Comment by generuso
12 hours ago
It is not clear what "full duration static fire" means, but if the stage was fully fueled, the fuel tank would have contained 1000 tons of methane. The heat of combustion of methane is 55 MJ/kg. TNT equivalent is defined as 4.2 MJ/kg. In terms of heat output (not blast or other effects) this would have been equivalent to 13 kilotons of TNT.
The first atomic bomb had yield of 20 kt TNT, of which about half was in heat, and the rest in the blast and radiation.
Depending on how full the rocket tank actually was, the fireball from the rocket explosion was in the same ballpark, or possibly even larger in the size and duration of afterglow compared to that from the Trinity nuclear test.
> TNT equivalent is defined as 4.2 MJ/kg.
It isn't this simple for liquid oxygen and methane mixtures, and there's a great deal of disagreement between industry and regulators over what the right percentage of TNT equivalence is. Naturally, industry thinks the percentage is low, and regulators are skeptical, so there's a government-run test campaign going on as we speak to collect data for proper modeling.
The TNT is relevant, because the atomic bomb energy output was defined in terms of TNT equivalent. Not the energy of the blast, but the total output. For Trinity this was 20 kt, or 20*4.2 TJ.
This serves as a basis of comparison for this deflagration. If we are considering specifically the appearance of the late fireball, the heat output is the relevant figure of merit.
Assuming about 10-15% of the total bomb energy remained in the heat of the late fireball (with the rest spent on the blast wave, peak thermal radiation and neutron/gamma radiation), the fireball of this rocket deflagration could have exceeded the late fireball from the bomb. But this assumes the tanks were fully filled, which we do not know yet.
The methane is not mixed with oxygen when it's still in the rocket tank, so it can't all explode - most of it will just burn off.
It's still a big boom, but not anywhere close to what world occur with optional mixing.
Counting frames on YouTube, I get about 0.3s for the blast front to reach the top of the 600ft towers. That gives an estimate of around 600 tons TNT so definitely nowhere near all the fuel exploding.
how did you compute that?
Sedov-Taylor-Rayleigh blast equation, though this video isn't high enough frame rate to more than ballpark it I think. Tower is ~180m high, so 0.2 sec would be a bit over 1 kiloton instead? But definitely not remotely 13 kt. Still serious of course, when SpaceX suffered launch complex damage during some of its incidents it took a solid 6-12 months to fix.
Everyone can be glad though that no hypergolics are involved at least!
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The estimate is roughly E = rho*R^5/t^2 with rho the density of air: https://en.wikipedia.org/wiki/Taylor%E2%80%93von_Neumann%E2%...
> not clear what "full duration static fire" means
You fire the rocket as if it’s going to space, but you keep it on the pad. (From the engine’s perspective, it did a full launch.)
I think the point is that that phrasing has been used by rocket companies to mean a whole range of different amounts of fuel load, it's not very precise wording in practice.
From the industry: I would expect to hear "mission duty cycle" in that case. "Full duration" doesn't have a consistent meaning (a fact which is sometimes used to the marketing team's advantage).
Flight computer tells engine to go. Full go, launch sequence. Engine goes. To me, anything but that isn’t a full-duration anything.
If clamped down, it’s a full-duration static fire. If clamps release, it goes to space. Basically, if the engine can’t tell (apart from atmosphere, which is a big apart) it isn’t going to space, it’s a FDSF. It’s a whole-engine show. If you’re running parts through a full duty cycle, that can be done in a lab (or on a stand).
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The problem is that there is no standard meaning for the "full duration" in this context.
Some reports say that this means "running all seven BE-4 engines at full thrust for up to 38 seconds".
In flight the engines fire for 190 seconds.
So what the full duration means, and whether they fill the tanks with just enough fuel for the firing, or with a larger amount to help the clamps to hold the stage down, all this we will probably only find out from the investigation, if the results are ever published.
I think it's amazing they can basically hold a rocket down and let it launch like that without things exploding or shearing apart from the forces. Are those the same bolts as the exploding ones they would use for a normal launch?
(on that note it's also amazing that these exploding bolts are so reliable, I can imagine even a single one not releasing would cause... Issues)
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Correction: The first stage of New Glenn carries only about 260 tons of methane. The 1150 tons is the full propellent load, liquid oxygen and liquid methane combined.
The heat from combustion of this amount would be about 3.4 kt, which is roughly the same as the heat in the late fireball of the Trinity test.
The mushroom cloud from the New Glenn explosion was also substantial: https://photos.app.goo.gl/a7uPVjsB5n453SJA7
the video is available, it's a large explosion, but nowhere near a trinity mushroom cloud