by SpaceX is building a souped-up version of its cargo Dragon spacecraft to propel the International Space Station out of orbit for a controlled reentry and breakup over an uninhabited stretch of ocean when the lab is finally retired in the 2030 time frame, NASA and company. officials said Wednesday.
The ISS Deorbit Vehicle, or DV, will be a one-of-a-kind spacecraft that will be necessary to ensure the space station re-enters the atmosphere at the precise location and orientation to insure any surviving wreckage. the re-entry heat will reach 3,000 degrees harmlessly in the sea.
In late June, NASA awarded SpaceX a contract worth up to $843 million to build the deorbit vehicle, which will be owned and operated by the space agency. The heavy lift rocket needed to launch it has not yet been selected, but NASA Administrator Bill Nelson has asked Congress for a total of about $1.5 billion to carry out the entire deorbit operation.
And it is not a trivial matter. The long axis of the space station is 218 feet long, consisting of multiple pressurized modules where visiting crews live and work. The lab’s solar power array and cooling truss, positioned at right angles to the long axis, stretch 310 feet end to end, longer than a US football field.
The entire laboratory complex has a total mass of 925,000 pounds and is moving through space at about 17,100 mph, or 84 football fields per second.
To carefully lower its altitude for a controlled re-entry, the dv will carry about 35,000 pounds of propellant that will power 46 Draco rocket engines, 30 of which will be located in an extended trunk section to perform most of the deorbit maneuvers.
“Once we make the decision to de-verify a station, we will launch the US dv about a year and a half before the final re-entry burn,” said Dana Weigelthe ISS program manager at the Johnson Space Center.
“We’ll dock it to the forward port, do a series of checks and then when we’re sure everything looks healthy and we’re ready, we’ll let ISS start drifting down.”
The crew of the final space station will remain on board until a periodic firing and the ever-increasing “pull” in the upper atmosphere combine to lower the laboratory to an altitude of about 205 miles. That milestone will be reached about six months before the final re-entry procedure.
As the then-unmanned ISS reaches an altitude of about 140 miles, the dv will “do a series of burns to set us up for that final deorbit,” Weigel said. “And then four days later, he’ll do the final re-entry burn.”
The space station’s large solar arrays will break and burn first, along with antennas, radiator panels and other appendages.
More massive components – the lab’s massive power truss and modules – will also break apart in the inferno’s high-speed descent, but a chunk the size of a small car is expected to survive all the way to an ocean splashdown for a slim 1,200 miles long . “seek.”
Remote areas in the South Pacific offer unpopulated splash zones, although a final destination has not yet been specified.
To achieve a precisely targeted entry, “The deorbit vehicle will need six times the usable propulsion and three to four times the power generation and storage of today’s Dragon spacecraft,” said Sarah Walker, a senior manager at SpaceX.
“It needs enough fuel on board not only to complete the primary mission but also to operate in orbit in partnership with the space station for about 18 months. Then, at the right time, it will make a series complex actions over several days to deorbit the International Space Station.”
A deorbiting spacecraft of some sort is needed because even at the space station’s current altitude of 260 miles, there are still trace amounts of atmosphere. As the station flies through that thin material at nearly 5 miles per second, collisions with those particles act to slow the craft slightly in a phenomenon known as atmospheric drag.
Over the life of the program, engines in Russian modules performed periodic thruster flares or attached Progress cargo ships to boost the laboratory’s altitude as needed to offset drag effects. More recently, Northrop Grumman’s Cygnus cargo ships have added modest reinforcement capability.
Without the carefully planned flares, the station would have crashed back into the lower atmosphere on its own.
The station flies over every point on Earth between 51.6 degrees north and south, covering the entire planet between London and the tip of South America. In an uncontrolled reentry, station debris that survived reentry heating could hit the surface anywhere in that area.
Although the chances of an impact in a populated area are slim, nothing as massive as the space station has re-entered and fallen to Earth, and NASA has no chance.
NASA and its station partners – the European, Russian, Canadian and Japanese space agencies – planned from the beginning to deliberately propel the laboratory into the atmosphere at the end of its life to ensure a breakup over an uninhabited stretch of ocean.
The original plan was to use thrusters in multiple Russian Progress cargo ships to lower the altitude of the laboratory and set up a targeted fall to Earth.
“Early in the planning of the station, we had considered deorbiting using three Progress vehicles,” Weigel said. “But the Roscosmos segment was not designed to control three Progress vehicles at one time. So that presented a bit of a challenge.
“And also, the capacity was not what we really needed for the size of the station. So we agreed together to go to the US industry to look at what we could do on our side with for the deorbit.”
Last year, NASA asked for industry proposals and two companies responded: SpaceX and Northrop Grumman. The agency announced last week that SpaceX had won the contract.
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