On 29 September, Elon Musk, the enigmatic founder of SpaceX,
took the stage at the National Press Club in Washington, DC
to announce that SpaceX would develop a fully reusable rocket. SpaceX, and Musk
in particular, have always been straightforward about their intention to build
such a vehicle, but this marks the first official announcement.
It is no coincidence that Grasshopper, a reusable suborbital
vehicle, was publically revealed only days before in a much less spectacular
manner - SpaceX had to reveal certain details to the FAA for a mandatory site
environmental review, and the FAA is legally obligated to publicise the
results.
Though Grasshopper is suborbital, it was immediately evident
that it was not a dedicated suborbital launch capability - it is evidently not
much more than a Merlin 1-D engine and fuel tank with struts to keep it off the
ground.
At a recent AIAA talk, Musk characterised building reusable rockets
as, "super-damn hard." And he meant it.
The Space Shuttle is the closest thing to a reusable
launcher ever built, but even that system barely meets the definition. The
Shuttle itself, the reusable part, is for practical purposes actually a
combination second stage/steerable capsule. The first stage consisted of
massive solid rocket boosters, which were separated when no longer necessary.
They tumbled back into the Atlantic Ocean, to
be hauled in by ships and never used for flight again.
After a flight, the Space Shuttle required extensive
processing to make it ready for another flight.
Musk said the first stage of a reusable Falcon will
separate, then fall back down, reignite the engines, and land vertically on the
launch pad. The second stage will carry the payload to orbit, fire thrusters to
re-enter the atmosphere, steer through
the atmosphere using its tiny lift quotient, then reignite its own engines and
land vertically on the pad.
The main obstacles of a reusable spacecraft are heat and
weight.
Heat is generated by the friction of moving through the atmosphere
at hypersonic speeds, and dissipating heat is one of the major challenges of
any spacecraft. The first stage will have to survive the flame from the second
stage once it separates, and the second stage will need extensive shielding to
re-enter safely, much less steer and execute a precision landing. Of course,
that extensive shielding adds a whole lot of weight. SpaceX declined to comment
on just how they would solve that problem, but they wouldn't spend the money if
they didn't think it could be done.
Even 'disposable' launchers can only launch payloads of
roughly 2-4% of their total weight. It requires an incredible amount of thrust,
which requires more powerful engines, which require additional fuel, which adds
more weight, which requires more thrust...
Not to say it's impossible of course, but it is, as Musk put it, "super-damn hard."
on September 30, 2011 11:17 PM | Reply
As noted, if you add up just the additional weight of the landing gear system and landing fuel load, and then subtract that from the payload mass, you'll find that such a reusable stage probably doesn't make financial sense.
There is also the additional structural weight penalty that will result from making the stage strong enough for multiple launches, as well as strong enough to handle the landing loads. With the typical expendable primary stage structure, the worst loading it will likely experience is due to the harsh acoustic environment right at engine ignition.
Finally, there is no such thing as a truly "reusable" liquid rocket engine. The highly stressed turbopumps will likely require extensive servicing between flights.
on October 1, 2011 3:15 AM | Reply
Weren't the Shuttle SRBs were recovered, refurbished and re-used. You could argue that re-using a solid rocket is not really 're-use', but it's not throw-away either. It's really somewhere in-between.
on October 1, 2011 2:18 PM | Reply
"They tumbled back into the Atlantic Ocean, to be hauled in by ships and never used for flight again."
While your overall conclusion that a fully-reusable launch vehicle would carry a lot of dead weight to orbit (or suborbit) is correct, the sentence above is wrong.
The Shuttle SRBs (which included the motor, the SRM, were fully reusable. After recovery, the SRMs were divided into segments, shipped back to the factory in Utah by rail, reloaded with propellant and shipped back to KSC for re-use. Part of an SRB for the last Shuttle flight, STS-135, was the same piece that had flown on STS-1, the first Shuttle flight.
The External Tank was the only piece that was no reusable. The ET became millions of pieces before it hit the Pacific Ocean south of Hawaii (earlier flights had used the Indian Ocean for disposal). That was the part of the Shuttle system that could not be reused and is also why there is only one ET left in the world, while many (unfueled) SRM segments and SRB parts still exist.
on October 3, 2011 12:59 AM | Reply
SRBs... "to be hauled in by ships and never used for flight again."
They were re-used unless severely damaged by parachuting into the sea...
http://en.wikipedia.org/wiki/Space_Shuttle_Solid_Rocket_Booster