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.
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.
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