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Monday, May 21, 2018

SpaceX's completely reusable rockets

It used to cost about $15,000 to $20,000 to lift one kilogram (2.2 lbs) to LEO (low earth orbit).   SpaceX can do it with the Falcon 9 for $2700 per kilogram, even when the Falcon 9 isn't reused.  SpaceX charges $62 million per launch, and that price assumes it's not reusable.   If the rocket could be reused several times without major refurbishment, then the costs would be slashed further, because most of the cost of the rocket is the hardware.  Only a tiny percentage is fuel.

With the "Block 5" version of the Falcon 9, SpaceX aims to reuse the first stage of the Falcon 9  for 10 launches with minimal or no refurbishment between launches.  This will cut the cost per launch to $5 or $6 million.  Note: the cost not the price.  SpaceX will still charge $50 million, and will use its profits to pay back the development costs of the Falcon 9 and to fund its new BFR.

At first glance, that Falcon 9 looked little different from the previous versions of the rocket. Its interstage section between the first and second stages was now black, as were the four landing legs at the base of the first stage. Those superficial aesthetic differences, though, hid more substantive changes to the vehicle in the company’s quest to make it more reliable and more reusable.

Elon Musk said that this “Block 5” version of the rocket is really the sixth version of the rocket. “Because we had version one; version 1.1, which was really like version 2; arguably a version in between that; and then a bunch of blocks,” he said.

Whether you call it Block 5 or Version 6, Musk said a major change in the vehicle was an emphasis on rapid reusability. “The key to Block 5 is that it’s designed to do ten or more flights with no refurbishment between each flight, or at least no scheduled refurbishment between each flight,” he said. “The only thing that needs to change is you reload propellant and fly again.”

SpaceX already demonstrated reusability with the previous Block 4 version of the rocket, with 11 reflown first stages launched to date. However, the Block 4 version required more maintenance between flights, and were only being flown a couple of times. “With Block 4, we’d optimized it to probably about a week’s worth of refurbishments if pushed. Maybe, call it about ten days of work between flights,” he said. But Block 5 is “really better in every way than Block 4.”

The no-refurbishment goal of the Block 5 first stage means it is at least theoretically feasible to fly it twice within 24 hours, and Musk has set a goal to do just that later next year.

“So it’s going to take some amount of time—we’re going to be very careful and deliberate about this—but that will be I think truly remarkable, to launch an orbit-class rocket—the same orbital-class rocket—twice in one day,” he said.

“There’ll be some moderate scheduled maintenance at ten [flights], but we believe that the Block 5 boosters are capable of on the order of at least a hundred flights before being retired, maybe more,” Musk said.

Musk said he expected that customers, once reticent to fly on previously-flown boosters, will soon come to prefer them. “I think the general sentiment will change from feeling like a flown rocket is scary to feeling like an un-flown rocket is scary,” he said. “Would you rather fly in an aircraft that’s never had a test flight before, or would you rather fly in an aircraft that’s flown many times successfully?”

With that kind of reusability, Musk said he anticipates building just 30 to 50 Block 5 first stages, even though he expects the vehicle to perform at least 300 launches before being retired in favor of the company’s BFR (officially, Big Falcon Rocket).

Another key motivator for the Block 5 upgrade, Musk said, was reliability. The Block 5 will be used for launching commercial crew missions for NASA—Friday’s launch was the first of seven launches of the new vehicle required before NASA will allow it to be used for crewed missions—as well as for national security missions for the Air Force.

“We need to exceed all of NASA’s human-rating requirements for Block 5, and they are quite extensive, as well as meet all of the Air Force requirements for extreme reliability,” he said. “I really don’t want to jinx fate here, but this rocket is really designed to be—the intent is to be—the most reliable rocket ever built. That is the design intent. I hope fate does not punish me for these words, but that is unequivocally the intent.”

[Read more here]

Some of the changes SpaceX has made to make the Falcon 9 safer and more reusable are:

  • The pressure tanks used to store helium ("the composite-overwrapped pressure vessels" or COPVs) have been redesigned and will now withstand twice the pressure they are subjected to.  The only two losses of Falcon 9s have come from failures in the COPVs.
  • A heat resistant coating has been applied to parts of the rocket which overheat during re-entry.  These are the black parts you see in the photos of Block 5 of the Falcon 9 (see image below)
  • The grid-fins, used to control the rocket during re-entry and landing have been redesigned and the aluminium alloy previously used has been replaced with titanium which is much more expensive but also doesn't catch fire during landing.
  • The landing legs have been made retractable so they don't need to be physically removed for transport after every landing.
  • The 'octaweb' which keeps all nine first stage Merlin engines in place has been strengthened and the alloy used changed, and the engines are now bolted in rather than welded to make replacement easier.  One aim of the stronger octaweb is to allow a single Merlin engine to fail without it also destroying the other engines.
  • Improved avionics.
SpaceX's aim is to 'fix' the design of the Falcon 9 so that all research and development effort can go into the BFR.  It's also a NASA requirement that the design be stabilised before manned missions can be started.  There will be no further 'blocks' or versions of the Falcon 9 except for minor tweaks.  Falcon 9 and Falcon Heavy will become cash cows that fund the BFR & BFS.  

Some interesting articles/videos:

Falcon 9 block 4 (left) vs block 5 after re-entry
Source: Scott Manley

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