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FGB-2


2013


Russian engineers tackle problems with MLM/Nauka module

In March, Russia's next addition to the International Space Station, ISS, hit another serious snag on the way to the launch pad in 2018. The 20-ton Multi-Purpose Laboratory Module, MLM, remains grounded, while engineers try to deal with a contamination problem in the spacecraft's propellant tanks.

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MLM

The design of the MLM module as of 2016.

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As of January 2017, the flight manifest for the International Space Station, ISS, called for the launch of the MLM module on December 6 of the same year, followed by its docking at the outpost on Dec. 15, 2017. To free the nadir (Earth-facing) docking port on the Zvezda Service Module, SM, for the new arrival, the Progress MS-06 cargo ship was to undock from the Russian segment, carrying along with it the Pirs Docking Compartment, SO1, which has occupied that location since 2001. The Pirs module would then be deorbited and burn up in the Earth's atmosphere over the Pacific Ocean along with Progress MS-06.

Following the integration of the MLM module, on Jan. 23, 2018, the Soyuz MS-06 transport spacecraft was to undock from the zenith (sky-facing) port of the MIM2 Poisk module, make a quick fly around of the ISS and re-dock at the nadir port of the MLM, becoming the first spacecraft parking at the new module.

To integrate the newly arrived MLM module into the Russian segment, Roskosmos promised to bump the Russian crew onboard the ISS back to three people beginning in March 2018. The cosmonauts were scheduled to conduct multiple spacewalks on the exterior of the MLM module, integrating its systems into the Russian segment. A total of 11 spacewalks were planned to accommodate the module: including four ahead of its arrival and seven more after its docking.

Tank problems threaten MLM schedule

By the end of January 2017, it became clear that the December 2017 launch date for the MLM module could not be sustained. The updated processing schedule to be issued by GKNPTs Khrunichev on January 27, was expected to push the launch to June 2018. However, as of mid-February, the status of the processing remained in limbo.

As it transpired, specialists were unable to clean the module's propellant tanks of the contaminants that had also plagued the MLM's propellant lines and valves.

Since 2013, engineers at RKK Energia and GKNPTs Khrunichev in Moscow have labored on replacing a jungle of pipelines, valves and thrusters, comprising Nauka’s sophisticated propulsion system, which got contaminated with metallic saw dust during the botched upgrades of the module.

tanks

The MLM spacecraft carries a total of six 400-liter cylindrical tanks designated 77KM-6127-0. Two of them are used to store high-pressure gas and four others contain fuel and oxidizer under low pressure with a total mass of 2,432 kilograms. This amount is enough for the MLM to make it to the ISS after separation from the third stage of the Proton launch vehicle. In case of a problem during the first rendezvous attempt, the module would still have fuel reserves to back away from the station, conduct an orbit correction and make another fully autonomous rendezvous attempt.

The repair team hoped to wash the tanks off their contaminants, but it had recently been determined that all efforts to cleanse the vessels had failed. To make matters worse, these tanks were built decades ago and are no longer in production. The cylindrical tanks are strapped around the module's exterior like bullets in a revolver drum. Because of their unique design, they fit neatly onto the module, leaving enough room to place radiator panels over them. But it also means that no modern tanks can get in their place without cutting through the radiators!

Fortunately, there is a second set of shorter tanks, which was removed from the module (previously called FGB-2) during its conversion from its original role as a backup to the Zarya FGB, the very first piece of the ISS. To make room on the exterior of the converted FGB-2/MLM module for the attachment of a European-built robotic arm and various scientific instruments, the extra set of shorter tanks was removed.

What are the options?

By March 16, experts at RKK Energia evaluated several options for replacing the contaminated tanks on the MLM module. First they looked whether they could install brand-new tanks developed for the new-generation Science and Power Module, NEM.

NEM

A pair of such vessels could be installed on the MLM to accommodate 880 kilograms of propellant. However, due to their size, the tanks would cut through the radiators and even protrude into the payload fairing of the Proton rocket, protecting the MLM spacecraft during ascent through the atmosphere. Even if the radiators and the fairing were modified, the module would still be lacking 1,291 kilogram of propellant required for its planned mission and contingency situations.

Option 2: Progress tanks

progress

Obviously, it was tempting for RKK Energia to consider fitting MLM with standard 210-liter refueling tanks from the Progress cargo ship. It would be feasible to install 10 such tanks into the MLM module in two groups of five accommodating a total propellant supply of 2,352 kilograms. It would be enough for the nominal mission, but, again, it would be also necessary to cut through radiators and Proton's fairing to accommodate them.

Option 3: FGB tanker tanks

short

Engineers then considered using six short tanks, identical to those used for refueling of the ISS on the Zarya FGB module. Six refueling tanks were removed from the former FGB-2 module, during its reconfiguration into the MLM to make room for attachment points of various payloads, instruments and a European robotic arm. If they were put in place of the contaminated long tanks, the short refueling tanks could hold 2,094 kilograms of propellant, just enough to fly MLM's nominal mission sans the orbital insertion maneuver after the separation from the Proton. Alternatively, the module could conduct the orbital insertion maneuver but would then have no propellant left for a contingency rendezvous maneuver in case of a failure of the original rendezvous attempt.

Option 4: Mix and match

mix

The best option was found to be to combine four (short) refueling tanks and a pair of longer (main) tanks which were left over from the FGB-2 project. Four short tanks would take 1,396 kilograms of propellant and two long tanks would take 808 kilograms. It will bring the fuel cache onboard the MLM to 2,204 kilograms, which would be enough to fly MLM's nominal mission. Most importantly, no changes would be required on the nearby radiators or on the rocket's payload fairing.

A summary of tank replacement options on MLM:

-
Description
Propellant capacity
Hardware modifications
Propellant requirements
1
2 tanks from NEM module
880 kilograms
Required
Not enough (1,291-kilogram deficit)
2
10 tanks from Progress
2,352 kilograms
Required
Enough for nominal mission
3
6 FGB refueling tanks
2,094 kilograms
Not required
Enough with limitations
4
4 refueling 2 main FGB tanks
2,204 kilograms
Not required
Enough for contingencies

 

All MLM tanks turned out to be contaminated. Repair team goes to Plan B

All things considered, the remaining extra tanks from the FGB-2 project could have provided a lifeline for the beleaguered MLM module. That’s is, of course, if they were not plagued with the same contamination problem, which was a million-dollar question.

However by the end of March, it was confirmed that, indeed, the extra tanks had been contaminated as well. As a result, on April 1, the MLM project managers made a decision to default back to the use of six original tanks, which would have to be opened up in order to be cleaned before being restored. The cylindrical metal vessel of each tank was expected to be cut open along its longitudal axis to give engineers the access to its flexible bellows inside. Resembling a harmonica, the bellows are used to force the propellant toward the engines with pressurized gas supplied on its opposite side. The most difficult part of the job would be a thorough cleaning of all the crevices in the internal cavity of the bellows.

The MLM repair team planned to limit the cutting of the tank to its metal vessel structure, which is not in contact with propellant. It will make it possible to extend the flexible internal bladder of the bellows to its full length. With the bellows in its unfurled position, the repair team will be able to thoroughly wash the contaminating metallic dust off its internal cavity.

If the plan works, there will be no need to cut the flexible membrane of the bellows, which will contain the propellant, or the bulkhead of the tank, which holds it.

As a result, it should be possible to conduct the thorough washing of the bellows without breaching the structural integrity of the hermetically sealed propellant-containing vessel.

Of course, in the complex and interdependent world of space technology, nobody can tell right now how long would it take to repair all six tanks, re-assemble the module once again or when it could launch, but it is probably safe to speculate that at least a year would have to be added to the current launch date.

In the meantime, the problems plaguing the MLM also block the path to orbit for the long-completed Prichal Node Module, UM, (prichal is Russian for "pier") and for the new-generation Science and Power Module, NEM. Not to mention, the ISS itself is not getting younger with its retirement projected between 2024 and 2028.

MLM repair scenario charted

By mid-April, the MLM project managers at GKNPTs Khrunichev prepared a new repair schedule, which aimed to complete the washing of the contaminated tanks in August. However, experts working on the repairs said that in addition to cuts of the tanks along their longitudal axis, another opening would have to be made on its bulkhead, which holds the bellows, in order to get access to its internal cavity.

In parallel with the work on the tanks, specialists will proceed with the assembly of the propulsion system, which includes newly manufactured components, experts said.

(To be continued)

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Propellant requirements for the MLM mission:

Orbital insertion maneuver after separation from the launch vehicle
205.4 kilograms
Nominal flight program, including:
1,255.2 kilograms
  • Entering ISS rendezvous orbit
701.2 kilograms
  • Rendezvous and docking maneuvers
554.0 kilograms
Backup rendezvous opportunity
710.0 kilograms
Total (all phases of the flight and backup rendezvous opportunity)*
2,170.6 kilograms

*Emergency deorbiting of the module would require additional 654.2 kilograms of propellant and it would be possible either without an initial maneuver of orbital insertion or before the second docking attempt.

 

Text, photos and illustrations by Anatoly Zak unless stated otherwise; Last update: April 18, 2017

Page Editor: Alain Chabot; Last edit: April 5, 2017

All rights reserved

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IMAGE ARCHIVE

The FGB-2 module, minus its solar panels, sits in the assembly shop at Khrunichev enterprise in Moscow in August 2001. Protective red boxes cover attitude control engines. Copyright © 2001 Anatoly Zak


closeup

A close-up view of the FGB-2's propellant section. Click to enlarge. Copyright © 2001 Anatoly Zak


tanks

Original propellant tanks onboard the FGB-2 module. Click to enlarge. Copyright © 2001 Anatoly Zak


proto

Click to enlarge. Credit: RKK Energia

tanks

A prototype of the MLM module during processing at RKK Energia's Checkout and Testing Station, KIS, in Korolev in 2011. Click to enlarge. Credit: RKK Energia