LMDE Fuel & Oxide Venting

[philwebb59]

Does anybody know how the fuel and oxidizer were vented from the lunar module descent engine? Was there a separate outside vent for each tank, or did they purge down the throat of the engine?

Also, all the LM Lunar Checklists include the item "FUEL & OXID VENT tb-bp" in the Powerdown Switch Configuration. What does the "tb-bp" stand for? An example of what I'm talking about is item 6 in the following checklist for Apollo 16.

history.nasa.gov/alsj/a16/a16surf1-6_7.jpg

[ajv]

What does the "tb-bp" stand for?

Probably "Talkback - Barber Pole" where the indicator shows the diagonal stripes.

[philwebb59]

That makes sense. tb-bp probably meant "check the talk back and cycle until the gray flag appeared."

On page 20 of a LM Orientation manual (link below) I found a mech drawing of the descent propulsion system. It shows squib valves on the helium side, which I assume fed a bladder in the tanks, and on the other side of the heat exchanger. The drawing is probably not very complete, since I know there was an inlet valve, not shown, that required a special coupling, that wasn't flown, to fill the tanks. So, it's possible an exhaust valve was also present.

er.jsc.nasa.gov/SEH/lemanual.pdf

[Bob B.]

Mar 20, 2010 17:33:17 GMT -4 philwebb59 said:

Does anybody know how the fuel and oxidizer were vented from the lunar module descent engine? Was there a separate outside vent for each tank, or did they purge down the throat of the engine?

According the engine schematic, there was a vent on the helium supply side of the system. The helium used to pressurize the tanks passed through quadruple check valves and then to the propellant tanks. The check valves protected upstream components against corrosive propellant vapors and prevented hypergolic action due to backflow from the propellant tanks. Between the check valves and the propellant tanks were a tees that branched off and then split with one line running to a pressure relief valve and the other to a vent valve (piped in parallel). The relief valve protected against over pressurization. The vent valve was used to vent both propellant fumes and helium. There were two relief valves and two vent valves, one each on the oxidizer side and again on the fuel side.

[ka9q]

The helium pressurization line to each set of tanks (two each, fuel and oxidizer) also fed a squib (pyro) valve followed by a solenoid valve and a vent line. These two valves were in parallel with another series combination: a blowout disc that ruptured at 288 psi followed by a reseating overpressure valve that cracked at 260 psi, reached full flow at 275 psi and reseated at 254 psi.

Each vent/overpressure valve combination had its own separate overboard vent. I presume they were placed as far apart from each other as possible to keep fuel and oxidizer vapors from reacting.

To vent the system after landing, the crew first fired the two squib valves with a single switch. Then they could individually open the two solenoid vent valves to bleed down everything in the DPS: the gaseous helium bottle, the supercritical helium tank and the ullage spaces in all four descent propellant tanks.

Operating either the fuel or oxidizer vent valves would vent the two helium tanks, assuming of course that their individual isolation valves had both been fired (which they would have been before the powered descent). But because of the check valves the fuel tank ullage could only be vented through the fuel vent, and the oxidizer tank ullage could only be vented through the oxidizer vent.

I'm pretty sure the oxidizer and fuel electromagnetic vent valves were opened and closed separately to further reduce the chances of an unwanted oxidizer/fuel reaction after landing.

[ka9q]

Mar 21, 2010 11:32:04 GMT -4 philwebb59 said:

which I assume fed a bladder in the tanks

I don't think the propellant tanks had bladders. They weren't needed as the RCS engines could be used to perform an ullage burn to settle the propellants in the tanks. So only the RCS propellant tanks needed bladders.