Rand Simberg had a conversation with Elon Musk about SpaceX’s plans for a fully reusable Falcon 9 launch vehicle. Some of the key highlights:
- “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 $1,000 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.
- 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.”
- 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.”
That all sounds great. And highly optimistic. Whether they can do all that, I don’t know. Given that SpaceX’s has often slipped to the right and how focused the company is on Dragon missions to the International Space Station, we’ll have to see how far they get on Grasshopper this year. On the other hand, the main holdup is the Dragon capsule, not the rocket. Which means SpaceX could have the personnel available to vigorously pursue Grasshopper this year.
One way SpaceX will attempt to mitigate some of the mass penalty is by continuing to upgrade its Merlin engines. If you can push those to their limits, then you can overcome some of the limitations of this approach.
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That second bullet point needs to be expanded on to make some sense. The 100 fold gain discusses paylod then flips to propellant cost with no obvious (to this reader) connection. I am lost in following the reasoning to reach the 100 fold gain.
What Elon is trying to say, is that theoretically, economies of scale will drive all the cost components of a reusable rocket to zero, except for the fuel costs. So at a high enough launch rate, the price per launch should be 0.4% of today’s price. So 250 times cheaper.
However, the context to this paragraph is Elon talking about launch costs in terms of price per pound.
This means that you have to take into consideration that payload loss from the conversion of the rocket to reusable.
So an expendable F9 can lift about ten tonnes to LEO, whereas a reusable F9 can lift about six tonnes to LEO.
This means that the price per pound falls by 250 but rises by the ratio of the reduced payload mass. ie. 250 x 6 / 10 = 150. (Theoretically.)
This is where the statement that “theoretically over a hundred times” comes from.
However, note in the third bullet point, that Elon caveats this idea saying “requires a very high flight rate”.
It’s interesting the see how SpaceX will do these next few years, they have some trouble with dragon mission to the Iss, and it has set them back. Could a reusable F9 dramatically change business in space?
I am actually thinking that Spacex needs to expand to meet it requirements for it launch commitments in the next few years. Maybe they should think along the lines of a Boeing type of assembley line. Even if the reuseable issue takes time to implement the number of rockets of similar type needed should bring the cost down.