Supercritical Helium

In previous posts I showed that the Apollo 10 descent stage orbit was stable over decades and that the lunar atmosphere could not have slowed the stage significantly. Is the stage still in lunar orbit today?

It seems clear that an inert object would still be in orbit today, but the stage was hardly inert. It was much closer to a flying bomb, with over 8 tons of highly reactive propellants, plus a tank with 40-odd pounds of liquid helium, which was slowly warming up, slowly increasing its pressure, slowly approaching the breaking point. Let's look at what would have happened next.

Film taken as it was jettisoned shows the ladder and footpad of the stage. Note the "upside down" lunar horizon above the pad.

To understand what might have happened, we need to understand the design of the descent engine. To keep things simple, reliable, and light, the Descent Propulsion System (DPS) employed a pressure-fed system. Helium gas was used to pressurize the propellant tanks, so that when valves were opened the fuel and oxidizer would flow into the combustion chamber. The propellants were hypergolic, so they would burn as soon as they came into contact. The image at right, excerpted from this presentation, shows a simplified view of the system.

To save weight, the helium was not stored as a gas. That would have required a very heavy tank, able to withstand very high pressure. Instead it was stored as a "supercritical" liquid, at very low temperature and modest pressure, inside an insulated tank along the lines of a big thermos bottle. This worked as a lightweight way to store the helium, but it was not designed to work for a long time. Heat would leak into the helium tank during the mission, raising its temperature and pressure. Eventually this rising pressure could cause a "burst disc" safety valve to open, and the helium would vent out to space.

On a normal mission the engine would be fired long before the pressure reached the breaking point. Once on the lunar surface the extra helium would be vented. For Apollo 10 most of the original helium was still in the tank when it was cut loose. (The Mission Report states that 44 pounds were loaded at launch, and the DOI and phasing burns consumed only 4% of the fuel.) Sometime after staging, the pressure would have climbed to the breaking point. How long did that take?

The pressure inside the helium tank was monitored by mission control, and the Mission Report states that the pressure was rising at 5.9 psi per hour after launch. A report on the DPS showed that the tank pressure at the end of the phasing burn was at 1160 psi (and still rising, due to the way fuel was piped through the helium during the burn) and that the burst disc was designed to open at 1881-1967 psi. From this it seems very likely that Snoopy's descent stage vented the helium between 4 and 6 days after staging, sometime between May 26-28, 1969.

How did the venting affect the orbit? Could it bring the stage down? This document can help answer the question. Figure 9.1-3 in the document, copied below, shows the time and force for a full tank to vent, and the resulting impulse. The total impulse to vent a full tank is ~1700 lb-secs, over the course of 120 seconds. Let's ignore the fact that the stage is tumbling, and assume this impulse all contributes to a change in velocity. With the stage weighing a total of 21,000 pounds, the venting could change the velocity by 2.6 feet per second at most.

This graph shows the thrust generated by helium as it vented to space after blowing the burst valves

Keep in mind, the stage was moving about 5600 feet per second in it's orbit. So even if this 2.6 fps change was exactly aligned to the direction of motion, it's much less than the variation I simulated in my Monte Carlo run. I simulated velocity variations of plus or minus 65 fps, and all the runs stayed in orbit for at least 10 years with no sign of decay. Furthermore, since the stage was tumbling in orbit, the overall impact of the venting would certainly have been much less than 2.6 fps. As it spun around, forward thrust would cancel out backward thrust. It seems very likely that the overall change in velocity was less than 1 fps.

There is another data point to consider. During Apollo 13, as they were returning to Earth using the LM as a lifeboat, the descent helium tank vented. (In addition to the serious problem Apollo 13 faced, this tank apparently had an insulation problem, and warmed up faster than it should have. It vented about 109 hours into the mission.) Although the venting did affect the roll rate of the combined LM/CSM stack, it did not affect the trajectory in any substantial way, and no final course correction was needed. This is good evidence that venting the tank would not strongly affect Snoopy's orbit.

My conclusion is that the venting of the helium didn't bring down the stage. Now we are left with a mere 8 tons of hypergolic propellants, held back by aging valves and decaying seals, and exposed to pure hot sunlight for one hour, then to the freezing black of space the next, orbit after orbit after orbit. In an upcoming post I'll examine what might have happened to all this fuel.

Introduction

The descent stage of the Apollo 10 Lunar Module ("Snoopy") may still be in lunar orbit today. This defies conventional wisdom. It goes against all expectation about how things behave in lunar orbit. It is the last thing I expected to find when I set out to look for an impact crater that I assumed would be the final resting place of the stage. Nonetheless, this is what simulations of the stage orbit show. In this blog I will show how I arrived at this surprising conclusion.

This picture of the descent stage ladder and footpad comes from the 16mm "DAC" film taken on May 22, 1969, during the dramatic moments when the stage was jettisoned. 

Snoopy's tail was jettisoned into lunar orbit on May 22, 1969, during a daring mission that paved the way for the first moon landing less than two months later. Apollo 10 was the first mission to take a Lunar Module to the moon; the first test of all the hardware and procedures. All except landing. It was the first demonstration of Lunar Orbit Rendezvous, the risky, radical, "sine qua non" of Apollo. 

When I started looking for the stage, I was expecting a quick orbital decay. Everything I read said that's what happened. I thought this would mean less uncertainty about the impact point...a smaller search area. So I was disappointed when I started running simulations, showing that the stage stayed in orbit for months. That was not helpful for finding a crater. My remaining hope was that some high piece of lunar terrain might have snatched the stage if it slowly drifted down to lower and lower altitudes. Perhaps I could focus the search on lunar mountaintops. So I kept looking.

The stage orbit was unusual, in terms of Apollo orbits. In order to demonstrate undocking, firing the LM descent engine to approach the moon, and then firing the ascent engine for the rendezvous, NASA had a problem. Without any landing, they needed a way to arrange for the right timing of the maneuvers. The descent would put the LM in a lower orbit, moving it ahead of the Command Service Module. (The "CSM".) Demonstrating the ascent and rendezvous required that the CSM be leading the LM. The solution was the "Phasing" maneuver. This special burn, never performed by any other Apollo mission, would raise the high side of the LM orbit to 190 nautical miles above the far side of the moon, slowing down the LM's orbital period enough to allow the CSM to overtake it.

This plot of the LM position relative to the Command Module, from mission planning documents, shows how the "Phasing" burn pushed the LM into a higher orbit, so that it would drop behind the CM, giving the right alignment for rendezvous.

While the LM was in the Phasing orbit, 12 n.m. at its low point, and 190 at the high point, the descent stage was jettisoned, with an initial velocity relative to the ascent stage of around 2 feet per second. (Both parts were zipping along at a mile a second at this point.) The goal was to kick the stage forward, but unexpected problems with the attitude controls during staging altered this, and the stage was pushed "upward" relative to the local horizontal at the time of staging. (Notice that the moon is "upside down" in the picture above taken during staging...it wasn't supposed to be this way.) Regardless of the extra drama, ten minutes later, the stage was at a safe distance, and the crew fired the ascent engine, slowing their velocity and lowering the high side of their orbit, putting them on track for a successful rendezvous and docking. The stage was left behind in the phasing orbit. It was assumed that this orbit would quickly decay, impacting the moon within days or weeks.

As I starting running simulations of the stage orbit, the hope for a quick demise did not pan out. I ran the simulations out longer and longer, out 10 years, and still the stage kept going. Finally I decided to run the simulation out to the present. This took about 40 hours on my laptop. At the end, the stage remained in orbit all the way to the present, with no sign of decay or orbital instability. As I build out this blog I will share more details, and show you how to try it to see for yourself.