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SpaceX fully stacks Starship/Superheavy in preparation for launch

Starship about to be stacked on Superheavy
Starship about to be stacked on Superheavy, using
the launch tower’s chopstick arms. Click for full image.

For the first time in six months SpaceX engineers have stacked Starship prototype #24 on top of Superheavy prototype #7, with the intention of running a dress rehearsal countdown and a full static fire test of Superheavy’s 33 engines, all in preparation for the first orbital test flight before the end of this year.

According to CEO Elon Musk, Booster 7 and Ship 24 will attempt Starship’s first full-stack wet dress rehearsal (WDR) once all is in order. The prototypes will be simultaneously loaded with around 5000 tons (~11M lb) of liquid oxygen and methane propellant and then run through a launch countdown. Diverging just before ignition and liftoff, a WDR is meant to be more or less identical to a launch attempt.

…If the wet dress rehearsal goes to plan, SpaceX will then attempt to simultaneously ignite all 33 of the Raptor engines installed on Super Heavy B7, almost certainly making it the most powerful liquid rocket ever tested. Even if all 33 engines never reach more than 60% of their maximum thrust of 230 tons (~510,000 lbf), they will likely break the Soviet N-1 rocket’s record of 4500 tons of thrust (~10M lbf) at sea level. It would also be the most rocket engines ever simultaneously ignited on one vehicle. SpaceX will be pushing the envelope by several measures, and success is far from guaranteed.

Depending on the results of these tests, the stacked rocket will either require further modifications, or could even proceed directly to launch.

We are thus seeing a true race between SpaceX’s privately developed and funded rocket and NASA’s government developed and funded SLS rocket. Which will launch first? Right now the race is neck-and-neck, though that is deceiving since SpaceX began development twelve years after NASA started work on SLS. Even if SLS launches first, SpaceX will have clearly shown that private enterprise does things faster (7 years vs 18 years) and for far less money (about $9 billion vs $46 billion).

Genesis cover

On Christmas Eve 1968 three Americans became the first humans to visit another world. What they did to celebrate was unexpected and profound, and will be remembered throughout all human history. Genesis: the Story of Apollo 8, Robert Zimmerman's classic history of humanity's first journey to another world, tells that story, and it is now available as both an ebook and an audiobook, both with a foreword by Valerie Anders and a new introduction by Robert Zimmerman.

The ebook is available everywhere for $5.99 (before discount) at amazon, or direct from my ebook publisher, ebookit. If you buy it from ebookit you don't support the big tech companies and the author gets a bigger cut much sooner.

The audiobook is also available at all these vendors, and is also free with a 30-day trial membership to Audible.

"Not simply about one mission, [Genesis] is also the history of America's quest for the moon... Zimmerman has done a masterful job of tying disparate events together into a solid account of one of America's greatest human triumphs."--San Antonio Express-News


  • Gary

    I can’t find it now, but I thought I read somewhere that SpaceX couldn’t fire all 33 at once in a test situation. I mis-remember more these days, but I thought the story was that if all 33 were fired simultaneously, the rocket would not be able to constrained from liftoff.

  • Gary: This was something I stated, based on other info I had gotten elsewhere (the source of which I do not remember). Apparently, that source was wrong, at least according to Elon Musk.

  • sippin_bourbon

    “…if all 33 were fired simultaneously, the rocket would not be able to constrained from liftoff.”

    That would be embarrassing.

    “We were doing a static fire, next thing we know, it is half way across the Gulf of Mexico…”

    SpaceX’s reduced pace, even since the green light following the forever enviro hearings/review, has been interesting.

    They had an impressive pace for getting these built, but it seems the bedeviled details of such a massive and complex machine have resulted in a slowed rate of work. I wonder if it is because they have so much riding on it now. No one cared when they were blowing up early starship iterations. But this is the full deal. The blowback may be bigger.

    I have watched most of the Musk interviews, but have not seen him address this concern directly.

    I still wonder if the 33 engine beast is just too complex. Too many possible points of failure.

  • Gary

    Bob, no worries. I assumed I had just dreamed it or mixed it up with another story.


  • Gary


    “We were doing a static fire, next thing we know, it is half way across the Gulf of Mexico…”

    That would be a neat way of evading the Biden admin restrictions.

    You could follow up the first statement with, “and since were off the ground, we figured we’d just go ahead and orbit the moon, lead the way for Artemis!”

  • “We were doing a static fire, next thing we know, it is half way across the Gulf of Mexico…”

    Shades of the Spruce Goose!

  • Alex Andrite

    … and Seabiscuit is chomping at the bit …..

  • pzatchok

    Man that thing is tall.

    When I get up that high standing outside I get a little apprehensive and quite nervous.
    Amazingly I can hang out of an aircraft at higher altitudes and not feel bad at all.

    I love the views though.

  • Concerned

    That beast is complex, but no more so than Falcon Heavy (which has 27 engines), and in some ways simpler. For example, only the center 13 engines of the booster gimbal, the outer 20 are fixed. Also, only the center 3 engines are restartable in the air compared to 9 engines on the Falcon Heavy. The 4 grid fins are huge, but they only move in one axis, whereas 12 total grid fins on the Falcon Heavy side boosters must first fold out before they can steer those boosters back to the surface, so SpaceX has eliminated that failure mode on the Starship booster (i.e. Super Heavy). A single rocket flying is much simpler to control compared to 3 very large rockets strapped together ala Falcon Heavy.
    The complexity comes in dealing with the new Raptor engines which are tricky to start up, particularly in multiples, hence the long ground testing campaign and iteration with the ground support equipment. Once they’re able to static fire all 33, I’m pretty confident the beast will fly soon thereafter.

  • MDN

    WRT keeping the beast on the ground during a 33 engine static fire test, I presume that is the primary reason why they are loading it up with 5000 tons of propellent (the secondary reason is likely to provide structural integrity for the booster tank structure with a StarShip atop with any significant fuel load as well).

    WRT complexity, I concur with Concerned’s analysis. I will also point out the recent focus of testing on maintaining adjacent Raptor integrity in the face of a possible RUD (Rapid Unscheduled Disassembly) event. SpaceX has blown up several dozen of these things looking for ways to eek out some more thrust and efficiency, but also to understand the characteristics of such disassembly events in great detail. Rocket engines are pushed to extremes by design, and Raptor more than most in history, but with clever design I think SpaceX has bounded this risk pretty well and have a high probability of success should even 1 or 2 engines not start or catastrophically fail in flight. And if there are failures, as long as they do not impact adjacent engines then the fuel they do not burn will simply be diverted to running the remaining engines longer, so the booster can and will still achieve its mission objectives.

    WRT scheduling, it will be interesting to see what the FAA has to say as they hold ultimate sway on the matter. And sadly I would not be at all surprised to see a dozen nit pik technicalities thrown out to give NASA one last chance to launch SLS first even should all of the StarShip testing go swimmingly. Call me cynical I guess.

    Kudos to Musk et al. Their process is a joy and pleasure to witness. It is how complex design and engineering should be done. Even so the risk of some spectacular failures is still decidedly non zero at this point, but that is why you have “prototypes” to begin with. Because in the end you Learn by Doing, not by naval gazing and running endless computer models that mostly reflect a confirmation bias of what we think we know, which itself is then protected by excessively conservative design that locks in exorbitant lifecycle costs and inefficiencies from day one.

    Go StarShip!

  • sippin_bourbon

    The concept of multiple engines providing redundancy is well established. Making the other engines perform a longer burn to compensate has been demonstrated in flight. (I believe as least one shuttle flight, and one Apollo).

    But 33 on a single booster vs 27 on three boosters, I think does not compare. Falcon Heavy has done it. Delta IV Heavy too. All with good success so far. This was done mostly because it was better to have liquid boosters over solid, for better control through throttling.

    I think that raptor reliability testing has been great. I think they will run, or shut down down before they RUD. But the complexity above the engine level, with the mass of fuel lines and other plumbing is near insane levels. This is where I get concerned. Between tank and engines. This is where the multiple points of failure begin to expand. This is where the configuration to make sure all 33 work is an unknown. And the management of all 33 as well. Is it one computer, or are they broken out by rings, inner and outer for control?

    I want to see it work, but at the same time, a RUD in Booster 7 will not surprise me.

    A 9 raptor engine booster to test and work out a lot of this might have been worth it.
    They did a single merlin Falcon early on, and then moved on from there.
    Also 9 engine/booster, that configuration and the computer to manage 9 is all known and done, and reliable.

    The raptor is still an amazing design. No matter what happens, I think it will be around a while.

  • Edward

    A few weeks ago, I did an analysis on the launch acceleration of Starship. That analysis was based upon a rocket weighing around 8 million pounds, but this report says that it will weigh much more, closer to 11 million pounds. I had calculated that the initial acceleration off the pad would be around 2G, but the math based upon this new report suggests that the acceleration will be much closer to 1/2G.

    You wrote: “I can’t find it now, but I thought I read somewhere that SpaceX couldn’t fire all 33 at once in a test situation. I mis-remember more these days, but I thought the story was that if all 33 were fired simultaneously, the rocket would not be able to constrained from liftoff.

    This is the advantage of having the tanks full for both stages. They will be at maximum weight, so the stresses at the hold-downs will be less, and the design should be able to hold the rocket during initial thrust, otherwise the hold-downs aren’t doing their job. The rocket must be held down while all the engines come up to launch thrust and stabilize. This can take a little time, during which the rocket must remain on the pad while they determine that it is safe to let it come off the pad. The thrust is supposed to be around 16 million pounds, and if the rocket weighs around 11 million pounds, then all the hold downs together only have to keep 5 million pounds in place.

    The problem that you mentioned, Gary, may have been for the tests in which there is not a full load of propellants. The hold-downs almost certainly cannot withstand the entire 16 million pounds, and even if the ground equipment can, the places on the rocket that are being held probably are not strong enough for that much force.

  • Ray Van Dune

    The guy designing the startup sequence for the Superheavy faces a daunting challenge. The engines cannot all be started simultaneously (even on an F9 or FH), but there can be little margin for delay since the forces involved in trying to hold the vehicle down would be huge.

    Thus I would suspect that any all-engine static fire could last only a few seconds – once all stage 1 engines are lit and running, they probably will have to be very quickly shut down, or the beast will have to be LET loose before it BREAKS loose!

    Just a hunch, but I n fact it would not surprise me if SpaceX decides an all-engine static fire is just too risky, and instead settles for firing a number of different multi-engine configurations from the overall startup sequence.

  • Star Bird

    Overload your ship and it will sink

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