posted 07-24-2016 01:47 PM               

However, on later Apollo flights and possibly Skylab flights some crews chose to not wear their helmets on reentry as well. This was true for the ASTP crew, which accidentally inhaled toxic fumes from thruster fuel as a result.

posted 07-25-2016 01:01 AM               

On subsequent Apollo flights crewmembers would re-enter without their helmets on.

posted 07-25-2016 09:20 PM            

The plan had always been for all the CDRs and LMPs to be soft-suited (with helmet and gloves) for LM liftoff through docking, was it not?

What changed as a result of the Soyuz 11 accident was that all the crewmembers on Apollo 15, 16 and 17 would now have to be soft-suited (with helmet and gloves) for the LM jettison also.

A third opinion anyone?

posted 07-25-2016 10:10 PM               

To a great extent, the Soyuz 11 tragedy also played on NASA's conscience. Within hours of learning of the disaster, astronauts and managers alike were wondering if exposure to the space environment for three weeks had caused the deaths of Dobrovolski, Volkov, and Patsayev. When decompression and a lack of proper space suits were blamed, a change was made to the Apollo 15 lunar mission, which was scheduled to launch a few weeks later, in July 1971. It was decided that astronauts Dave Scott and Jim Irwin would wear their space suits during their ascent from the lunar surface.

"The decision," read a NASA press release, dated 19 July, "was based on a re-evaluation of the requirements for crew members to wear pressure suits during different phases of the mission. The evaluation was conducted following the Soyuz 11 accident..."

posted 07-25-2016 11:10 PM            

The National Aeronautics and Space Administration, following an evaluation of operational procedures for Apollo 15, has decided that the astronauts will wear their pressure suits during jettison of the lunar module.

This maneuver is scheduled for 6:55 p.m., EDT, August 2, shortly after the lunar landing crew has returned to the command module following their expedition to the surface of the moon.

The decision to have astronauts David R. Scott, Alfred M. Worden, and James B. Irwin fully suited at that time was based on a reevaluation of the requirements for crew members to wear pressure suits during different phases of the Apollo 15 mission.

The evaluation was conducted following the Soyuz 11 spacecraft accident which resulted in the deaths of three Soviet cosmonauts.

The Apollo 15 flight plan had called for the crew to be in "shirt sleeves" (wearing the inflight cover garments) during jettisoning of the lunar module ascent stage from the command module.

The evaluation included a review of the design, test and flight data of the windows, hatches, valves and tubing and wiring that penetrate the cabins of both the lunar module and command module.

In addition, studies were performed on reentry effects on crew and cabin with a completely failed window, structural loading during lunar module jettison, cabin pressure decay caused by various sized holes, suit donning times, and postlanding emergencies.

The results of this review reconfirmed high confidence in the hardware and that operational procedures reduce to a minimum the possibility of damage to critical hardware through incorrect use.

The Command Module has the capability to maintain 3.5 psi for leak rates equivalent to a 1/4-inch hole for approximately 29 minutes and the Lunar Module even longer for an equivalent leak. Suit donning times for an emergency average about 11 minutes for the two crewmen in the Lunar Module to suit-up and approximately 19 minutes for the three crewmen in the Command Module to suit-up.

As a result of the review, NASA concluded that:

• The Lunar Module jettison event will be added to those events already requiring the crew to be fully suited.

• No change would be made in plans for the crew to be unsuited during reentry and splashdown. Although wearing suits would increase safety during reentry down to approximately 50,000 feet, the time from that altitude to the water is insufficient for removal of suits before splashdown. Since the more probable malfunction would occur at water impact, when wearing suits would decrease crew safety, the overall level of crew risk is lower on a nominal mission, by conducting reentry with the crew unsuited.

The reentry event, except for the splashdown phase, is fairly predictable, and stress loads are well within the safety factor of the hardware. The stress loads imposed by the water impact are not so predictable and vary, for example, with wind velocity and direction, wave heights, wave velocity, wave rising or falling and direction.

Therefore, although the probability of a malfunction occurring at splashdown is still low, it is higher than a malfunction occurring during reentry into the earth's atmosphere. In the event that such a malfunction did occur on splashdown and emergency egress were necessary, a suited crew would be handicapped.

Furthermore, should the command module remain upside down, egress through the upper hatch under water would be required. It would be difficult and time consuming to attempt to remove suits in this condition, and if an emergency condition should exist, the crew would be severly (sic) handicapped.

posted 11-13-2018 12:42 AM               

Other concerns included the risk of a pressure seal or valve failing during reentry, or a structural failure of the pressure vessel.

The key point here is risk assessment and risk mitigation. What are the chances of experiencing a cabin pressure loss during reentry? What are the expected results of such a failure? What mitigation steps can be conducted to reduce such risks? What problems do these mitigation steps introduce? Are these mitigation steps worth adoption?

Hindsight is 20/20, and the risks of an Apollo command module pressure failure were calculated, and the management and engineers evaluated these risks as presented. The question can be expanded further into what are the risks of the spacecraft ever experiencing a pressure loss.

The end result was that during the initial launch phase was identified to be the highest risk of a cabin pressure failure, because the rate of change in cabin differential pressure was high, and there were 5 very large and powerful rocket engines accelerating the vehicle up at high rates. meaning the time required to identify and correct a failure was short.

During this phase, the astronauts were pre-breathing 100% oxygen as the oxygen/nitrogen mixed gas was purged from the cabin.

Once the spacecraft orbit was established as safe, and that the cabin air had been purged, the removal of the pressure helmets becomes possible, however this depends also on what the next task requirements was.

If this task involved a risk of pressure loss, the crew kept their helmets on, so docking, undocking, opening the hatch for EVA, etc, a risk assessment is done, and the result actioned.

The chance of pressure loss on orbit or during trans lunar flight had a similar risk assessment done, as did activities on lunar orbit and on the lunar surface. Should a lunar landing be done in shirt sleeves or pressure suits etc.

posted 11-13-2018 03:24 AM               

quote:

---

Originally posted by oly:
...as the crew time of useful consciousness would be low (less than 30 seconds) and donning oxygen masks would be useless during the reentry phase.

---

For aircrew, the time of useful consciousness at 40000 ft is given as 12 secs and above 50,000 feet or so you need the oxygen to be pressure fed so a normal crew O2 mask is not useful.

posted 11-13-2018 03:51 AM               

The Valsalva device was added to later helmet designs.

posted 11-14-2018 02:13 AM               

After Apollo 7 however it was decided that all crews could be in just their "shirt-sleeves" meaning no suits or helmets at all. I have never understood this decision.

One explanation that was offered was a fear of water leakage aboard the spacecraft at splashdown. The suits were considered cumbersome and a hindrance as far as egressing goes. Perhaps this is the on;y explanation but during reentry, a depressurization would have been fatal.

Any thoughts on this?

posted 11-14-2018 07:45 AM               

1. "The Command Module has the capability to maintain 3.5 psi for leak rates equivalent to a 1/4-inch hole for approximately 29 minutes."
   That is more than enough time to complete entry.

2. "...although the probability of a malfunction occurring at splashdown is still low, it is higher than a malfunction occurring during reentry into the earth's atmosphere. In the event that such a malfunction did occur on splashdown and emergency egress were necessary, a suited crew would be handicapped."
   This is a risk assessment. They felt that there was more risk at splashdown than from entry. The confidence in the hull integrity of the spacecraft increased as the mission duration progressed. Leak probability was likely no different during entry than during coast to and from the moon, when the crew was also not wearing a suit. A hull breach from entry gases means that there are other problems that are likely not survivable.

3. Shuttle could deal with a hole even longer. The suits on the shuttle were more for high speed bail out than for depressurization.