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  #11  
Old 12-10-2009, 06:50 AM
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Quote:
Originally Posted by wilsotr
Blackshire I'm not sure I understood your first point, but in general you are correct: the higher Isp offered by LH2 is offset, at least in part, by the additional vehicle mass required for high-volume LH2 tankage, cryo insulation, and etc. Drag eats into the Isp advantage too if the vehicle core diameter is increased to grow tank volume - this unless payload requirements drive a large-diameter shroud of some kind anyway. Everything else being equal, a short-burn, high-thrust RP-1 first stage coupled with a high-Isp LH2 second stage (SSME class) would be the theoretically optimum way to maximize mass to orbit.
The exhaust gas of a hydrogen/oxygen rocket engine is very light, and at sea level it encounters the back-pressure of the ambient air. The exhaust of a kerosene/oxygen engine is heavier and is less affected by the surrounding air molecules.

The difference between a non-afterburning turbojet (pure jet) engine and a non-afterburning turbofan (fanjet) engine illustrates this ISP versus thrust situation nicely. A turbojet accelerates a small mass of very hot, low-density gas (fuel combustion products plus air) to high velocity, while a turbofan accelerates a larger mass of relatively cool , denser gas (fan bypass air mixed with the exhaust gas from the core engine) to a lower velocity than a turbojet does.

The turbojet's higher-velocity (but lower mass flow) exhaust is quite efficient at high subsonic velocities and high altitudes, but is inefficient at low altitudes and low airspeeds. The turbofan's high-thrust (high mass flow) but lower-velocity exhaust is more efficient at lower altitudes and airspeeds. A turbofan is not as efficient as a turbojet at high subsonic velocities and high altitudes, which is why (along with lower drag at lower airspeeds) today's turbofan-powered jetliners do not cruise quite as fast or as high as the early turbojet-powered airliners did.

A kerosene/oxygen rocket engine is analogous to a turbofan engine, while a hydrogen/oxygen rocket engine is analogous to a turbojet engine (regarding the engines' most efficient velocity and altitude ranges).
Quote:
Originally Posted by wilsotr
RP-1 infrastructure doesn't exist at LC-39 though, so it would have to be designed and installed. That drives up initial system procurement costs. Then it has to be maintained in parallel with the LH2 infrastructure ... that has implications for long-term recurring costs. If the RP-1 design has a high-enough initial thrust to eliminate the solids there's a savings buried there to offset those, but that's all part of the trade. There is no domestically-produced high-thrust RP-1 engine available. You can continue with the licensing / foreign-produced approach taken for the RD-180, of course, but really need to think about the long-term implications of that for this vehicle and the US industrial base and weigh those against the development cost associated with producing something new.
With RP-1 kerosene, they don't need a fuel infrastructure at LC-39, since RP-1 doesn't require vacuum-insulated cryogenic dewar storage tanks. Supplying RP-1 to a launch complex is more like supplying jet fuel at an airport. Running a pipeline from a tank farm to the launch pad or using tanker trucks to fuel the first stage is all that is needed. (I believe SpaceX's Falcon launch vehicles are fueled with RP-1 from tanker trucks.)
Quote:
Originally Posted by wilsotr
As it turns out, the Isp of SSME's is high enough that a long-burn SSME-core coupled with 5-segment solids produces a vehicle with about the same mass-to-orbit capability as the RP-1 liquid-only vehicle - this without the RP-1 infrastructure and engine development (or licensing) risks, but a whole different set of design issues, procurement and recurring costs. The choice of one or the other as "optimum" isn't very clear until you've done the trades and factored in all those other considerations. Even then it isn't very clear.
The Space Shuttle Main Engine (SSME) achieves adequate all-altitude performance at the cost of a very high chamber pressure (3,000 pounds per square inch [versus only 700 psi for the J-2 engine], if memory serves), which makes the engine very expensive. Even so, it is still less powerful than a kerosene/oxygen rocket engine of comparable size. The SSME produces 375,000 pounds of thrust at sea level and 475,000 pounds of thrust in space. The RP-1/LOX Rocketdyne E-1 (predecessor to the F-1) was about the size and weight of the SSME, but produced 750,000 pounds of thrust at sea level. (Ironically, the E-1 was abandoned in favor of the F-1 because the E-1 wasn't considered to be powerful enough! We could certainly use an RP-1/LOX engine of that size and thrust today.)
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  #12  
Old 12-10-2009, 11:03 AM
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Quote:
Originally Posted by blackshire
With RP-1 kerosene, they don't need a fuel infrastructure at LC-39, since RP-1 doesn't require vacuum-insulated cryogenic dewar storage tanks.


Propellant has to be transferred from wherever it is to the vehicle. That requires pumps, plumbing, a valve complex of some kind, firex, and a control system ... thus the infrastructure. I doubt you could do without a storage tank(s), even if you bought into the complexity introduced by multiple direct tanker offloads, because you'd need a place to offload all that propellant in event of an emergency. The Saturn V held something like 200,000 gallons of RP-1 ... that's a lot of liquid to just dump on the ground.

Quote:
Originally Posted by blackshire
We could certainly use an RP-1/LOX engine of that size and thrust today.)


Yep ... one advantage of the SSME is that it exists. An RP-1/LOX engine of that class would have to be developed or licensed at risk. Again, it's doable, but those development-recurring cost trades (among many others) drive the architecture choices as much as the engineering.
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  #13  
Old 12-10-2009, 11:45 AM
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Quote:
Originally Posted by luke strawwalker
What's amazing to me, in having read and followed a lot of the design issues and tradeoffs over the last couple years or so on nasaspaceflight.com, is just how good Von Braun and his team actually were... they REALLY hit the sweet spot on so many levels... the Saturn V was REALLY a great design... The RP-1 first stage had AWESOME performance and the choice of RP-1 gave it the smallest size possible for a slightly reduced ISP by using kerosene, but which was a good tradeoff considering the superior thrust characteristics blackshire noted above from liftoff until exiting the lower atmosphere. It then staged at just about the perfect point, getting rid of the massive first stage and igniting the high-performance hydrogen fuelled common bulkhead, exceptionally light for it's size second stage to take the vehicle almost to orbit, before staging to the restartable third stage which rapidly completed the ascent before shutting down and later restarting to push the stack through TLI, minimizing the 'dead weight' which directly reduces lunar performance pound for pound...


And it had problems with combustion instability and pogo -- among many others -- which proved exceedingly difficult to resolve. The latter nearly took out Apollo 13 long before the SM LOX tank came apart. Here's a good reference ....

http://ntrs.nasa.gov/archive/nasa/c..._2008018688.pdf

That's not to say SV wasn't a good design, only that it wasn't perfect. Funny how the unpleasant parts of the good ole days seem to fade from memory, huh?

Quote:
Originally Posted by luke strawwalker

Von Braun and his team designed the Saturn V at a time when rocketry was basically in diapers and designed it all without benefit of computers, using slide rules and their considerable talent.



Don't forget national political support and an open checkbook, which were arguably as beneficial as slide rules and talent.
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  #14  
Old 12-10-2009, 12:46 PM
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Acchhhh !
Pogo is a highly OVER-blown phenomenon that has NEVER caused the loss of a spacecraft.
Mix a healthy amount of Pogo in with some RANDOM engine gimbaling and then you would have an exciting, anything BUT boring, flight.
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  #15  
Old 12-11-2009, 03:09 AM
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It came very close on Apollo 6 (S-IC first stage pogo) and Apollo 13 (S-II second stage pogo) to causing aborts.

Apollo 6's pogo was so severe that one of the four "petals" of the fairing that covered the Lunar Module (a mass simulating dummy LM on this flight) was shaken off the Saturn V stack. The damage from the vibration also caused one of the S-II second stage's J-2 engines to shut down early, and the vehicle nearly went unstable when another J-2 on the same side of the S-II as the damaged one also shut down because its control system wiring was mistakenly connected to the damaged engine's wiring. It flew in a severe nose-up attitude due to the thrust imbalance. The S-IVB third stage ignited and injected the vehicle into low Earth orbit, but it failed to re-ignite later because the pogo vibration damaged the hydrogen feed line to its J-2 engine's igniter. The Apollo Service Module engine was used to complete a modified mission profile (the re-entry was slower than the planned 25,000 mph simulated lunar return without the Delta-V that the S-IVB could have provided.

The center J-2 engine on Apollo 13's S-II shut down early after nearly breaking its mounting brackets due to the engine surging and slamming against them several times per second. Had that happened, it would have ruptured the S-II's tankage and caused the stage to explode. The remaining four J-2s burned longer to compensate for the loss of thrust.
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  #16  
Old 12-11-2009, 10:36 AM
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Awww Pshaw !
Minor glitches that provided some excitement to an otherwise boringly nominal flight...
Apollo missions had an expected precision amount of RANDOMABILITY as a built-in FEATURE !
Close only counts in horse-shoes, hand grenades, and themonuclear warfare.
No permanent irreversible harm=NO FOUL !
Good Enuff/Ity'll Doo !
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When in doubt, WHACK the GAS and DITCH the brake !!!

Yes, there is such a thing as NORMAL
, if you have to ask what is "NORMAL" , you probably aren't !

Failure may not be an OPTION, but it is ALWAYS a POSSIBILITY.
ALL systems are GO for MAYHEM, CHAOS, and HAVOC !
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  #17  
Old 12-11-2009, 11:21 AM
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Quote:
Originally Posted by ghrocketman
Minor glitches that provided some excitement to an otherwise boringly nominal flight... [/SIZE][/B]


LOL ... you haven't experienced high-frequency oscillation at 1/2g have you? A friend of mine used the phrase "significant testicular discomfort" when describing it to me ......
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  #18  
Old 12-11-2009, 01:19 PM
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That 1/2g oscillation was uncomfortable because he was not fitted with the proper supportive equipment !!

PO-go is MUCH better than a NO-go !!!!

That was back when NASA did not issue an IDIOTIC delay anytime a gnat lands within 50 miles of the mission director's panel. The only agency I have heard more unique reasons for delays than the Michigan DOT/Road Commission is NASA. If you have seen the roads here in Michigan, that says a LOT !
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When in doubt, WHACK the GAS and DITCH the brake !!!

Yes, there is such a thing as NORMAL
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  #19  
Old 12-11-2009, 06:39 PM
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The KSC gnat exclusion zone is 52.5 miles, expanding to 55.3 as a function of moon phase. Get your facts straight.
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