The SpaceX Raptor engine is still of unproven reliability.

 Copyright 2025 Robert Clark

 The explosion of the upper stage Starship during the IFT-7 test flight came as a surprise since SpaceX has promoted the idea the Starship is close to being operational to carry passengers.

 I had previously written in 2023 the Raptor engine was insufficiently reliable at least in regards to an engine intended for manned rockets:

SpaceX should withdraw its application for the Starship as an Artemis lunar lander, Page 2: The Raptor is an unreliable engine.

https://exoscientist.blogspot.com/2023/12/spacex-should-withdraw-its-application.html

 That conclusion has to be said still holds. Multiple lines of evidence lead to the conclusion SpaceX has not been completely forthright in regards to the Raptor reliability. 

 SpaceX has been disingenuous in regards to the Raptor reliability from the beginning of the Starship testing. In describing static fires of the Raptor, SpaceX referred to short 5 to 7 second burns as “full duration”. But in the industry the term “full duration” is understood to be short for “full mission duration.” It refers to static fires that last for the full length and full power level of an actual mission. They are meant to give confidence to the rocket company, and importantly also to potential customers, the engine can indeed fulfill the mission requirements needed during flight.

 Defenders of this use of the terminology have argued SpaceX is using it to mean “full planned duration”. But in the industry, if a rocket engine manufacturer wants to do a test for a shorter length they just call them tests of that shorter length. There is no logical reason for using a term well accepted in the industry with the meaning changed. The only reason that comes to mind is that SpaceX wanted to provide an unwarranted assessment to the Raptor reliability.

  The unreliability of the Raptor engine was seen in prior tests of the Starship landing procedures:

  As this video shows, the leaks and fires are seen quite commonly during restarts, though they do occur during the initial burns also. This points out another area where SpaceX has not been fully forthright about the Raptor reliability. For the SpaceX plan using multiple refuelings for their Moon and Mars flights it is absolutely essential the Raptor be reliable for 3-burns during a single flight, the initial burn, the boostback or reentry burn, and finally the landing burns.

 But astonishingly SpaceX has not done a single static test of the Raptor able to do all 3 burns for the full mission lengths, full mission wait times between burns, and full mission power levels.

 SpaceX has done a static test showing a quite large number of restarts in succession:

Adam Cuker  @AdamCuker

Guess that was only an appetizer. We end up having another test with 34 Raptor firings back-to back

https://x.com/AdamCuker/status/1849176567785967989

 This was offered as evidence of the Raptor able to do the needed burns for reusability. But actually it does the reverse. The Raptor will never have to do this number of burns in quick succession for a real flight. In contrast, the Raptor will have to do the cited 3-burns for both stages for every single flight. Why test an engine usage that will never happen in place of one that will always happen?

 The only apparent answer is SpaceX has no confidence in the Raptor to do the necessary burns for the needed burn times, wait times, and power levels.

 Several Raptors also exploded or otherwise failed on the first Superheavy/Starship test flight IFT-1. SpaceX has argued the Raptor reliability has improved with the Raptor 2. But a key failure shows the Raptor 2 still is lacking in reliability. Indeed this failure provides further support for the contention SpaceX has not been completely forthright on the Raptor reliability. This was the failure on IFT-4 during the booster landing burn.  

 

 During this booster landing burn a Raptor 2 actually exploded. SpaceX still has not “come clean” on this fact. By not acknowledging this explosive failure they are giving an inaccurate assessment of the Raptor reliability.

 There are other important implications of this failure however. SpaceX had previously told the FAA the Superheavy booster was expected after ocean touchdown to tip over and float. 

Starship/Super Heavy Vehicle Ocean Landings and Launch Pad Detonation Suppression System, p.5

https://www.faa.gov/sites/faa.gov/files/20230414_Starship_ReEvaluationEA.pdf

  It should have floated like the Falcon 9 did after a soft ocean landing:

 But in point of fact the Superheavy booster actually exploded. It appears likely the Raptor explosion during the landing burn compromised the vehicle integrity causing it to explode after ocean touchdown.

 But this has important consequences for the other booster landings over ocean and land. In IFT-6 the booster was waved off from the booster catch and did an ocean touchdown. Elon said it was likely that it would explode after ocean touchdown, which it did. But what about the SpaceX claim to the FAA that after soft ocean touchdown and tip over the booster would survive and float?

 Note that in this landing burn of the booster in IFT-6, flames were also seen shooting out the side of the booster. Even though there was no apparent engine explosion during this landing burn, as happened in IFT-4, that it also exploded after ocean touchdown suggests in this case also the booster was damaged. Then rather than the flames shooting up the side of the booster being an intentional venting it may be indicative of fires occurring in the engine bay as had been seen previously, thus compromising the vehicle integrity.

 In IFT-5 however, the booster was able to successfully complete the tower catch, despite the flames shooting up the side. In IFT-7 as well the tower catch was successful despite the flames also seen shooting up the side:

  We may hypothesize that it is the forces of the booster toppling over and impacting the ocean that cause the explosions that do not obtain during the tower catch, even though fires inside the engine bay occur in both scenarios.

 There is further evidence to suggest that fires occurring inside the engine bay are the underlying cause of the flames seen shooting up the sides of the booster during the landing burns.

 

 After the Starship explosion in IFT-7 Elon suggested the flames inside the Starship may have caused pressure build up that released flames, in this case small near the rear flap hinge:

 Elon suggested the position of the fire based on the position of this hinge:

Elon Musk @elonmusk

Preliminary indication is that we had an oxygen/fuel leak in the cavity above the ship engine firewall that was large enough to build pressure in excess of the vent capacity. 

Apart from obviously double-checking for leaks, we will add fire suppression to that volume and probably increase vent area. Nothing so far suggests pushing next launch past next month.

8:14 PM · Jan 16, 2025

 However, it should be noted because of their large size the Raptor Vacuum engines powerheads are also close to this area, so the leak could still have originated from them. You see in the image below the middle sea level engines’ powerheads are surrounded by fire shielding, and are below an apparent firewall. But the longer vacuum engines powerheads extend above this firewall.

 

 In that statement by Elon, it is also notable that Elons says the oxygen/fuel leak and resulting fire was in excess of the vent to handle. Applying that logic also to the Superheavy booster, the flames seen shooting out the side during the booster landing burns may have been due to propellant leaks and fires that were within the capacity of the larger vents on the booster to handle. 

 The second part of Elon’s statement also suggests this. Elon makes SpaceX sound sanguine about the leaks and flames within the rocket as long as they can be controlled.

 But whether they are controlled or not does not contradict the fact the flames seen shooting up the side of the booster during the landing burns are due to leaks and fires within the rocket.

 An earlier statement by Elon also suggests SpaceX had accepted the leaks, and resulting fires, by the Raptors and just sought to contain them:

Elon Musk @elonmusk

We could build a lot more, but the next version of Raptor is really the one to scale up production. We begin testing it in McGregor within a week or so. 

Regenerative cooling and secondary flow paths have been made integral to the whole engine, thus no heat shield is required. Nothing quite like this has ever been done before.

Taking away the engine heat shields also removes the need for 10+ tons of fire suppression behind the engine heat shield, as any gas leaks simply enter the already super-heated plasma surrounding the engines, rendering the leaks irrelevant.

Raptor 3 also has higher thrust and Isp.

9:38 AM · Jun 23, 2024 · 404.1K Views

https://x.com/elonmusk/status/1804871620114214978

 

 SpaceX managed to convince the FAA not to require mishap investigations when an engine didn’t start or or fire the expected length of time, as long as the public was not endangered. This was a mistake because it allowed even IFT-4 not to require a mishap investigation. But this meant SpaceX didn’t have to admit an Raptor exploded during the booster landing burn on this flight. This had the effect of giving a misleading understanding of the reliability of the Raptor.

 However, with Starship exploding in IFT-7 with a chance conceivably of the public having been endangered, it must be noted the possibility it was an engine explosion can not be ruled out, along with the possibility it was a plumbing explosion. Then the Raptors tendency to leak and catch fire must be given more serious review. 

 For these reasons, the FAA should require SpaceX to release any and all videos of the engine bays of both stages while the engines are firing, most specifically during restarts.