SuperHeavy+Starship have the thermal energy of the Hiroshima bomb. UPDATED, 3/8/2023.

Copyright 2023 Robert Clark

 I was curious what the thermal energy release would be if there is a mishap during the launch. I found this during a web search:

https://forum.nasaspaceflight.com/index.php?topic=52656.msg2189985#msg2189985

This calculated only for the Superheavy and got about 10 kilotons. When you add on also the Starship it comes to nearly that of the Hiroshima bomb at 15 kilotons.

See also the link given there of the estimate of the blast damage:

https://nuclearsecrecy.com/nukemap/?&kt=9.8&lat=25.9971256&lng=-97.1553612&hob_opt=2&hob_psi=5&hob_ft=98&casualties=1&psi=20,5,1&rem=&therm=_1st-50,_noharm-100,35&cloud=1&zm=13

 It might be argued that even if the total energy is released it won't be as bad as the blast damage from Hiroshima because it would only be slow deflagration. Unfortunately, it is now acknowledged the July, 2022 SuperHeavy fire during a fueling test was indeed a fuel-air detonation:

https://scanalyst.fourmilab.ch/t/spacex-super-heavy-explosion-beneath-booster-during-test/1676

 So the possibility of a detonation can not be ruled out. Likely, that July explosion was helped by the small volume close to the ground. But that possibility can not be ruled out during a launch. The only thing we have is the SpaceX hope it doesn’t:

“The real goal is to not blow up the launch pad. That is success.”

https://spacenews.com/shotwell-says-spacex-ready-for-starship-static-fire-test/

 Fuel-air explosions are very dangerous because the simplicity by which they can be detonated and for their power, without needing high explosives such as TNT:

 https://en.m.wikipedia.org/wiki/Thermobaric_weapon

 All that is needed is for a flammable substance to be widely and finely dispersed and for an ignition source to be present. Methane being cryogenic will flash to a gas and rapidly expand if the fuel tank is breached. Note an explosion can even happen if the liquid oxygen tank remains intact. Fuel air explosions can occur just using the surrounding air.

 It might be thought the July, 2022 explosion was just a one-off event. Unfortunately, it was not the only time a fuel-air detonation occurred during a Starship test. The explosion of the SN4 Starship in 2020 after a Raptor static test had already ended also was a fuel-air detonation. Two fuel-air detonations in their ground tests and the second one when there was not even supposed to be any ignition involved is disconcerting.

Scott Manley described the SN4 explosion here:

Scott Manley @DJSnM
Also, we know the dimensions of the structure, which means we can estimate the lateral speed of the flames, and I get 2-3 times the speed of sound in air. Suggesting the combustion was a detonation. Also note the shockwave shape rising up, supporting this being a detonation.
12:24 AM · May 30, 2020

https://twitter.com/djsnm/status/1266586181392674817

 In the SN4 explosion there was a failure of the quick disconnect valve. Either the methane was spewing out the Starship methane tank from piping at the bottom of the ship or from the quick disconnect valve from a ground methane tank. Either way it shows the extra danger when the fuel is cryogenic, unlike for kerosene: you can get flash, rapid dispersal.

 Videos of the explosion shows the methane that leaked below the ship flashing to a gas and becoming finely and widely dispersed. This is analogous to what would happen if there is a methane tank breach while in flight and if the methane has enough time to spread out before it ignites.

 Scott Manley in the video notes when the methane tank at the top of SN4 breaches, it gushes out methane at high pressure that immediately flashing into gas, resulting in an additional fireball. It doesn't appear though from this video that it detonates, merely appearing as large fire. Likely the methane here  caught fire before it became thoroughly mixed with the surrounding air to cause a fuel-air detonation. 

 By the way, on the question on whether leak came from the ships methane tank via piping at the bottom or from the quick disconnect valve from a ground methane tank, another video showed an earlier SN4 static test that completed successfully without an explosion had a methane leak at the bottom of the ship. This suggests the leak that resulted in the later explosion may have been from the ship:

STARSHIP SN4 FIXED FOR HOP, Loverro Resignation Riddle, Space Force New Mission.
https://youtu.be/sqbmLbRKzrE

 So fuel-air detonations happened twice in Starship tests by accident, with the July, 2022 SuperHeavy spin up test and the 2020 static fire test of Starship SN4. It may have also happened with the SN9 test landing crash:

SpaceX Starship: Slo-mo SN9 flight video shows explosion in stunning detail SpaceX's Starship tackled its latest "hop test" — and it didn't end well.
BY MIKE BROWN
FEB. 3, 2021
Ryan Chylinski, co-founder of Cosmic Perspective, tells Inverse he and his team were on a hotel balcony on South Padre Island during the launch — around five miles away. “We could certainly feel the rumble of the Raptors [the craft's engines] at this distance," he says. "And that explosion shockwave, wow!” he adds.
https://www.inverse.com/innovation/spacex-starship-sn9-flight

 Careful analysis of the SN9 explosion video may allow it to be determined if a fuel-air detonation did indeed occur, as Scott Manly was able to do with the SN4 explosion.

 There haven’t been all that many SuperHeavy/Starship tests where large amounts cryogenic methane fuel was released, probably less than 10. It should be a matter of concern that at least 2 and perhaps 3 fuel-air detonations occurred. It may be whenever there is large cryogenic methane release fuel-air detonations have a high probability of occurring.

 That is an additional risk of using a cryogenic fuel like methane rather than say kerosene. A cryogenic fuel will rapidly disperse as it flashes to gas unlike kerosene which will remain liquid at room temperature. Careful analysis by experts with fuel-air explosions have to be made to quantify this additional risk.

 It could be argued that a previous case of the Soviet N-1 rocket that exploded in flight resulted in a far less powerful detonation than the full energy content of the propellant: 

N1 launch explosion[edit]

Main article: N1 (rocket) § Second failure, serial 5L

On 3 July 1969, an N1 rocket in the Soviet Union exploded on the launch pad of Baikonur Cosmodrome, after a turbopump exploded in one of the engines. The entire rocket contained about 680,000 kg (680 t) of kerosene and 1,780,000 kg (1,780 t) of liquid oxygen.^[58] Using a standard energy release of 43 MJ/kg of kerosene gives about 29 TJ for the energy of the explosion (about 6.93 kt TNT equivalent). Investigators later determined that up to 85% of the fuel in the rocket did not detonate, meaning that the blast yield was likely no more than 1 kt TNT equivalent.^[59] Comparing explosions of initially unmixed fuels is difficult (being part detonation and part deflagration).

https://en.wikipedia.org/wiki/Largest_artificial_non-nuclear_explosions#N1_launch_explosion

 However, the extra danger of a cryogenic fuel like methane is that it will rapidly flash to a gas if the tank is breached, increasing the danger of a fuel-air explosion, unlike for kerosene which will remain as a liquid at room temperature. There is also an additional danger of a cryogenic fuel that I'll address later.

 A recent report is in approximate agreement with the assessment of the explosive force of the the N-1 rocket explosion:

Explosive Equivalence of Hydrocarbon Propellants. 
April 15, 2019
E. J. Tomei and James T. Nichols Launch Enterprise Engineering Launch Systems Division
https://apps.dtic.mil/sti/pdfs/AD1104981.pdf

 The primary propellants it considers are LO2/RP-1(kerosene) and LO2/ethanol. Note: it measures the explosive potential as a weight(in pounds) of TNT as a percentage of the weight(in pounds) of the propellant, rather than as a proportion of the total thermal energy possible. For example, for large amounts of LO2/RP-1, greater than 300,000 lbs, it multiplies the propellant weight times 0.30 to get the weight of TNT equivalent the explosion would have. Unfortunately, the report does not consider cryogenic methane since it did not have that among the rocket propellants that had been used for its review.

 However, the report does note there have been higher estimates found from prior testing for the propellants it does review:

2. Background 

No definitive analysis has been found defining the current explosive yield values in the Defense Explosives Safety Regulation (DESR) 6055.09 for the liquid oxygen/hydrocarbon propellant combinations, i.e., TNT equivalences of 10% for static test stands and 20%/10% for launch pads. However, per [6] these values can be traced to the 1961 Joint AF-NASA Hazards Analysis Board report. It is believed that these values resulted from extrapolating the Atlas propellant testing done by Broadview Research Corporation during 1957 and 1958. Reports can be found, however, from the 1961 period that ascribe yields ranging from 25% to 56% [24] and from 20% to 38% [52] at 250,000 lb of LO2/RP-1 propellant.

"Explosive Equivalence of Hydrocarbon Propellants", p. 10

 Using the 30% weight of TNT estimate for the N-1 rocket would give a value of 750 tons of TNT, while the current estimate is 1 kilotons (1,000 tons) or higher. This post to the NasaSpaceflight.com forum cites a U.S. intelligence report that gives it as 1.2 kilotons:

Re: Forty-five years ago today the first N-1 launch ended in an explosion

« Reply #14 on: 05/11/2014 11:04 pm »

I wish to begin by saying that the 6 kiloton figure is wholly incorrect.

I would like people to engage and do their "due diligence," and not depend on wikipedia for anything accurate.  There is historical research published about the Soviet space program, and one has to go looking for it.

I hereby provide a pathway to help answer this question under discussion in this thread about 5L.

There is indeed accurate information about this 5L explosion.  (The data comes from the NASIC boys.)

There is declassified documentation that talks to the power of the explosion of the N-1, and it is not 6 kilotons, or anywhere near it.  The NASIC report (title redacted, but issued 30 November 1971) has a table that talks to this 5L event, and it states unequivocally:  "1.2 kt explosion measured." 

Additionally, the table has measurements for:  "Explosion on Pad"; "Low Velocity Fallback"; and "High Velocity Fallback."

This table is reproduced in full in an article that appeared in the BIS (UK) periodical "Space Chronicle," appearing in the Autumn 2012 issue.  Here is the complete bibliograpihc citation:

The Ghosts of Tyuratam: Wright-Patterson, the “SL-X,” and What the US Intelligence Community Knew During the Moon Race. Space Chronicle 65 (JBIS Supplement 2): 71-90, 2012.

https://forum.nasaspaceflight.com/index.php?PHPSESSID=cf8smkflmsnmtfmj5kebkdi92h&topic=34080.msg1196912#msg1196912

 Then the 30% TNT equivalent estimate is too low by a factor of 1.6. So it should be 50% TNT equivalent for LO2/RP-1.

 The estimate it gives for LO2/ethanol of 35% TNT equivalent may also be too low by a similar amount. But taking the 35% estimate, LO2/methane has twice as much energy so we can estimate it as 70% TNT equivalent. For the 4,800 ton total propellant load of the SuperHeavy+Starship that would put it at 3.4 kilotons TNT equivalent. And if such estimates are likewise too low by a factor of 1.6 as in the LO2/RP-1 case that would put it for LO2/methane in the 112% TNT equivalent range, so to 5.4 kilotons. 

Blast Damage Beyond the Exclusion Zone.

 This then is quite concerning. Non-nuclear accidental explosions in this kiloton range caused extensive damage even kilometers away. See for example the Halifax explosion and the Texas City disaster. The damage would be even worse if it occurred in the air as in an air blast. This is how the Hiroshima bomb was designed for example in order to create the maximum extent of damage.

 The damage possible is likely to extend beyond the hazard area or exclusion zone for Starship:

 This is only an area 3 minutes, 15 seconds of latitude wide, that's 3.7 miles, 6 km. But this means the radius from the launch site is 2 miles, 3 km. Based on the Halifax and Texas City explosions the damage from such a blast is likely to extend beyond that.

Engine Failures on Starship Test Flights.

 In regards to the SuperHeavy flight, SpaceX has only done one test firing of all 33 engines together, and this was only at half power and barely more than 5 second duration. It is notable that 2 of the engines failed. Elon suggested this is ok because with 2 engines shutdown, it still has enough thrust to loft the vehicle to orbit.  But for all we know it could be that every 5 seconds or so, two additional engines could fail. We can't say one way or the other because tests of longer duration with all the engines together weren't done. 

 Because of the large number of engines, SpaceX should construct a separate engine test stand for all 33 engines together to test all the engines being able to fire for the true, full length of the booster flight, which would be minutes long, not seconds.

 In regards to those engine failures during the SuperHeavy test, it is notable that multiple times during Starship test flights it has happened that at least one engine experienced a fuel leak and exhibited a fire about the top portion of the engine, i.e., not due to the exhaust from the engine nozzle. In at least one of these cases, SN11, it resulted in explosion and complete destruction of the vehicle:

 

 So in more than one Starship test flight, 1 of the 3 engines caught fire. This raises the possibility that in case with multiple engines such as the SuperHeavy in an actual flight of several minutes length 1/3rd of the engines could catch fire or for some reason have to be shutdown. 

 In such a scenario SpaceX would be in a Catch-22 if it happened early in flight. If SpaceX shutdown that many engines, it would not have enough thrust to get positive lift, and the vehicle would come crashing down to Earth. Well then, they should initiate the Flight Termination System. The problem is the vehicle is nearly full of fuel, and exploding the vehicle could initiate a detonation of that large amount of propellant.

 In regards to that failure of SN11 being due to an engine fire, Elon Musk suggested it caused a hard start on that engine during a relight for the landing burn, which caused damage to the vehicle propellant tanks. However, another possibility comes to mind. 

 I mentioned previously that cryogenic propellants introduce another failure mode where detonation can occur: that's in the case of a BLEVE, Boiling Liquid Expanding Vapor Explosion:

https://en.wikipedia.org/wiki/Boiling_liquid_expanding_vapor_explosion

 In this case the fuel does not have to ignite. The BLEVE is due to a pressurized tank being exposed to heat which causes the tank to rupture explosively. In this case the fluid does not even have to be flammable. It can happen with steam for example. However, if the fluid is flammable it can cause additional detonations when the tank is breached.

 So an alternative explanation of  the SN11 explosion is the engine fire caused heating of the piping from the propellant tanks which led to excessive heating of the propellant and pressurization of the tanks until they ruptured.

 Scott Manly in his video on the SN11 failure was flummoxed in trying to give an explanation of the explosion and concludes it looks like it must have been some failure of the tanks in flight where they exploded from the inside:

 In the comment section to his video Manley, noted that the two haves of the methane header tank are widely separated in the debris field on the ground which supports the idea of an overpressure event:

Since recording this it’s been noticed that the two halves of the Methane header tank were separated by quite a distance, which likely meant that an over pressure event occurred inside the tank. - I.e. there might have been an explosion in the methane header tank meaning oxygen must have leaked in somewhere.

 See this image of the debris field:

 Manley, suggested oxygen may have leaked in but the over pressure just as well may have been due to overheating of the methane tank.

SpaceX ignored FAA safety warnings.

Two very, surprising and disturbing facts about the story here:

SpaceX ignored last-minute warnings from the FAA before December Starship launch.
Elon Musk’s company was told SN8’s launch would violate its FAA license, but SpaceX launched anyway.
By JOEY ROULETTE
Jun 15, 2021, 4:16 PM EDT|
https://www.theverge.com/2021/6/15/22352366/elon-musk-spacex-faa-warnings-starship-sn8-launch-violation-texas

The first surprising fact is that SpaceX ignored the FAA’s safety recommendation not to launch. This is about the SN8 flight remember that did crash and explode on landing. The FAA officials are described as being quite unhappy that SpaceX disregarded their safety recommendations:

SpaceX’s violation of its launch license was “inconsistent with a strong safety culture,” the FAA’s space division chief Wayne Monteith said in a letter to SpaceX president Gwynne Shotwell. “Although the report states that all SpaceX parties believed that such risk was sufficiently low to comply with regulatory criteria, SpaceX used analytical methods that appeared to be hastily developed to meet a launch window,” Monteith went on.

But a second fact about the article is actually alarming:

The FAA’s models showed that if the rocket exploded, its shockwave could be strengthened by various weather conditions like wind speed and endanger nearby homes. As a new launch countdown clock was ticking, SpaceX asked the FAA to waive this safety threshold at 1:42PM, but the FAA rejected the request an hour later. SpaceX paused the countdown clock.

 That’s really quite surprising that just due to weather conditions the FAA believed the Starship alone without the SuperHeavy could damage nearby homes if there were an explosion.

 This is actually alarming because with only having 3 of the earlier Raptor 1 engines, the propellant load on this test likely was 400 tons or less for it to be able to lift off with the propellant weight and dry mass weight. But this is 1/10th the propellant mass of the full Superheavy/Starship. So we can imagine for the full launch the effects could be 10 times worse if those same weather conditions obtained.

 By the way, the phenomenon of the weather extending the severity and extent of a blast wave is a known one. It's known as atmospheric focusing. It can happen when there is an inversion layer for example that reflects a shock wave back down to the ground.

  Robert Clark