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Water vs. Land: NASA Weighs Landing Options for Orion Spacecraft
NASA expects to decide sometime in 2008 whether the Orion Crew Exploration Vehicle, the agency?s space shuttle replacement, will typically splash down off the California coast or touch down on dry land when it returns from space.
Time is of the essence because the choice will determine a number of other design decisions that need to be made between now and a scheduled review in September, NASA officials say.
It is a decision NASA officials have likened to choosing the shape of the space shuttle?s wings or the International Space Station?s orbit. As might be expected in cases where the long-term consequences of a decision are not always obvious, NASA is not of one mind on the Orion landing question.
?The simple answer is we have not picked a landing mode for Orion yet. Both options are still on the table as we head into the coming year,? Rick Gilbrech, NASA?s associate administrator for exploration systems told reporters Dec. 10.
For the better part of the past two years, NASA had been leaning heavily toward dry landings for Orion. That approach was endorsed in the landmark Exploration Systems Architecture Study (ESAS) that NASA Administrator Mike Griffin commissioned upon his arrival in 2005 to kick the agency?s return-to-the-Moon planning into high gear.
The ESAS embraced dry landings in the Western United States ?for ease and minimal cost of recovery, post-landing safety and reusability of the spacecraft.? While the ESAS planners wanted Orion to be capable of making water landings if necessary, they reasoned that ?a vehicle designed for a primary land-landing mode can more easily be made into a primary water lander than the reverse situation.?
Subsequent studies by Denver-based Lockheed Martin Space Systems, which NASA picked in mid-2006 as the Orion prime contractor, also favored having the crewed capsule touch down on land for similar reasons. In submitting its winning proposal to build Orion, Lockheed Martin left the landing option open and won praise from NASA for an innovative airbags-and-retro-rockets solution that kept land-landings on the table without imposing a significant weight penalty on the spacecraft design.
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But by the time NASA gave permission this fall to begin Orion?s detailed preliminary design phase, the assumption was that Orion would routinely land in water with only a contingency capability to touch down on land.
Jeff Hanley, program manager of NASA?s Constellation Program, which includes Orion and other hardware needed to return astronauts to the moon, said the team has made a strong case that landing in water offers safety and performance advantages over land landings.
?There are a couple of aspects that pop out to us,? he told reporters during the Dec. 10 media roundtable. ?Looking at the landing event itself -- the event of actually touching down -- water comes out to be preferable. And that kind of makes sense.
?Now when you add in the risk of what happens after landing, if you?ve nominally landed on land in a place you intended to land, then obviously getting out of the capsule is a lot easier and a lot safer,? Hanley continued. But from a broader safety perspective, water landing still came out the winner because the landing event itself, he said, is ?the riskiest part of doing crew return.?
When the Constellation Program looked at how the various landing options affected Orion?s performance for lunar missions, water landing again came out the winner.
?Every pound you send toward the moon is a precious thing ? from an efficiency point of view -- a performance point of view -- carrying 1,500 pounds (680 kilograms) of landing bags to the moon and back when I have a perfectly viable mode of landing in the water near a U.S. coastal site didn?t seem like a good trade in terms of its performance,? Hanley said.
For an Orion team under the gun to shed weight from its spacecraft design, the 680 kilograms of airbags the vehicle would have to carry to enable routine land landings proved a fat target.
?That?s a pretty big amount in terms of the mass challenge that the Orion team was facing last summer,? Hanley said. ?That 1,500 pounds represents quite a bit of the amount that they were trying to burn down.?
No panacea
Land-landing advocates inside NASA argue that the added weight is worth it if it means the difference between using each Orion capsule up to 10 times instead of throwing it away after every mission. By treating Orion as expendable, they argue, NASA could find itself spending hundreds of millions of dollars annually keeping production lines open. That money, they say, would be better spent establishing a robust presence on the moon.
?It?s the difference between going to the moon to stay and making one two-week trip a year,? one such advocate said.
Gilbrech said the Constellation Program is being asked to assess the long-term cost implications of a water- versus a land-landing scheme. A key factor in the analysis is the cost of maintaining a permanent Orion production capability versus building enough vehicles and spares to last 30 years and then shutting down the assembly lines.
?That?s one of the knobs we?ve asked them to go back and turn is keeping the production line open longer depending on how reusable you make the spacecraft,? Gilbrech said. ?That?s one of the things we want to hone in on in some of these trades.?
Hanley said the analysis will test the assumption that touching down on dry land offers the greatest reusability and the lowest cost. ?Landing on land is not a panacea in any sense of the imagination,? he said. ?There have been a lot of assumptions made assuming that landing on land is going to be better. There are a lot of people in the technical community who don?t buy into that.?
For example, NASA still does not fully understand the loads Orion would experience in a land landing versus a water landing, Hanley said. And while a lengthy stay in salt water certainly is not good for anything as sensitive as a spacecraft, advances in navigation and positioning since the Apollo program bode well for rapid recovery efforts.
?In a nominal water landing, how long does the capsule stay in the water? If we have a fairly rapid recovery, where the recovery ship would be pulling the spacecraft out of the water very, very quickly -- assuming with our targeted landing capability we are going to land very close to the recovery ship -- then maybe most of the spacecraft is reusable in a water landing case,? Hanley said. ?We don?t know.?
NASA has learned a thing or two about recovery at sea since Apollo, Hanley said.
?The philosophy the Orion team has adopted is to have a targeted landing zone off the coast of California with one or two recovery vessels,? Hanley said. ?We?ve got a couple of recovery vessels for the [space shuttle solid-rocket boosters] today off the coast of Florida, so that is not an infrastructure cost we don?t understand at this point, since we have been in that mode for 30 years with the space shuttle.?
While a quick recovery would bode well for reusing a capsule that splashed down in the ocean, Hanley said he still has questions about just how much reusability is desirable from a cost perspective.
?The life-cycle cost trade between the two is not at all clear,? he said. ?We have a lot of battle scars, if you will, from re-using space shuttle. We need to go really interrogate that.?
With Orion?s preliminary design review slated for September, Hanley said it is necessary to lock down something as basic as the landing mode so that the vehicle?s various design teams can do their job.
James Reuther, project manager for advanced development of Orion?s heat shield at NASA Ames Research Center at Moffett Field, Calif., agreed. He said the water versus land question does not make much difference for Orion?s ablative main heat shield since it is a single-use system either way. But he said it does make a difference for Orion?s back shell, whose current design relies on space shuttle-heritage ceramic tiles for thermal protection.
?There are those in the community who say if you stick a tile in water, forget about it. You are never going to reuse it,? he said. Others, he said, simply do not think the tiles are worth saving under any circumstances and favor treating them as expendables by mounting them on easy-to-change detachable panels instead of directly onto the capsule?s rigid hull.
Reuther said thermal protection system designers also are expecting a decision from the program in the coming weeks on whether Orion will jettison its main heat shield before landing or keep it in place through touchdown.
For a dry landing, the assumption has been that Orion would jettison its heat shield in order to deploy the airbags and retro-rockets necessary for a soft touchdown. But if land landing is only going to be used in the event of a launch-abort situation, Reuther said, big airbags and retro rockets might not be necessary.
Small airbags probably still would require Orion to jettison its heat shield. But if Orion engineers decide that parachutes and retro-rockets alone can guarantee a safe touchdown in the event of an abort situation that shoots the capsule back over land, Reuther said it is possible to design a heat shield where the retro rockets would be able to blow through the shielding and bring the crew down to a safe, if somewhat rough, landing.
What is important at this stage of the game, Reuther said, is that the program freeze some design assumptions so that subsystem teams can continue to move forward.
Hanley said that while Constellation is assuming a water landing for now, it has not closed the door on land landing.
?By the time we get done looking at what that minimal capability would be to land on land safely and have the crew walk away, we will see what the design looks like then,? Hanley said. ?And if the design is in fact robust enough that we could in fact return to having nominal land landing then we will make that choice at that time. We are allowing that to now fight its way back in.?
As for the cost estimates the Constellation Program has come up with for reuse versus expendable, and land versus water, Hanley declined to comment.
?As far as the specific cost numbers, I?m not going to share them because I don?t believe them,? he said.