Elon Musk, founder of Space Exploration Technologies (SpaceX), has always said that he wants his rockets and spacecraft to be fully reusable. That's why it stoked our interest to see Musk tweet this recently: "Design completed for bringing rocket back to launchpad using only thrusters. Yay. Wings r just dead weight in space."
In a press release a few months ago, SpaceX had shown an animated simulation of a Falcon 9 that returned both its first and second stages all the way to land vertically at a recovery site. Recently we spoke to Musk so he could elaborate on the design in more than 140 characters.
Reusable Rocket Parts
As Musk's tweet implied, there are two primary ways to recover a rocket part: It can either fly back to a recovery location with wings (as the shuttle did) or return under rocket power. Wings add structural weight but are reasonably reliable, while the rocket approach is versatile but adds weight because of the extra propellant that must be carried to fly back. Plus, there's a risk of the rocket engines not restarting, or shutting down permanently.
Despite the dangers, Musk is clearly a fan of the rocket-powered approach. He told PM that SpaceX has come up with a solution to make both the lower and upper stages of the Falcon 9 reusable. (The Dragon capsule that will fly atop the rocket has already demonstrated that it can be recovered in the ocean after it splash-lands with a parachute, though SpaceX is building vertical-landing capability into that as well.)
The key, at least for the first stage, is the difference in speed. "It really comes down to what the staging Mach number would be," Musk says, referencing the speed the rocket would be traveling at separation. "For an expendable Falcon 9 rocket, that is around Mach 10. For a reusable Falcon 9, it is around Mach 6, depending on the mission." For the reusable version, the rocket must be traveling at a slower speed at separation because the burn must end early, preserving enough propellant to let the rocket fly back and land vertically. This also makes recovery easier because entry velocities are slower.
However, the slower speed also means that the upper stage of the Falcon rocket must supply more of the velocity needed to get to orbit, and that significantly reduces how much payload the rocket can lift into orbit. "The payload penalty for full and fast reusability versus an expendable version is roughly 40 percent," Musk says. "[But] propellant cost is less than 0.4 percent of the total flight cost. Even taking into account the payload reduction for reusability, the improvement is therefore theoretically over a hundred times."
A hundred times is an incredible gain. It would drop cost for Musk's Falcon Heavy rocket—a scaled-up version of the Falcon 9 that's currently rated at $1000 per pound to orbit—to just $10. "That, however, requires a very high flight rate, just like aircraft," Musk says. "At a low flight rate, the improvement is still probably around 50 percent. For Falcon Heavy, that would mean a price per pound to orbit of less than $500." Falcon Heavy is particularly amenable to reuse of the first stage—the two outer cores in particular, because they separate at a much lower velocity than the center one, being dropped off early in the flight.
Turnaround Time
Bringing down the cost of rocket launches isn't just about reusability; as Musk's quote suggests, it's also about turnaround time. The original premise of the space shuttle program was that the vehicle would be turned around within days; it ended up being months, which is one of the reasons that it never met its cost goals.
What about a reusable Falcon? Musk says he expects "single-digit hours" between landing and next flight, at least for the lower stages. "For the upper stage, there is the additional constraint of the orbit ground track needing to overfly the landing pad, since cross-range [the distance to a landing site that it can fly to either side of its original entry flight path] is limited. At most this adds 24 hours to the upper-stage turnaround."
Translation: One of the other reasons that the shuttle was so expensive was that it had very large wings to give the vehicle a thousand miles of cross-range. The Air Force demanded this feature, which would have allowed the shuttle to return to its launch site after a single orbit, though it was never used. But SpaceX doesn't mandate that cross-range feature. Therefore its craft would have to wait a little bit for the Earth to rotate and bring the landing site around again, but this would make SpaceX missions cheaper because the rockets don't have to carry so much propellant in this stage.
What does it imply for flight rate? "Multiple flights per day for first stage and side boosters," Musk says. "At least one flight per day for the upper stage" (which costs much less, anyway).
Last fall, the company announced an experimental test vehicle called Grasshopper to prove and refine the reusability concept. "There is no question in my mind that it will work," Musk says with trademark confidence. "It's just a question of how quickly the testing progresses. We expect to do several vertical-takeoff, vertical-landing (VTVL) flights this year and hopefully go supersonic in the fourth quarter."
SpaceX is trying to improve the reusability of the Dragon capsule, too. Last week, the company announced the successful test of its new SuperDraco rocket engines, which will power the launch–abort system for the Dragon, making it safer for human occupation, and also act as the landing engines. The idea is that Dragon will land vertically on the pad, like the Falcon rocket components, as opposed to landing in the water with parachutes.
So what does that mean for ticket prices in the future? Musk tells us that with daily flights, the cost will run about $100 per pound. For the average male, that means about 20,000 bucks. Start saving your money.
