high school in 1963 and he said that someday people would fly into space and back on planes that were designed by things we learn from the X-15.
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William Barton
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« Reply #485 on: 12/26/2008 01:35 PM » |
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Drag losses are much worst with small vehicles.
Depends on how you do it. As it is, drag losses are roundoff errors on typical launch vehicles (I think I was hearing ~200m/s for a typical EELV). There are options like air launch or other things that can help deal with that. Or you can just bite the bullet and deal with it. Admittedly if you get *too* small, drag losses get truly heinous, but I'm not talking about a 5kg to orbit vehicle, a 1000kg to orbit vehicle is still a respectable sized craft, but small enough to keep the development cost cheap enough to be feasible.
A small RLV is even more difficult.
But it's easier on the one dimension that matters most--the financial one. Raising hundreds of millions or billions of dollars for a bigger RLV is a lot harder than raising a smaller sum for a vehicle that is a lot smaller. There still are technological challenges with small RLVs (especially if you try to go too small), but if you don't overcome the financial challenge, you don't get to even try on the technical side. And quite frankly, I have higher confidence in being able to solve the technical issues than the financial ones.
~Jon
Many of the questions about small RLVs would have been answered by K-1, including viability of RTLS recovery and wings v. no wings. When people talk RLVs, it's usually winged rocketships no that different from Von Braun (or Tom Swift, Jr., for that matter), or maybe a DCX-like VTOL. Parachutes and airbags sometimes seems like a cheat, because you can't gas and go (like DCX) or restack, gas, and go (like a winged TSTO), but it still would have been an RLV is the general sense of the phrase.
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kkattula
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« Reply #486 on: 12/26/2008 04:41 PM » |
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People generally confuse RLV with SSTO. A vehicle can be both, either or neither.
Both is the most difficult.
The big issues for SSTO are:
1) High mass fraction
2) Altitude compensation for nozzles.
3) Thrust reduction (throttling) late in flight.
Going RLV helps with none of these, in fact it hurts 1 & likely 2 & 3.
Typically, staging is the solution for all three, at the cost of discarded hardware.
RLV first (& maybe second) stage has the potential to overcome that disadvantage.
In summary, IMO:
- RLV makes SSTO much harder & maybe a little cheaper.
- RLV makes TSTO much cheaper & maybe a little harder.
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kevin-rf
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« Reply #487 on: 12/28/2008 02:13 PM » |
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3) Thrust reduction (throttling) Only if it is manned. People also see SSTO, RLV, and assume manned. The best first step RLV X program would be one that is designed to demonstrate reuse, rapid turn arround, and low man hours between flights. Once that is done you can think about a follow on program with payloads. The problem is someone would have to pay for something that has "zero" chance of "directly" making money. It would be gen 2 or gen 3 before you "may" have a commercial vehicle.
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tnphysics
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« Reply #488 on: 01/03/2009 05:11 PM » |
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What about borane fuels? They were supposed to offer the performance of hydrogen at the density of kerosene, but I believe that their was a problem.
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tnphysics
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« Reply #489 on: 01/03/2009 05:24 PM » |
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The key to a gas-and-go SSTO RLV IMO is airbreathing engines-somewhat like the Forerunner V business jet proposed somewhere on the forum (afterburning ultra-high-bypass turbofan to Mach 8, then LNG scramjet to Mach 15, then switch to LH2 to Mach 20), with a small rocket added for EOI. A metallic TPS should be used.
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Eerie
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« Reply #490 on: 01/03/2009 05:38 PM » |
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The key to a gas-and-go SSTO RLV IMO is airbreathing engines-somewhat like the Forerunner V business jet proposed somewhere on the forum (afterburning ultra-high-bypass turbofan to Mach 8, then LNG scramjet to Mach 15, then switch to LH2 to Mach 20), with a small rocket added for EOI. A metallic TPS should be used.
That is really too complex.
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mlorrey
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« Reply #491 on: 01/03/2009 05:49 PM » |
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What about borane fuels? They were supposed to offer the performance of hydrogen at the density of kerosene, but I believe that their was a problem.
Boranes combustion products tend to be thick sticky/solid boron compounds which are generally impossible to remove from engine surfaces and make turbopumps impossible to use. Kerosene/boron powder slurries offer similar performance without as much coking issues but still should only be used with pressure fed or piston pumps fed engines. If SpaceX used such a mix on, say, the Kestrel pressure fed engine, the Falcon 1 payload performance might be significantly increased.
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mlorrey
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« Reply #492 on: 01/03/2009 05:53 PM » |
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The key to a gas-and-go SSTO RLV IMO is airbreathing engines-somewhat like the Forerunner V business jet proposed somewhere on the forum (afterburning ultra-high-bypass turbofan to Mach 8, then LNG scramjet to Mach 15, then switch to LH2 to Mach 20), with a small rocket added for EOI. A metallic TPS should be used.
That is really too complex.
Agreed. The GTX RBCC launcher (which would have launched by 2006 but was cancelled) would have demonstrated stage and a half air breathing rocket based combined cycle RLV technologies. Personally I'd like to see a more basic POGO-like ram/rocket RLV, though IMHO provided polywell fusion moves along as expected, we'll be seeing fusion SSTO RLVs within a decade.
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khallow
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« Reply #493 on: 01/03/2009 06:28 PM » |
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The key to a gas-and-go SSTO RLV IMO is airbreathing engines-somewhat like the Forerunner V business jet proposed somewhere on the forum (afterburning ultra-high-bypass turbofan to Mach 8, then LNG scramjet to Mach 15, then switch to LH2 to Mach 20), with a small rocket added for EOI. The gravity and air resistance losses would be much greater than for a standard rocket trajectory. But those are known weaknesses. Perversely, using the rocket to accelerate the vehicle to viable scramjet speeds may be better than adding a turbofan. It still means that a majority (as above) of your delta v comes from the scramjet. And you are carrying fewer systems as a result.
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mlorrey
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« Reply #494 on: 01/04/2009 02:28 AM » |
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The key to a gas-and-go SSTO RLV IMO is airbreathing engines-somewhat like the Forerunner V business jet proposed somewhere on the forum (afterburning ultra-high-bypass turbofan to Mach 8, then LNG scramjet to Mach 15, then switch to LH2 to Mach 20), with a small rocket added for EOI.
The gravity and air resistance losses would be much greater than for a standard rocket trajectory. But those are known weaknesses. Perversely, using the rocket to accelerate the vehicle to viable scramjet speeds may be better than adding a turbofan. It still means that a majority (as above) of your delta v comes from the scramjet. And you are carrying fewer systems as a result.
Good point. Also note that air resistance losses are greatly mitigated by use of sharp design. Using SHARP materials for leading edges (hafnium diboride and zirconium diboride as is used in IRV steering fins) allows for mach 7 flight at sea level, mach 11 flight at 100k feet and minimal hypersonic plasma issues.
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