In a rare Saturday news item, Elon Musk today confirmed that the next test flight of the Starship/Super Heavy combination would use a different staging method than they've been designing for.
In an online discussion with Bloomberg journalist Ashlee Vance on Twitter, the social media company Musk owns, he said that SpaceX had recently decided to switch to a “hot-staging” approach where the Starship upper stage will ignite its engines while still attached to the Super Heavy booster.
“We made sort of a late-breaking change that’s really quite significant to the way that stage separation works,” Musk said, describing the switch to hot staging. “There’s a meaningful payload-to-orbit advantage with hot-staging that is conservatively about a 10% increase.”
While I can't think of a launch vehicle I've watched that uses the technique, Space News points out that Russian launch vehicles have been using this approach "for decades." I'll take that to mean that it might be in use on other vehicles around the world as well. Instead of the way we see it done routinely: cut off booster engines, drop the booster, start the upper stage, this will start the second stage while the first stage is still attached. They can save a few seconds of flying without having thrust, but I've never timed one.
That makes me wonder how that can be done and still keep the first stage reusable.
Musk said that, for Starship, most of the 33 Raptor engines on the Super Heavy booster would be turned off, but a few still firing, when the engines on the Starship upper stage are ignited. Doing so, he said, avoids the loss of thrust during traditional stage separation, where the lower stage shuts down first.
Doing so requires some modifications to the Super Heavy booster. Musk said SpaceX is working on an extension to the top of the booster “that is almost all vents” to allow the exhaust from the upper stage to escape while still attached to the booster. SpaceX will also add shielding to the top of the booster to protect it from the exhaust.
Musk also acknowledged this might be the riskiest change for the next launch -
now generally referred to as around the start of August (Elon Standard Time). We've seen
Booster 9 (B9) before but the top of the booster didn't look any different
than other boosters seen so far.
Besides the stage separation, Musk said they've made a “tremendous number” of other changes to the vehicle, “well over a thousand.” We've heard of many changes to the Orbital Launch Mount itself, the water deluge system and its "steel sandwich" construction, as well as actual tons of concrete. He pointed out that the engines are different but not in the kind of detail I'd like to see; which would have something like a count of how many of which version are flying, and the more descriptive the better. He referred to the booster flown in April as using a “hodgepodge” of engines, but there's no more detail on the engines on B9.
Starship on the first Integrated Flight Test, April 20th. SpaceX photo.
So now they intend to let Starship do to the top of Booster what Booster previously did to the Pad, eh?
ReplyDeleteSix engines have a lot less effect than 33 (or how ever many were working, I think it was 29 in the first few seconds).
DeleteHe's done the math, but I have a hard time internalizing that saving a couple seconds of zero thrust makes a 10% load-to-orbit difference.
That's way I've been thinking about it, Malatrope. So they go from full thrust, 33 engines, to "a few still firing" and then light the upper stage. They're still losing thrust and slowing down until they drop that booster and fully light the top 6.
DeleteYeah, I guess they won't slow down as much, but is it really 10%? I guess it comes down to defining "a few still firing." If 3 engines are still firing out of 33, that's about 10% of the thrust, but does that turn into a 10% "payload to orbit" advantage?