[AR] Re: LOX-Methane Kabooms

  • From: Peter Fairbrother <zenadsl6186@xxxxxxxxx>
  • To: arocket@xxxxxxxxxxxxx
  • Date: Thu, 26 May 2016 02:12:54 +0100

On 26/05/16 00:25, Paul Breed wrote:

mixing with buffer gas (air).
Not 100% sure this helps your case... just methane mixed with air could
make a really big bang....
The largest non nuke explosive has just 7 tons of fuel...
(Russian FOAB) 44Tons TNT equivalent...


Yep.

Worst I can think of:

Methane tank BLEVEs, methane vapour and droplets spray around. This cracks the LOX tank, perhaps half of the LOX flies all about, mixing with the methane and air, and the mechanical reaction expels any remaining methane.

Pause. Something ignites the cloud of methane/LOX/oxygen/air. Ka-boom.

The MR for a rocket would usually be about 2.8, whereas the MR for stoichiometric combustion is 4.0 - so there would be excess methane to react with the air even if all the oxygen was used, but I'm supposing about half the LOX remains in the remains of the rocket.

Some of the LOX is mixed in with the methane, so the methane-air-mixture is oxygen-rich compared to methane-air, which would make it burn better.

(it would be much easier to get a fast deflagration in methane/air/oxygen than in straight methane/air - fuel/air bombs use metallised propylene oxide, iirc. They disperse the fuel in droplet form, then ignite it. Propylene oxide is pretty reactive - but nasty - stuff and burns easily in air over large concentration ranges - the useful concentration range for methane in air is a bit inconveniently small for fuel-air explosives, unless the air is enriched with oxygen...)

In terms of destructiveness, it doesn't matter whether the methane (and LOX) is in droplets - for fuel-air explosions we are not looking for a detonation, but a large-scale fast deflagration; in order to get the most destructive and longest lasting blast wave possible, droplet form is actually more effective.



This dispersed cloud explosion gives a long-duration, well-coupled blast wave, which is generally lethal at distances where a condensed explosive of similar mass would be survivable.

In a condensed explosive, too much energy gets wasted near the explosion, where you are going to be dead anyway - local overkill, resulting in reduced effectiveness at distance.

Nitrogen in the air actually helps to couple the explosive power to the blast wave, and increases the blast effect at distance.

Also the long duration blast wave tends to push debris (~shrapnel) further than in a similar mass condensed explosive event.

This makes comparisons of dispersed cloud explosions with conventional explosives a bit meaningless, as the effects are qualitatively different. They are also different for different scales of explosion; here I am talking about the 50-1,000 ton range.

However, supposing all the methane and half the LOX reacted, I estimate the effect at lethal and highly damaging distances would be similar to about 3.9 times the mass of propellent in terms of TNT.

That's a bit of a BOTE SWAG, but for medium-large rocket sizes it would be somewhere above 3.4x and less than 4.5x TNT equivalent.


Ouch indeed.


20% ? piffle.


Oh, and then the blast spreads the remaining LOX over anything remotely flammable. Grass, aluminium fuel tanks, steel launch tower...



-- Peter Fairbrother







On Wed, May 25, 2016 at 2:48 PM, David Masten <dmasten@xxxxxxxxxxx
<mailto:dmasten@xxxxxxxxxxx>> wrote:

    The latest document is from 2008 with correction 1 Sept 2, 2011.
    6055.09-M, volume 5 is the one of interest, specifically table
    V5.E4.T5 (which is nearly identical to 2005's 6055.09-STD C9.T18).

    It may very well be the case that LO2/LCH4 should be treated more
    like LO2/LH2 than LO2/RP-1. The greater of 8W^2/3 or 0.14W for LH2
    gives greater than one adjustment at smaller quantities (<~500lb)
    while only 14% at huge quantities.  Is this because any credible
    mixing would be limited due to pure volume and time? I'm thinking if
    the BLEVE shock case it occurs before time to mix, otherwise in the
    open and sufficient time to get full mixing it would also involve
    dissipation and mixing with buffer gas (air). I don't know, just
    trying to think through it.

    Thanks,
    Dave


    On 5/25/2016 2:12 PM, George Herbert wrote:

    That's the standard, yes; the question is if it's right or not.

    The difference between 10 tons HE equivalent probably typical
    (plus satisfying fireball), and 225 tons at C9.T18 (20% to 500k
    lbs 10% above that), to a possible 1.3 kilotons for a Falcon Heavy
    at 100% TNT efficiency, to a worst case ~ 4 kilotons for the full
    load of Kerosene at 43 MJ/Kt...

    I am not convinced the scenarios *past* 100% RE are
    implausible...  The propellant energy density is ~3x higher than TNT.

    Assuming it will always necessarily go very inefficiently,
    especially if something falls back into a flame trench and
    ruptures and mixes before ignition, seems unwise.  That's the
    point.  A Falcon-9 contains over a kiloton of unreacted energy, a
    Falcon Heavy about 4 kilotons...


    George William Herbert
    Sent from my iPhone

    On May 25, 2016, at 1:33 PM, Randall Clague
    <<mailto:rclague@xxxxxxxxx>rclague@xxxxxxxxx
    <mailto:rclague@xxxxxxxxx>> wrote:

    Has no one on this thread read DDESB-6055.9? It's the standard
    reference on the topic. LOX/hydrocarbons, all of them, are rated
    at 20%. If you have 40,000 lbm of LOX/methane, and it goes bang,
    it will have a TNT equivalent (NEW) of 8000 lbm.

    The Quantity-Distance for blast is 40*(NEW^(1/3), units are feet
    and pounds. For 8000 lbm, that comes to 800 feet. That's not
    useful, because the Hazardous Fragment Distance for even the
    smallest quantity of explosive mixture is 1250 feet.

    Jeff Greason and I did have some success convincing AST that if
    you could dump one of your propellants, you need only consider
    the stoichiometric complement to whatever you hadn't been able to
    dump before impact. For example, if kerosene (assuming O:F of
    2.6:1) remaining at impact was 1000 lbm, but LOX remaining was
    only 520 lbm, your NEW would not be 304 lbm, as 6055.9 would have
    it. It would be 144 lbm.

    That's only useful if you can dump a propellant, of course.

    -R

    On Tuesday, May 24, 2016, Keith Henson
    <<mailto:hkeithhenson@xxxxxxxxx>hkeithhenson@xxxxxxxxx
    <mailto:hkeithhenson@xxxxxxxxx>> wrote:

        On Tue, May 24, 2016 at 10:08 PM, Henry Vanderbilt
        <hvanderbilt@xxxxxxxxxxxxxx> wrote:

        > Seems to me it might be worthwhile to come at this from the
        opposite
        > end: What conditions would be necessary to produce thorough
        mixing of
        > the majority of a vehicle's LOX-CH4 propellants, followed
        by detonation?

        At least once in a test the tank valves opened without
        ignition and
        the whole load of RP-1 and LOX drained out through the
        engine.  The
        mix was called gel and tended to be treated like
        nitroglycerine.  In
        this particular event (which I don't remember even what
        rocket it was
        or when), the aftermath was a rocket sitting in a large pool
        of gel.

        According to the story related to me, the test engineers just
        walked
        away and waited a few hours till all the LOX evaporated.

        There are people who have fun by filling two liter soda
        bottles with
        LOX and propane.  Apparently if you let the bottles warm up a
        little,
        the propane becomes miscible with the LOX.  If you shoot at the
        bottles with a 30-06, about half the time they will detonate
        (with an
        impressive bang).  So apparently that hydrocarbon and LOX
        isn't as
        sensitive as nitro.  On the other hand, tracer ammo will set
        off such
        mixes every time.

        Keith


    --
    David Masten
    CTO
    Masten Space Systems
    415-244-9171 <tel:415-244-9171>




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