NASA classes MMOD as primary threat to commercial crew vehicles

by Chris Bergin

NASA has claimed the threat of MicroMeteoroid and Orbital Debris (MMOD) – small pieces of space debris capable of punching bullet hole wounds into spacecraft – is the “primary” threat to the safety of Commercial Crew vehicles. The claim was made as the Agency works on closing the gap between the current analysis of the Loss Of Crew (LOC) numbers with the Program’s goal.

Commercial Crew Advancing:

NASA’s latest status overview of its Commercial Crew Program was provided to the Aerospace Safety Advisory Panel (ASAP) during its recent meeting at NASA’s Headquarters in Washington DC.

The meeting saw the chair of the panel change hands, with VADM (Ret.) Joseph Dyer handing over the reins to Dr. Patricia Sanders.

The ASAP is NASA’s primary oversight body focused on crew safety. It was particularly outspoken during the Shuttle extension debate and was actively opposed to any additional missions past Atlantis’ STS-135 swansong.

Since the Shuttle’s retirement, the ASAP has refocused its attention on NASA’s current and future programs involving astronauts, namely the International Space Station and the missions involving Orion and the Space Launch System (SLS), all while playing an active role in the formulation of certification plans for NASA’s Commercial Crew Program.

The panel was skeptical of the link up between NASA and commercial providers during the early days of CCP. However, its opinion has since thawed, not least following the observation of the progress being made between NASA and its commercial partners on elements such as transparency and data sharing – and is on “a trend that the panel appreciates”.

2015-06-10-014231The latest CCP status update was provided to the ASAP by Kathy Lueders, CCP Program Manager, focusing on the two vehicles – SpaceX’s Dragon 2 and Boeing’s Starliner (CST-100) – that will be tasked with launching NASA astronauts to the ISS.

“Both providers are working to the schedule, which is challenging. The next year or so will be a defining moment,” noted the minutes from Ms Lueders’ overview.

“The Program is about to enter the hardware testing phase. At this point, the providers are tracking to schedule and making significant progress.”

2015-06-10-014319While the ASAP will have been pleased by the report card, the panel’s gaze is mainly fixated on paper elements such as programmatic risks.

This includes areas such as “requirements changes” where commercial companies attempt to find a “commonality of process” within NASA’s strict rule book.

Both SpaceX and Boeing have proposed changes to NASA’s “standards and policies” – although no examples were cited to the ASAP. However, the CCP overview noted the requests are currently being evaluated by NASA.

The other item of interest is also related to requirements, with NASA’s standards for crew safety working via a probability matrix known as the LOV/C (Loss Of Vehicle/Crew) ratio.

2016-08-22-221328LOV/C is a product of the Shuttle era, where a number of LOV scenarios resulted in a “no win” situation for the crew, equating to a LOV/C. In a number of cases, that was the fault of the vehicle’s design, such as the lack of crew escape options during key phases of the mission.

Shuttle had improved its rating via the post-Return To Flight (RTF) modifications, allowing for a LOC ratio of “about 1 in 90”. Although the vehicle’s safety was improved throughout the final flights, the options for saving the crew – such as ISS “Safe Haven” – helped improve the LOC ratio.

For the new breed of vehicles, a number of the Shuttle-style threats have been removed, not least during launch.

2016-06-23-002835Should a commercial crew launch vehicle, or indeed SLS, suffer a major issue during ascent, the crew will be pulled free of the failure by the Launch Abort System (LAS) on the crew vehicle.

Ascent and re-entry/landing were two of the three major LOC threats for the Space Shuttle. The third, however, will remain a threat for the commercial crew vehicles, along with Orion and any spacefaring vehicle.

That threat is MicroMeteoroid and Orbital Debris (MMOD) – the unseen threat that all spacecraft need to be protected from, not least because they are impossible to track and thus avoid.

This is in contrast to the larger debris threats that the ISS can dodge via a Debris Avoidance Maneuver (DAM).

Impacts from MMOD strikes are usually noticed when they strike areas such as windows of the spacecraft.

Such strikes were observed on a large amount of Space Shuttle missions, especially late into the mission when the orbiter had undocked from the protection of the ISS and was preparing to head home.

Although spacecraft are designed with a level of protection from such impacts, MMOD was the third biggest threat to losing an orbiter during her mission – second only to launch and re-entry. With the improvements to crew launch via the new era of vehicles, launch and landing are not close to the Shuttle’s danger levels.

During the Space Shuttle era, all of the orbiters would receive flesh wounds from MMOD strikes.

The most serious event was during STS-109 when a small piece of debris was lodged in Columbia’s coolant loop 2 and restricted the flow of Freon-22 in that coolant loop. While the option for an immediate return to Earth was discussed, mission managers opted to press on with what was a successful flight.

Atlantis and Endeavour both suffered “bullet hole” impacts to their radiators, with Atlantis’ damage sustained when she was hit by a tiny piece of circuit board on orbit – likely from a destroyed satellite. The damage held no mission impact and was only noticed once she had returned home and was in post-flight processing inside her Orbiter Processing Facility (OPF)

Extensive evaluations took place ahead of each shuttle mission, not least Atlantis’ STS-125 flight into space, a mission that provided one final servicing mission to the Hubble Space Telescope.

This mission held the largest risk to the orbiter, given it was the only post Return To Flight mission that did not have the “safe haven” option of docking with the ISS, ahead of a rescue orbiter being flown to pick up the crew.

STS-125 would also see an increase in the concentration of MMOD, due to the region of Low Earth Orbit (LEO) Atlantis flew in for the majority of her mission.

The Program Requirements Control Board (PRCB) results – which took into account satellite breakups and a variety of other components evaluated to be in Atlantis’ orbital neighborhood – showed that the overall risk of a Loss Of Vehicle and Crew (LOC/V) scenario due to MMOD impact(s) to the Thermal Protect System (TPS) was 1 in 185, with an error factor of 1.35 based on MMOD distribution, velocity, and density uncertainties.

As noted, most of the risk was associated with impacts on the TPS, as opposed to the windows. In the end, Atlantis completed her mission successfully and without any major MMOD concerns.

2015-05-04-131404Data has already been gained on the Orion spacecraft’s performance, via the Exploration Flight Test -1 (EFT-1) mission.

The review of Orion’s TPS did show a slightly higher than predicted level of MMOD impacts to the outer shell of the vehicle. This was based on computational predictions and post-flight inspections.

However, being a test flight, engineers have taken lessons learned and have already been applying them to the Exploration Mission -1 (EM-1) Orion, with further refinements to the TPS and outer shell of the vehicle.

EM-1 will also be an uncrewed flight, allowing for more refinements head of EM-2 in the 2020s.

2015-05-31-020608MMOD also played a role in dismissing a level of post-Shuttle bravado from the Constellation Program (CxP), which was quick to boast the crews of Orion – launching on the Ares I rocket – would enjoy a LOC of 1 in 1000, based on a 2005 study.

However, as the Constellation system design process began and the program started looking at hazards and threats, they found that 1 in 1000 was going to be an impossible number to meet, not least due to MMOD threats.

The LOC was reduced to 1 in 270 – or three times better than Shuttle at end-of-life – which was still thought to be an exaggeration by the ASAP. However, it was used as a benchmark for the Commercial Crew vehicles, to “ensure an even playing field.”

2016-08-22-224300“NASA needs to be careful as it picks what the controls will be and to be smart about how to make up the gap,” noted the ASAP last year.

“Bottom line, there is still a 1 in 270 requirement; some of that has been allocated to the contractors and some to the Program.”

It is understood both Dragon 2 and Starliner – designed to be extremely safe vehicles – have yet to satisfy NASA on the LOC ratio, again due to the factors surrounding MMOD.

“(There is an) effort involved with closing the Loss of Crew (LOC) gap between what the Program goal is and what the current analysis indicates that the systems will achieve,” noted the minutes from the ASAP meeting during which Ms. Lueders was speaking.

2016-08-22-224421“The primary threat to both vehicles is MMOD damage for the long-duration time in orbit.”

However, there are factors that can help the MMOD threat numbers, such as the period in which the vehicles will be docked with the ISS.

As seen with the recent addition of the new International Docking Adaptor (IDA-2), in the same port the Space Shuttle utilized, the Station can provide a level of protection from MMOD strikes, with the structure of the Station in the direction of travel, effectively providing a shield to vehicles docked to the US segment.

The main challenge will be at the beginning and end of missions, where the spacecraft are alone on orbit.

2016-08-22-224826While Starliner has yet to fly in space, some data will have been gained from the cargo version of Dragon during her missions to the Station, albeit with a different OML (Outer Mold Line).

The key will be to refine the MMOD threat data, which is based on historical flight information and may be – due to NASA requirements – overly conservative.

“The MMOD damage analysis depends on the modeling of the environment, which is in many aspects speculative and quite robust,” added the minutes.

“There are discussions regarding gathering additional historical information to determine if the environmental model is perhaps too robust. All answers are yet to be determined.”

(Images: NASA, L2 Shuttle and numerous L2 Renderings by Nathan Koga. The full gallery of Nathan’s (SpaceX Dragon to MCT, SLS, Commercial Crew and more) L2 images can be *found here*)

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