ISS partners – NASA and Roscosmos – have conducted a bilateral Technical Interchange Meeting (TIM) to refine procedures relating to the disposal of the Station at the end of its service life, or in the event of an emergency. The deorbit burn capability – which won’t exist until at least 2017 – requires multiple docked vehicles firing in unison to push the Station to its fiery demise.
ISS EOL:
Providing the ISS can avoid a critical failure, the Station still has many years ahead of her before the day comes when a deorbit burn will be the only remaining option.
The Station is expected to continue operations until at least 2024, a date that could be extended as far as 2028. However, the financial and logistical support from its international partners may eventually be overtaken by the condition of critical hardware required to keep the orbital outpost operational.
Technical evaluations have shown the Station is likely to be able to press through to 2028, around the time NASA is expected to start dedicating major elements of its funding towards missions to Mars.
While the hardware-limiting factor requires no immediate demand for a deorbit plan, an emergency scenario – where the Station becomes crippled and has to be evacuated – would potentially call for its disposal within a relatively short timescale.
NASA’s Aerospace Safety Advisory Panel (ASAP) members have taken a keen interest in the safe disposal of the Station, known as the EOL (End Of Life) game plan.
Back in 2013, the ASAP heard from (former) ISS Program manager Mike Suffredini, who described the evaluations on what would be an EOL scenario for the Station.
“NASA now has a plan so that in the event the Station must be evacuated, there will be a 14-day period in which to make a decision on whether or not to bring the ISS down,” noted Mr. Suffredini at the time. “The Program is setting the contingency plan in place, although there is still a lot of work to be done.”
That initial plan called for a period of 180 days to allow the Station to decay its path towards the deorbit altitude.
During this period, Russia would launch two Russian Progress vehicles to autonomously transfer propellant to the Service Module thrusters and to prepare themselves to provide additional deorbit propulsion.
However, further evaluations noted this was a simplified emergency plan and further considerations had to be made to ensure the huge structure would be destroyed over a safe area of ocean.
Almost three years later, the ASAP heard of a new TIM-level meeting between NASA and Roscosmos, held at the Johnson Space Center (JSC) at the end of April.
Both agencies have created a co-written “Strategy Document” and a “Contingency Action Plan”, which they will continue to refine before signing a multilateral agreement on the EOL strategy.
“The old adage of what goes up must come down is equally applicable to the ISS as it is to a baseball,” noted the ASAP at the latest meeting conducted at the Marshall Space Flight Center this week.
“So by in large, preparing for the process we will undergo when EOL occurs – or how to respond in an emergency – we felt was extremely important.”
Classing the TIM as a major milestone, the ASAP members were told that the two key partners – NASA and Roscosmos – are now in unison on the EOL requirements, while the other ISS partners have also agreed to the forward plan.
Work that will be evaluated over the near-term include estimating the footprint size of the debris field, which will be better known per the final burn Delta-v parameters. The aim is to “minimalize” the potential of surviving debris impacting populated areas.
Additional work is required on the attitude thrust control estimates, given attitude control will be a key player in the final moments of the ISS’ life.
Other elements that were provided out of the TIM include a NASA requirement for the avionic systems to be in a working condition for at least six months after a crew evacuation. Also, work is being conducted on mitigating against onboard propellant “freezing” in the cargo area of the Station.
Multiple Russian vehicles will be involved with pushing the Station towards its demise, via the deorbit burn. Previous plans called for two Progress cargo vehicles and the Station’s own Service Module thrusters working together to shove the giant structure out of orbit towards entry interface.
However, the inclusion of a Soyuz vehicle has also now been noted, not least based on the emergency scenario where the docked Russian vehicles at the time of the EOL call will be tasked with a major part of the deorbit requirement.
Roscosmos has said it will aim to ensure additional Progress vehicles could be launched and docked to ISS following a contingency call, in tandem with any subsequent evacuation conducted via Soyuz.
The Russians have noted they will implement certain software modifications necessary to allow integrated burns of multiple engines on ISS, as well as the final deorbit burn sequence from the Progress and potential Soyuz vehicles docked on the Station. at the time of any incident.
Roscosmos will also conduct further analysis of the “dual-axis burn concept” that will be employed to deorbit the Station.
This forward work will take some time to complete, meaning the ISS does not currently have the capability to safely deorbit.
It is expected that gap in capability will be mitigated by September 2017. However, the ASAP are pleased by the progress made on a subject matter that has been on great interest to them for some years.
“There is no question that the TIM and the agreement that was reached is an absolute victory for the entire program and the safety planning for this major system.”
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