Bob B.
Bob the Excel Guru?
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Post by Bob B. on Jun 30, 2010 16:55:26 GMT -4
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). Just to be clear, I mean that the APS oxidizer tank was equal in size to the APS fuel tank, and the DPS oxidizer tank was equal in size to the DPS fuel tank, etc. I don't mean that the APS tanks were equal in size to the DPS tanks which were equal to the SPS tanks.
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Post by banjomd on Jun 30, 2010 17:46:52 GMT -4
Sorry for the nitpicking, Bob! Your explanation for the asymmetric LM is correct (explained in Tom Kelly's book.) Ever since I read that book, I get chills when I think of the LM crew standing next to a firing rocket engine!
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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 19:46:13 GMT -4
I've checked some of my resource books and I've found that some of my previous comments were correct and some were not. So let's set the record straight.
My description of the LM was correct. The tanks were arranged as described and the fuel and oxidizer tanks were of equal capacity. Both the APS and DPS used nitrogen tetroxide and Aerozine 50 at a mixture ratio of 1.60. The RCS used the same propellant but at a mixture ratio of 2.0. The RCS was interconnected with the APS propellant tanks and could run off that supply while the APS was operating.
I didn't do as well remembering the layout of the SPS; furthermore, my resources give conflicting information. There were six sectors inside the Service Module with four of them containing the propellant tanks. There were two smaller tanks and two larger ones with the small tanks across from each and ditto for the large ones. The angle between the tanks was 65 degrees. The propellant was nitrogen tetroxide and Aerozine 50.
The conflict is in regard to which tanks contained oxidizer and which contained fuel. The Apollo Spacecraft News Reference states that "the fuel tanks have a smaller diameter." On the other hand, Virtual Apollo by Scott Sullivan indicates that one of each size tank contained oxidizer and one of each fuel. If the former, the mixture ratio was 1.97, and if the later, the mixture ratio was 1.60. Scott gives pretty detailed information, including the capacity of each tank, so I find it difficult to believe his facts could be wrong; however, if his description is correct, then the SM propellant was unbalanced about the centerline. If the former source is correct, than the propellant mass was balanced.
The SM RCS also used nitrogen tetroxide and Aerozine 50, but the CM RCS used MMH for the fuel.
Does anybody have futher information that could resolve the conflict?
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Post by banjomd on Jun 30, 2010 23:32:30 GMT -4
"The two-tank design gave the LM ascent stage an unusual asymmetric appearance because, to maintain the center of gravity on the centerline of the rocket engine, the distance between the fuel and oxidizer tanks had to be in inverse ratio to their weights when fully loaded. (This ratio is the product of the oxidizer/fuel [O/F] density ratio times the O/F ratio required for proper combustion.) The result, Dandridge admitted, looked like the LM had the mumps-on one side only." Moon Lander; Thomas Kelly 2001 p60 Sorry I couldn't find mention of the tank size.
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Post by Tanalia on Jul 1, 2010 2:47:27 GMT -4
Everything I've been able to find agrees with your Virtual Apollo description (also the Wiki Apollo Command/Service Module page), one large (sump) tank and one small (storage) tank for each of oxidizer and fuel, giving the 1.6 ratio.
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Post by echnaton on Jul 1, 2010 9:59:54 GMT -4
Everything I've been able to find agrees with your Virtual Apollo description (also the Wiki Apollo Command/Service Module page), one large (sump) tank and one small (storage) tank for each of oxidizer and fuel, giving the 1.6 ratio. What is the difference between a sump tank and a storage tank?
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Bob B.
Bob the Excel Guru?
Posts: 3,072
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Post by Bob B. on Jul 1, 2010 10:44:36 GMT -4
Everything I've been able to find agrees with your Virtual Apollo description (also the Wiki Apollo Command/Service Module page), one large (sump) tank and one small (storage) tank for each of oxidizer and fuel, giving the 1.6 ratio. From what I recall, that Wiki page mirrors what I read in Virtual Apollo. I've run some numbers and the center of gravity offset isn't as bad as I feared it might be. I don't have all the dimensions, but I estimate the center of each tank was about 45 inches from the SM centerline. This places the propellant CG about 7.4 inches off center in the direction of the oxidizer tanks. Of course, this is the CG of the propellant only. When we have a complete stack of SM, CM and LM, the SPS propellant is only about 40% of the total mass. If all the other mass is balanced around the centerline, then the CG is offset about 2.9 inches when fully fueled. As propellant is burned, the CG moves closer to the centerline. When the LM is separated, the CG would suddenly shift away from the centerline as the propellant now becomes a larger percentage of the remaining mass. However, by the time of LM separation about 2/3 of the SPS propellant had been consumed. With the lighter fuel load, the CG offset would be about 2.5 inches. Assuming I haven't messed up my calculations, it seems the CG offset was somewhere between 0 and 3 inches depending on how much propellant was remaining and whether or not the LM was attached. This doesn't seem to me to be a huge problem. The SPS engine could be gimbaled, so surely small adjustments could be made to account for the shifting CG.
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Post by Tanalia on Jul 1, 2010 18:43:05 GMT -4
A sump tank is really just a storage tank with a defined purpose. In the case of the SPS, the sump tank was the one that fed the engine, the storage tank was used to keep the sump tank filled as long as possible.
As for the center for gravity, also note that the CM itself was a little off-balance (to provide a tilt for aerodynamic control during reentry), and I would assume was mated to the SM with that in mind.
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Post by ka9q on Jul 2, 2010 20:08:44 GMT -4
What is the difference between a sump tank and a storage tank? The Apollo SM had four SPS propellant storage tanks, two each for fuel (Aerozine-50) and oxidizer (nitrogen tetroxide). Each propellant had a "storage tank" and a "sump tank" arranged in series so that the sump tanks directly fed the engine and the storage tanks fed the respective sump tanks. In the drawings that I've seen, the bottom of each storage tank fed the top of the sump tank through a long tube in the latter tank. Helium pressurized the top of the storage tank through a long internal tube so it would drain first. The sump tank stayed full until the storage tank emptied. Then the storage tank would feed helium to the sump tank, and it too would drain. I wondered why the two tanks weren't simply interconnected at the bottom, but then I realized why. When the storage tank emptied, helium began to flow through the interconnection. If not for that long tube, the helium would vigorously bubble up from the bottom of the sump tank and possibly be ingested by the engine. The tube ensured that the helium came out in the ullage space at the top of the sump tank and the engine drew solid liquid.
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Post by ka9q on Jul 2, 2010 20:18:11 GMT -4
Jay, what's a "cold fire"? I know it's practice to test at least some hypergolic engines (e.g., the 400N MBB thrusters in the amateur Phase III satellite series) by putting isopropyl alcohol in the tanks, loading the helium tank, and "firing" the engine. I'm not sure if they expended a pyro valve, or simply bypassed it for the test after checking that the firing circuit was working.
Although the engine does not actually fire, this tests the major system components - the control circuits, the engine propellant valves and the helium subsystem, especially the pressure regulator. And it provides an opportunity to check for leaks.
Isopropyl alcohol is used because its density is similar to hydrazine and it's non corrosive (though it is hygroscopic, so you have to be careful not to allow water into the system.)
Is this what you mean by a "cold fire"?
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Post by ka9q on Jul 2, 2010 20:31:03 GMT -4
Come to think of it, I don't think a cold fire test was performed on the Phase III-D satellite launched as Oscar-40 in 2000. Had one been done, it would have detected the "remove before flight" cap left on a vent port in the helium valves.
These pressure-fed engines, though vastly simpler than large launcher engines with turbopumps, are still rather complicated, with various "gotchas" like this one. The solenoid that controls the engine does not directly drive the propellant valves. Instead it controls a helium valve that in turn drives pressure-operated valves that control the propellants.
The cap in question was on a vent that relieved helium pressure in the operating line to the oxidizer valve when the control valve was in the non-firing position. So when the engine was commanded to fire, it fired normally. But when it was commanded to stop, the oxidizer propellant valve remained open; the helium holding it open could not vent to space. Because the engine was hot from having been fired, the pure oxidizer rapidly corroded and burned through the combustion chamber, streaming into the spacecraft. This was followed by pressurized helium when the oxidizer tank emptied. It blew off the bottom panel of the spacecraft, tearing some cabling.
The amazing part is that one transponder on the spacecraft continued to function for several years after this incident.
The point of all this? Take the claim that hypergolic engines are "simple" with a large grain of salt. And never forget that there's simply no alternative to testing...
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Post by echnaton on Jul 2, 2010 23:14:40 GMT -4
Thanks guys for the answers to my question. It makes a lot of since.
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Post by JayUtah on Jul 15, 2010 10:48:30 GMT -4
Is this what you mean by a "cold fire" Yes, that's it.
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Post by ka9q on Jul 18, 2010 16:47:47 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!) Jettisoning the descent stage would also discard most of their batteries, water and oxygen supply. That was not an option...
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Post by banjomd on Jul 19, 2010 6:24:27 GMT -4
Doh! (the other choice would be to take the chance that the SPS engine would work properly) Die if you don't, possibly if you do!
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