Why use hot staging? It’s all about ullage and simplification.
While the second test flight of the SpaceX Starship space launch system ended with the explosion of both booster and spacecraft, perhaps the most technically challenging aspect of the mission, hot staging, operated perfectly. Hot staging simplifies rocket design by eliminating ullage rockets and associated sequencers, but it’s hard on the booster stage, which at SpaceX, is expected to not only survive, but fly back for a soft landing for eventual reuse.
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Episode Transcript:
The recent second test flight of SpaceX’s gigantic Starship space launch system experienced, like the first flight, what the company describes as a “rapid, unscheduled disassembly.” In this second flight, both the booster and Starship spacecraft exploded independently. As a result, neither stage achieved the primary mission objective of a controlled water landing.
There were some positives, however. The redesigned launch stand worked well, and booster engines operated well right through hot staging, which was a change in staging procedure compared to the first light.
Now we’ve all seen the familiar images from Apollo missions of the first stage dropping away, then the interstage adapter, followed by ignition of the second stage engines. There are similar images from the second stage looking upward, as the third stage departs under the power of three small “ullage” rockets, before it’s J2 engine ignites.
“Ullage” is an old brewer’s term, describing the amount of empty space inside a full barrel of beer. In the rocket business, it refers to the empty space in a propellant or oxidizer tank. With space launch systems consuming literally tons of propellant per minute, the liquid levels in these tanks drop rapidly, and the empty space is backfilled by pressurized helium to prevent the stage from collapsing, and to help settle the propellants.
At launch, gravity keeps the liquids at the bottom of the tanks, and when pre-valves and valves open, gravity feeds the inlet side of the turbopumps. For the first stage, that’s fine, since thrust produces the necessary positive Gs to keep the propellant where it needs to be.
For upper stages, however, it’s a little more complicated. At booster engine cut-off, acceleration of the stack goes to zero, and the upper stages are in freefall. This means the propellants are free to float around inside the tank, creating a very real possibility that at turbopump start up, the pumps will gulp a slug of helium gas or a froth of helium bubbles entrained in the liquids.
If you’re an engineer who works in the pump industry, you know what happens when the impeller encounters a gas instead of a liquid: it overspeeds. In the case of a rocket engine, it can overspeed itself into catastrophic failure in a matter of seconds, and this was a major reliability issue with early launch vehicles.
To get the propellants settled at the bottom of the tanks, small solid fuel rockets, logically called “ullage” rockets, were fired before upper stage engine ignition. This process had the added benefit of pushing upper stages away from the tank dome of the lower stage, for added safety.
Hot staging eliminates the need for sophisticated sequencers and ullage rockets by simply igniting the upper stages, while the lower stages are still thrusting, ensuring that the entire stack operates under positive Gs throughout the climb to orbit. When you’re discarding the boosters, it’s relatively easy, since the lower stages are going to plunge into the ocean anyway. But SpaceX reuses boosters, which are programmed to pitch over and fly away from the Starship immediately after its engines ignite.
There are all kinds of ways that this can go catastrophically wrong. But in the case of the recent test, the hot staging, paradoxically, appeared to operate perfectly.
So why did both the upper and lower stages explode? Well, that remains to be seen, but from an engineering perspective, one interesting thing about the way SpaceX is developing the Starship launch system is that there doesn’t seem to be a standard configuration, at least not yet. Staging methods and flight profiles are going to have to be standardized at some point, and then a test mission flown more or less flawlessly.
The last time a rocket of anything like this size was crew-rated was the Apollo Saturn 5 system in the late 60s, which was tested in Apollo 4 and 6 before a crew was committed on Apollo 8. Apollo 8 was considered a high-risk mission—and it was—but unlike the Apollo program, SpaceX has time on their side. With regular Falcon firings paying the bills, and no deadline like the one sent by President Kennedy, they can be patient and minimize risk.
But with two catastrophic failures, the pressure to execute well on the third test is going to be enormous. This test made it to space before exploding, but NASA never lost a Saturn 5, and that vehicle integrated three stages built by different contractors, all built by engineers working with slide rules and less computing power than your smart phone.
Third time really needs to be the charm.
