#11
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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:
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__________________
Black Shire--Draft horse in human form, model rocketeer, occasional mystic, and writer, see: http://www.lulu.com/content/paperba...an-form/8075185 http://www.lulu.com/product/cd/what...of-2%29/6122050 http://www.lulu.com/product/cd/what...of-2%29/6126511 All of my book proceeds go to the Northcote Heavy Horse Centre www.northcotehorses.com. NAR #54895 SR |
#12
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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:
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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Tim Wilson |
#13
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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:
Don't forget national political support and an open checkbook, which were arguably as beneficial as slide rules and talent.
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Tim Wilson |
#14
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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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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, TURMOIL, FIASCOS, and HAVOC ! |
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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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Black Shire--Draft horse in human form, model rocketeer, occasional mystic, and writer, see: http://www.lulu.com/content/paperba...an-form/8075185 http://www.lulu.com/product/cd/what...of-2%29/6122050 http://www.lulu.com/product/cd/what...of-2%29/6126511 All of my book proceeds go to the Northcote Heavy Horse Centre www.northcotehorses.com. NAR #54895 SR |
#16
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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, TURMOIL, FIASCOS, and HAVOC ! |
#17
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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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Tim Wilson |
#18
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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, 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, TURMOIL, FIASCOS, and HAVOC ! |
#19
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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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Tim Wilson |
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