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The prevalent choice by NASA since Gemini for reaction control jets (and the Shuttle OMS engines) has been Monomethyl Hydrazine for the fuel and Nitrogen Tetroxide for the oxidizer which are "hypergolic" (ignite on contact with each other). The fuel is of no use without the oxidizer. Both are extremely hazardous/corrosive chemicals and require elaborate care when handling (SCAPE suits are mandatory). I have often felt that using such chemicals in the confines of the pod bay would not be a wise choice; that there would have to be a special propellent handling room completely separate from the pod bay, with its own airlock. These chemicals are SO dangerous that any leaks in a confined area would lead to contamination of all exposed surfaces that come into contact with free floating liquid, which in turn would continually outgas from there on out. Even in very small quantities, MMH and N2O4 are extreme health risks; there is no known safe exposure limit. Safer options would be the high energy LH2 and LOX combo, or lower energy (but still considered high) monopropellent Hydrogen Peroxide (H2O2) with a catalyst such as silver. LH2 and LOX are difficult to store, although a long duration mission would certainly already have the equipment to store liquid or solid O2. Hydrogen Peroxide can be stored at room temperature so it does not require heavy refrigeration equipment to liquify it. LH2, LOX and H2O2 are non-toxic, but will severely burn surfaces, requiring SCAPE suits to handle. For comparisons sake, the H2O2 in your medicine cabinet is at most 3% concentration which WILL burn skin. Rocket grade H2O2 is about 90% concentration. But the good thing about H2O2 is that it is non-toxic and will evaporate and separate into H20 and one O-atom. Since the pod is essentially a low-thrust vehicle, H2O2 is perfectly viable. Nitrogen, cold-gas thrusters could also be used (N2 already being available for the spacecraft atmosphere), but this is a very low energy solution perhaps not applicable to a relatively massive pod in close proximity to objects where collision avoidance requires quick changes in momentum.

Another issue is MMH/N2O4 contamination of exposed surfaces from thruster activity. Remember that the pod must use thrusters to get off of the ramp. The gases from the thruster activity impinge on the ramp surface and deflect and spread out into the pod bay through the open pod bay doors. The amounts are sufficient enough to contaminate the pod bay and make it a dangerous place to breath. Even if there was no deflection into the pod bay, there is certainly severe contamination of the ramp upper surface which will continually outgas from then on.

As for the lower engine (even showing up in dramatic use, in Robert McCall's famous pod & Discovery painting), I think there is some misconception about its capabilities. I am praying to the math gods that I have done the math correctly here as I haven't had time to REALLY check these numbers more thoroughly. The pods were really intended only for maintainance tasks outside Discovery and intimated limited recon, according to A.C. Clarke in his novel. A big rocket engine would be used for what is referred to as Delta-V maneuvers; amounts of thrust that are large enough to change the shape of trajectories and orbits. Since the pod is intended for close-range operations well within the proximity of Discovery and limited Delta-V, the size of the engine is limited. In fact, A.C.C. gave the acceleration of this engine as only 1/5g and an unknown pod range, but it couldn't be much given the pod's size, even at 9ft diameter (vs. the movie's 7ft). But we do know Bowman was 50 miles above the surface of Iapetus (roughly 84% of Earth's moon in diameter, but 1/3 its density) and descended to near the surface. That's equivalent to only 6.4 ft/sec^2 engine thrust hovering above the surface, with Iapetus' surface gravity of 0.76378 ft/sec^2 (1/42 g). For comparison, the Lunar Module (with an essentially empty Descent Stage) hovering above the lunar surface has 18 ft/sec^2 maximum acceleration to counteract the moon's 1/6 gravity (5.36 ft/sec^2). Even when hovering above the surface of the small moon, Iapetus, Bowman only has a few minutes of hover time, according to A.C.C. who was quite the mathematician. Also, there simply is no room inside the pod for the amount of propellents necessary to perform Delta-V burns powerful enough to get you anywhere relatively far away, let alone the amount necessary to get you back to the Discovery from such a "long" trip. Therefore, given the movie pod's size of roughly 7ft, it ain't going to stray too far...unless you get hijacked by a Stargate). Consider as well, the volume taken up by life support air tanks, and cooling fluid tanks.

Incidentally, if you haven't wondered about it already, the pod has great translation capability in all three axes, and great yaw and roll control but lowsy pitch control, because there are no thrusters dedicated to pitch (unless they're hidden inside the cone below the pod). This means wasted propellent since it takes far more to fire the other thrusters in a rocking motion fashion to get the equivalent pitch, than just to have dedicated pitch thrusters. The only other possibility is a gimballed engine to handle pitch control.

THX11138 > Yah, I agree. If the pod could be considered as a "Universal Bus" as they call it in the satellite biz, it could be attached to any number of plug-on modules.

EVAPOD man > I agree as well. It is quite common for thruster leaks or stuck thrusters to occur; it's happened during Gemini, Apollo, Shuttle and Soyuz. Doing the external docking thing is the only thing that makes real sense, even for H2O2 or even N2. Why risk a stuck thruster in an enclosed area, or even a burst tank. If an N2 leak is bad enough, it is no less dangerous than fuels and oxidizers, even though it is not intrinsically "poisonous", and can kill you just as quickly. Many people may not recall the deadly incident at KSC on March 19, 1981, before the 1st Shuttle launch. It was just after the Countdown Demonstration Test, when they sent a team of 6 back to the pad. Unfortunately, there was an unforseen problem lurking in the post-test procedures, and they had already started an N2 purge to flush out any possible H2/O2 vapours around the aft engine compartment:

"1981 March 19: anoxia: During preparations for STS-1, at the end of the 33-hour-long Shuttle Dry Countdown Demonstration Test, Columbia's aft engine compartment was under a nitrogen purge to prevent the buildup of oxygen and hydrogen gases from the propulsion system. Six technicians entered the aft engine compartment and five of the six lost consciousness due to the lack of oxygen in the compartment. Two died. John Gerald Bjornstad, a 50-year-old Rockwell employee, was pronounced dead at the scene, and Forrest Cole was brought to the hospital where he later died. The other four workmen were treated and released."

I'm not a doctor (but I do play one on TV), but in my shady and murky memory I seem to recall that if you are not getting enough oxygen, it's not the lack of oxygen that causes you to breath faster, it's the buildup of CO2 that your body senses. The key is the partial pressure of CO2. If you are breathing, say, pure nitrogen (which is odorless), your body is not metabolizing CO2, so the body does not see a problem and you just breath normally as your body slowly purges itself of CO2 with each breath. Very quickly, without O2, you pass out without ever knowing what hit you. I think that was the general idea, anyway.

The second pod (the one that Dave blew the hatch on), could still be recovered if it was in Attitude Hold mode when the hatch was blown. In fact, it was still there when he closed the airlock hatch.

I'm a fence -stradller on this one. Although I agree in principal to leave well enough alone, I really WOULD like to see someone do a good digital rendition of the hatch and other bits and pieces jetting into the airlock. Would be cool, just for the fun of it. Bloodshed not withstanding.

Concerning the pod door, there is exactly what you suggest...a door inside of a door. If you look at most clear shots of the pod back, you can see the seam between inner door and outer frame...it lies right along the outer edge of the E-Bolts red squares. On the interior side, the "seam" is the depression just inside the E-Bolts square inner edges.