Monday, September 19, 2022

SpaceX Aces Seven Engine Static Fire on Starship Booster 7

Today, at 12:46:56 Central, SpaceX tested the largest number of Raptor 2 engines it has ever static fired at once.  It was seven engines on booster seven firing for seven seconds.   

Screen capture from - as you can see. 

Earlier in the day, they had performed another spin prime test that appeared to be on the center engines.  There are 13 engines in the innermost ring that can gimbal to steer the rocket, and 20 fixed engines around the perimeter of the booster that aren't steerable.  The seven tested today appeared to be in the steerable center.  Shortly after the test, Elon Musk Tweeted "Chamber pressure looked good on all 7 engines." 

While seven is a record for SpaceX - the most they've fired at once on a Super Heavy booster has been 3 out of the potential 33 engines the booster can use, and you'll find that immediately all the watchers switched to thinking "how soon do we get all 33 engines?"  It would seem to be something off in the distance, but everyone is thinking "if seven engines look like that, what will almost five times that number of engines look like?"  Everyday Astronaut Tim Dodd, who has interviewed Elon enough times that his Tweets probably get recognized by Elon, chimed in and Elon's reply followed quickly. 

That mention of a "full stack wet dress rehearsal" should capture your attention.  The "full stack" consists of Starship atop a Super Heavy booster rocket, which has been done before for fit checks and photo opportunities, but never for a test like a WDR - which does everything up to igniting the engines.  The full stack is the world's tallest rocket at 395 feet tall (120 m) as well as the most powerful.  While up to date numbers are hard to get, Raptor 2 engines have been mentioned as exceeding 500,000 pounds of thrust - half a million pounds - so let's use that and know they might do more or be throttled back to less.  With 33 engines, that puts it well over 16 million pounds of thrust, more than twice that of the Saturn V. 

You may wonder about why they use 33 smaller engines while the Saturn V and other rockets of its day used fewer, more powerful engines.  I've read that the main argument against lots of smaller engines was difficulty in controlling them, a concern which has been obsoleted by today's better electronics and control systems.   


  1. I only counted 6 seconds, but who gives a rip...
    At 235 tons (metric) that comes to 1645 TONS of thrust (again, metric) and they didn't put that much prop and oxy in the booster! Very good hold downs! NOW LET'S DO A FULL STACK FIRE already, people.

    So, Elon is kind of (?) putting off lighting the candle on the whole stack until SLS gets off the ground - IF it gets off the ground in the next dozen days or so. Otherwise, it's off to the races.


    1. The way I figure it, SLS is almost sure to launch first but whichever launches first will be the biggest rocket in history. The difference is if it's SLS, they'll be the biggest rocket in history for a month or two. If it's Starship, the "biggest rocket" trophy won't go away in a month or two.

  2. One of the things going for multiple small engines is an incredible redundancy. Not to mention it's easier to build, test and perfect smaller engines over larger ones (which is why the Aerojet M1 was never finished, one engine at 6.67 M N.)

    And we can see that with the Raptor vs the BE4, somewhat, kind of.

  3. As I understand it, the huge F1 engines had serious combustion stability problems. The vibration levels on Saturn V were uncomfortably large. I would suspect that the individual vibration levels of the Raptors will tend to cancel each other out and result in a very smooth ride.

    1. I pity the craft if ALL of the engines decide to hit matching amplitude F-sub-0 at the same time! IF the booster can take it for 2-1/2 minutes, it's golden.