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Post by JayUtah on Jun 27, 2010 10:53:05 GMT -4
A 10-15 second qualification firing on a single-engine test stand for each engine flight article is common in the industry if the engine is human-flight rated. We no longer hot-fire entire stages as a flight qualification test.
Each APS flight article was cold-fired. But because corrosion in the valve seat sealant material (either Nylon or Teflon) caused by oxidizer vapor the APS was certified for human flight only for a 3.5-day flight life after first fire. Because the APS is confined within the LM interstage space, propellant leakage constraints were extremely high in the design requirements. The only way to satisfy the leakage limits in light of the propellant valve-seat behavior was to forego a hot-fire flight qualification test.
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raven
Jupiter
That ain't Earth, kiddies.
Posts: 509
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Post by raven on Jun 29, 2010 17:31:11 GMT -4
Whoa, that makes the repeated firing of the Lunar Module decent stage that Apollo 13 undertook all the more tense.
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Post by banjomd on Jun 29, 2010 22:35:47 GMT -4
Great point, Raven! Now does anybody know if the LM DPS engine was ever tested for restarts? (I suppose if it failed they would've jettisoned the descent stage and used the ascent engine; but no throttle, then!)
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Post by PhantomWolf on Jun 30, 2010 7:04:02 GMT -4
Great point, Raven! Now does anybody know if the LM DPS engine was ever tested for restarts? (I suppose if it failed they would've jettisoned the descent stage and used the ascent engine; but no throttle, then!) They used the Desent Engine multiple times, including a firing while attached to the CSM, on Apollo 9.
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Post by drewid on Jun 30, 2010 7:13:47 GMT -4
Completely different engine design, (it had a throttle for a start, the first manned-flight rocket engine to do so IIRC) , and it used a different fuel/oxidiser mix.
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Bob B.
Bob the Excel Guru?
Posts: 3,072
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Post by Bob B. on Jun 30, 2010 8:37:13 GMT -4
Completely different engine design, (it had a throttle for a start, the first manned-flight rocket engine to do so IIRC) , and it used a different fuel/oxidiser mix. Different from what? What engines are you comparing to each other with that statement? I assume one is the LM DPS, is the other the APS? Regarding your comment about the fuel/oxidizer mix, do you mean a different fuel and/or oxidizer or do you mean a different mixture ratio using the same fuel/oxidizer? I don't think I agree with your fuel/oxidizer comment but I need to know specifically what you're talking about before I reply.
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Post by banjomd on Jun 30, 2010 8:54:53 GMT -4
Great point, Raven! Now does anybody know if the LM DPS engine was ever tested for restarts? (I suppose if it failed they would've jettisoned the descent stage and used the ascent engine; but no throttle, then!) They used the Desent Engine multiple times, including a firing while attached to the CSM, on Apollo 9. Whoops; forgot 9!
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Post by banjomd on Jun 30, 2010 8:59:10 GMT -4
Completely different engine design, (it had a throttle for a start, the first manned-flight rocket engine to do so IIRC) , and it used a different fuel/oxidiser mix. Wasn't the X-15's XLR99 engine the first throttlable man-rated rocket? Found a fact sheet: www.nationalmuseum.af.mil/factsheets/factsheet.asp?id=890
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Post by JayUtah on Jun 30, 2010 10:47:27 GMT -4
Whoa, that makes the repeated firing of the Lunar Module decent stage that Apollo 13 undertook all the more tense. Not really. Pressure-fed hypergolic engines would have to be very badly designed not to be restartable. Typically you have an initial-arm operation where the pressurant is released to the tanks: a pyrotechnic valve or disc is fired that has kept the pressurant at bay during launch and initial flight. The pressurant arm valve is usually an extremely tight seal and prevents the fuel system from being pressurized prematurely. You typically have manifold isolation valves if you have multiple propellant tanks. They are normally open and stay open during nominal operation. The reaction valves are the key on-off valves for the system, and these are those whose valve seats suffer from prolonged contact with nitrogen tetroxide. To keep them pliable in a vacuum they have to be made of synthetic elastomers that don't often have good chemical inertia properties. In the APS you have a flow regulator orifice, but in the DPS the pintle injector that acts as the throttle performs the task of regulating fuel flow rates under varying pressure. All human-rated engines need favorable blowdown characteristics. "Blowdown" means varying circumstances of propellant exhaustion. If you run the engine until the propellant or pressurant is exhausted, it has to shut down cleanly. To keep combustion from creeping around the system under low injection pressure you need check valves whose seats must also be rated for exposure to oxidizer. The bottom line is that once oxidizer first touches some of those valve seats, you have only 3-4 days of safe operation before the valves exceed functional limits on leak rate, and possibly 7 days longer before they can no longer retain propellant pressure.
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Post by banjomd on Jun 30, 2010 11:02:28 GMT -4
Thanks, Jay. Very informative.
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Post by drewid on Jun 30, 2010 13:09:14 GMT -4
Completely different engine design, (it had a throttle for a start, the first manned-flight rocket engine to do so IIRC) , and it used a different fuel/oxidiser mix. Different from what? What engines are you comparing to each other with that statement? I assume one is the LM DPS, is the other the APS? Regarding your comment about the fuel/oxidizer mix, do you mean a different fuel and/or oxidizer or do you mean a different mixture ratio using the same fuel/oxidizer? I don't think I agree with your fuel/oxidizer comment but I need to know specifically what you're talking about before I reply. Yeah, APS vs DPS and different fuel and oxidiser ratio of the same fuel and oxidiser, wasn't it? Brain fried, too many 80 hour weeks. edit - Ah - I see I'm wrong then, same fuels + helium. Also been reading some interesting stuff on the two horse race to make the throttleable engine. Fascinating that it seemed to come down support and management rather than the actual designs (which were both equally valid).
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Bob B.
Bob the Excel Guru?
Posts: 3,072
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Post by Bob B. on Jun 30, 2010 14:11:31 GMT -4
Yeah, APS vs DPS and different fuel and oxidiser ratio of the same fuel and oxidiser, wasn't it? I'm going from memory, but I'm almost certain the APS and DPS used the same propellants - nitrogen tetroxide and Aerozine 50. In fact, it's my recollection that the entire Apollo spacecraft used this propellant - APS, DPS, SPS and RCS. Regarding mixture ratio, I think it was probably the same for all the main engines, though I can't swear to it. One of the advantages of Aerozine 50 (and also MMH) is that the optimum mixture ratio is very close to the oxidizer/fuel density ratio - about 1.6. This means that tanks of the exact same size can be used for both oxidizer and fuel. When the tanks are filled to equal volume, you'll have the correct mass of each propellant to operate the engine at the optimum mixture ratio. This is a common practice to simplify manufacturing. From what I recall of the Apollo schematics, I believe equal size tanks were used for the APS, DPS and SPS (don't know about the RCS). In the case of the DPS and SPS, there were two oxidizer and two fuel tanks. The oxidizer tanks were placed equidistant from the spacecraft centerline and 180-degrees apart. The two fuel tanks were similarly arranged and offset 90-degrees from the oxidizer tanks. This arrangement balanced the mass around the spacecraft center of gravity centerline so that no moment was produced. In the case of the APS, there was only one oxidizer and one fuel tank. In order to balance the mass around the center of gravity centerline, the lower mass fuel tank had to be placed further from the spacecraft centerline - about 1.6 times further. This is what gives the LM ascent stage its distinctive unsymmetrical appearance.
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Post by drewid on Jun 30, 2010 15:06:48 GMT -4
Fantastic, thanks Bob. I love this place
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Post by banjomd on Jun 30, 2010 15:09:49 GMT -4
...In order the balance the mass around the center of gravity... ?Centerline/thrustline?
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Bob B.
Bob the Excel Guru?
Posts: 3,072
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Post by Bob B. on Jun 30, 2010 16:29:49 GMT -4
...In order the balance the mass around the center of gravity... ?Centerline/thrustline? Good catch, I misspoke. The propellant mass is balanced around the spacecraft's centerline. Any spacecraft must be packaged so that the thrustline runs through the center of gravity, and the the thrustline generally corresponds to the vehicle's centerline. Since the propellant is such a large portion of the total mass, you want it balanced around the centerline so it doesn't cause a big offset in the location of the center of gravity in relation to the centerline.
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