Bob B.
Bob the Excel Guru?
Posts: 3,072
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Post by Bob B. on Jan 18, 2011 15:14:15 GMT -4
Here's what I've decided on...
Command Module * Power: batteries; 3 × 40 Ah, 28 V
Service Module * Power: fuel cells; 3 × 400-1420 W, 2 kW average; 415 Ah battery Apollo 14-17
LM Descent Stage * Power: batteries; 4 × 415 Ah H-series, 5 × 415 Ah J-series; 28 V
LM Ascent Stage * Power: batteries; 2 × 296 Ah, 28 V
Short and simple and, I believe, correct. I'll go into a bit more detail when I get around to revising the specifications web pages.
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Post by PhantomWolf on Jan 18, 2011 17:20:19 GMT -4
I kind of remember there being an incident on Apollo 11 where the LM fuel lines started to freeze while they were on the Moon. It caused some anxious moments before they eventually warmed up and melted prior to ascent. Does anybody recall anything about that and can confirm, deny, or elaborate on the incident? (Had those lines frozen, I bet they wished they used MMH instead of Aerozine 50.) It was in the descent just after the landing. I have a feeling that venting the helium caused the freeze when it sprayed the fuel lines, but couldn't find the information about that. Here's the relevent part from the ALSJ though. ---------- 103:15:44 Duke: Tranquility, Houston. Please vent the fuel. It's increasing rapidly. Over. (Pause) 103:16:00 Armstrong: We show 30 psi in the fuel and 30 in the oxidizer (which are not unusual readings). 103:16:05 Duke: Roger. We're reading somewhat different than that. Standby. 103:16:14 Armstrong: The fuel temperature is reading 64 in the descent, and the oxidizer...(Clarifying what he has just said) That's descent (tank) 2. And the oxidizer is off-scale low. Descent 1 is showing 61 in the fuel and 65 in the oxidizer. 103:16:41 Duke: Roger. Standby. (Pause) Tranquility, Houston. Please take the fuel vent switch and hold it open. Over. [Armstrong - "That switch was momentary to the vent position, which would account for why you couldn't just leave it open."]103:17:01 Armstrong: Okay. We're holding it open. Indicating about 24 psi on board. 103:17:09 Duke: Roger. (Long Pause) 103:17:34 Armstrong: Now indicating 20 psi on the fuel side. 103:17:37 Duke: Roger. 103:17:38 Armstrong: And 22 in the OX. 103:17:41 Duke: Roger. [Comm Break] 103:18:47 Armstrong: Now indicating 15 psi in both tanks. 103:18:51 Duke: Roger. (Long Pause) 103:19:17 Duke: Tranquility, Houston. If you haven't done so, you can release the fuel vent switch now. Over. 103:19:25 Armstrong: Roger. [Comm Break] [Aldrin - (Chuckling) "You were holding the switch all that time?"]
[Armstrong - "Yes."]103:20:52 Duke: Tranquility, Houston. We have an indication that we've frozen up the descent-fuel helium heat exchanger - and with some fuel trapped in the line between there and the valves...And the pressure we're looking at is increasing there. Over. 103:21:10 Armstrong: Roger. Understand. (Long Pause) [Later, the NASA Public Affairs commentator tells the press that there is a small amount of fluid trapped in a line and, if the pressure had continued to increase, the worst that could have happened would have been a small leak which would then have relieved the pressure. A fuller discussion is linked here.]103:22:04 Duke: Tranquility Base, Houston. If you have not done so, please close both Fuel and Ox vents now. Over. 103:22:17 Armstrong: They're closed. -----------
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Post by PhantomWolf on Jan 18, 2011 17:27:21 GMT -4
Still looking for more. They started venting the propellants at about 103 hrs in just after landing.
-------- 103:00:44 Duke: Tranquility Base, Houston. All your consumables are solid. You're looking good in every respect. We copy the DPS (Descent Propulsion System) venting. Everything is copasetic. Over. --------
This was to prevent the pressure build up in the tanks as they warmed from rupturing them, so they had to vent the excess pressure off. Still looking but pretty sure it was something venting onto the fuel line heat exchanger that caused the problem.
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Post by ka9q on Jan 19, 2011 1:47:09 GMT -4
Any gas will cool as it expands adiabatically (insulated from the outside world). The helium in the supercritical helium tank was already extremely cold -- to cram as much in as possible -- and it got even colder as it expanded from the tank during the descent burn.
The purpose of the helium was to pressurize the fuel and oxidizer tanks, to force the propellants into the engine against the combustion pressure. But as the helium cooled, its pressure dropped. To maintain pressure, fuel lines were looped between the regenerative cooling jacket on the engine and the supercritical He tank to carry waste heat from the engine to the He tank. This also helped cool the engine, thus killing two birds with one stone.
After landing, the DPS engine stopped producing heat and fuel stopped flowing through the lines. So when the remaining helium was vented, there was nothing to warm it up. The fuel remaining in the line froze.
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Post by ka9q on Jan 19, 2011 2:59:42 GMT -4
Looks good. I'd make one change here: insert a '+' in front of the battery, otherwise it looks like the fuel cells were replaced with the battery on Apollos 14-17...
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Post by ka9q on Jan 19, 2011 4:04:10 GMT -4
I just checked the Apollo Experience Report on the LM Descent Propulsion System. I had the design of the heat exchanger a little wrong. Fuel did not circulate back to the SHe tank after reaching the engine; it was the helium that circulated back to the SHe tank.
The fuel (Aerozine-50) passed from the tanks through the fuel/helium heat exchanger just once before going to the engine.
The helium passed from the SHe (supercritical helium) tank through a first set of coils inside the fuel/helium heat exchanger, returned to the SHe tank where it passed through a set of coils inside the tank, and then passed through a second set of coils inside the heat exchanger. The helium then passed through filters, valves and regulators to a low pressure manifold from where it pressurized the four propellant tanks through check valves.
The problem on Apollo 11 resulted from the combination of a design flaw and the venting procedure used after landing. Venting the helium manifold allowed the remaining helium in the SHe tank to expand as it passed through the heat exchanger, absorbing heat from the now-motionless fuel remaining there and eventually freezing it. This trapped the residual fuel in the line between the heat exchanger and the (now closed) engine valves. As this line absorbed heat soaking back from the engine, its pressure rose rapidly. The concern was that if the line fractured, it could cause some real damage.
One of the suggestions made at the time was to momentarily fire ("burp") the descent engine to relieve the pressure in the fuel line. While they were debating it in the MOCR, the pressure began to drop. Either the heat exchanger thawed, or the fuel line cracked without doing any damage, but the crisis was over.
On missions after Apollo 11 a bypass line was added to allow some fuel to pass directly from the tanks to the engine without passing through the heat exchanger. This allowed the fuel lines to the engine to vent through the fuel tanks without having to pass back through the heat exchanger should it freeze again. Also, the valve on the SHe tank was closed and the remaining helium left in that tank until right before lunar liftoff; only the helium manifold and propellant tanks were vented right after landing.
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