SpaceX

John Kraus @johnkrausphotos:

In a Twitter Space with @ashleevance, @elonmusk shares that Starship will hot-stage during the next flight, lighting engines on the ship with some engines still running on the booster, as to Never Stop Thrusting!™️

"Hot staging" is firing the upper stage engines while it's still nominally attached to the lower stage (like resting on or loosely attached). The advantages that I gather exist: It's fast. It takes care of stage separation without needing springs or little rockets or a flip or anything. Before firing a liquid-fueled stage that may have gases in a tank ("ullage"), you have to settle the contents so that the engine intakes suck only liquid (maybe using "ullage rockets"), but if you're still accelerating at separation, that's automatically taken care of.

But if you intend to reuse the first stage, well, I wonder whether six engines igniting will be too hard on it.

Apparently U.S. Titan rockets, a lot of Soviet / Russian ones (Soyuz, Progress, N-1), and (some?) Chinese Long March rockets were designed with hot staging.

Joe Barnard @joebarnard replies: "'okay so when I hot stage it’s “an anomaly” and I’ve “torched another flight computer” but when SpaceX does it it’s fine???'

Edit: There's now an article up at SpaceNews, "SpaceX changing Starship stage separation ahead of next launch", which includes

"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."...

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.

“This is the most risky thing, I think, for the next flight,” he said of the new stage separation technique.

Besides the change in stage separation, Musk said SpaceX made a “tremendous number” of other changes to the vehicle, “well over a thousand.” He didn’t go into details about the changes, ...

SpaceX also made improvements to the Raptor engines, with Musk describing the vehicle launching in April as using a “hodgepodge” of engines built over time. The Raptors on the new vehicles include changes to the hot gas manifold in the engine to reduce fuel leakage.

Those changes, he said, gave him more confidence in the success of the next launch. “I think the probability this next flight working, getting to orbit, is much higher than the last one. Maybe it’s like 60%.” In an online conversation in late April, he estimated a “better than 50% chance” of success on the next launch.

In another note, Musk finally learned some caution!

Musk, asked about any plans for a Starlink IPO, declined to comment. “It would not be legal for me to speculate about a Starlink IPO,” he claimed. “I think it’s against regulations to talk with any kinds of specifics about a future public offering.”

Edit: Peter Hague PhD @peterrhague: Thus far Musk estimates $2-3bn invested by SpaceX so far in Starship. The price of a single SLS launch