Tank system for the MLM module
In the spring of 2017, Russian engineers approved a new plan to cleanse the propellant tanks of the MLM module of severe metallic dust contamination. It required a delicate surgery-like dissection of the tanks to cleanse them of dangerous contaminants, while avoiding damage to their irreplaceable parts.
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.
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How Nauka's tanks will be repaired
By mid-April 2017, the MLM project managers at GKNPTs Khrunichev prepared a new repair schedule, which aimed to complete the washing of the contaminated tanks in August.
The key goal of the operation was to thoroughly wash the expanded internal cavity of the flexible metallic bellows, which will contain the propellant, without actually cutting or otherwise disrupting the delicate accordion-resembling structure. (The metallic tape used to build the bellows is no longer produced in Russia.) To remove the flexible structure, a total of four cuts would be made in its holding framework, including two in the bulkhead of the tank and two in the propellant expeller, which is attached to the opposite end of the accordion structure. The first cut would be made in the pressurization cavity of the tank's bulkhead to provide access to the propellant cavity below. After installation of a special fixation ring, the bellows could be removed, expanded and washed. The tank would then be reassembled and seams welded together.
As of mid-June 2017, the actual dissection of the tanks was yet to begin. Because the failure was not an option in the repair effort, engineers at GKNPTs Khrunichev practiced the tank surgery on a prototype before touching the flight-worthy hardware.
After resolving the problem of how to disassemble the contaminated tanks of the MLM module, specialists at GKNPTs Khrunichev began working on the construction of a special rig for automated washing of the internal cavities of the tanks's flexible bellows.
Towering more than seven meters, the system was designed to hold the tank's bellows vertically in fully expanded position. A carousel-like platform holding a spray head, along with a high-resolution camera and a lighting system to document the cleansing operations, will be inserted into the internal cavity of the bellow on a special column connected to a water supply tank. The carousel can rotate 360 degrees and move up and down to wash each of the 310 grooves of the accordion-like bellow.
The assembly of the washing system and its testing was largely completed by the end of July. Next, come the irreversible operations of cutting the flight-worthy tanks to extract their bellows and begin the cleaning effort.
By mid-September 2017, specialists at GKNPTs Khrunichev completed the first test operations of disassembling, washing and re-assembling a prototype simulating the contaminated tank of the MLM module. However, after the final welding, leaks were discovered in the test article. As a result, GKNPTs Khrunichev had to reassess and upgrade its welding procedures, further delaying the repairs of the flight-worthy tanks. Possibly, some additional equipment would be required before welding operations with the tanks could be resumed, industry sources said.
Facing insurmountable problems with repairs of the original tanks on the MLM Nauka module, Russian officials made a decision at the end of March 2019 to use tanks from the Fregat upper stage developed at NPO Lavochkin. That decision was reversed later in 2019.
Known specifications of propellant tanks for the MLM module:
Propellant requirements for the MLM mission: