Wildstar:
>The Saturn had some pretty darn big flame trenches to contain that
>blast and make sure it doesn't interfere with the rocket. Do *you*
>really want to land in the middle of a 10km wide area that's been under
>fusion gun attack for the last however long?
*I* wasn't even talking about landing! *I* assumed sensibly that
landing would be done with scramjets. *I* was talking about torching
cities. And I don't have to get down close enough to the ground to be
affected by the backblast to torch a city...
>Go fly some real rockets and try different blast deflector designs.
>I'll give you a clue: a flat plate is one of the worst offenders.
>Something that is angled away from the rocket is much better.
I've flown model rockets for years. I have yet to see one get damaged
by the backblast once it was ten feet off the ground...and you don't
torch cities by landing on them, then taking off again. You come down
to within a few Km, then HOVER for a while...
DJohnson:
>So basically, with Fusion drives for take-off, you're going to want
>landing pads about 10Km across for each ship! Not to mention the fact
>that they're going to need some *major* resurfacing after each use...
It would require a lunatic to take off under fusion flame unless he
WANTED to torch a city. Remember Niven's Known Space? It was an
Instant Organ Banks offense to light off a fusion drive in atmosphere.
>Of course, what GDW should do if they're going to give the ships this
>sort of drive is address the above issues in the combat rules....Which
>isn't happening...
I doubt GDW is aware that the problem exists...
>>DREAM ON, Bertil! Look at a REAL rocket launch! MUCH lower exhaust
>>velocity, MUCH lower pressures, and they DO _NOT_ form fireballs -
>>they form long tongues af flame. Which this will do as well. Only
>>this will make them longer, hotter, FAR more lethal!
>How long are these going to be, exactly? This could cause some *major*
>traffic hazards :) I think the fusion drives would, "realistically",
>change the nature of both combat and general starship practices pretty
>heavily. Things ought to be different, but probably the official GDW
>word won't be that much different (sort of like how they probably won't
>notice the "demise" of Gas Giant refueling...)
Who knows? Depends too much on nozzle design, Isp, atmospheric pressure
(though the last won't do too much terribly soon). George, you are a
rocket guy, so your fluid dynamics is more current than mine is. What
kind of exhaust profile would you get from an Isp=2,000,000 rocket with
a 100:1 mouth:throat ratio nozzle? Politely assuming that the nozzle
shape was optimized for the fluid moving through it, of course...
Bertil:
>> Loud laughter is heard in the distance! Really loud!
> Mail me if you start getting a stomach ache after all that laughter
>and I'll stop provoking it.
Actually, these last couple, the feeling is more disgust than
laughter...
>> > Ok, I'd probably (in my campaing) say that it is a BAD IDEA to run
>> > the main drive of a ship inside any atmosphere denser than code 1.
>> > And that the drive has much too short range to be used in space
>> > combat unless you manage to bluff your way in to 1km or less from
>> > the enemy.
>> Why should it be bad??? Your reasoning is specious, as can be seen
>> by looking at pictures of any Saturn launch. The blast punches DOWN!
>> Hard! Way too hard for anything trivial like atmosphere to slow it
>> down.
> You mean like nuke punches the atmopshere out of the way without
>forming a fireball? The only example of a large fusion reaction in a
>normal atmosphere we have is the hbomb and it sure forms a fireball.
Of course, this is an assinine example, since a nuclear weapon doesn't
have a combustion chamber or nozzle, or any of those good things you
need to get reasonably efficient thrust out of the thing. BTW, in case
you are interested, if you mix LH2 and LO2 in a big puddle on the
ground, then light it, IT FORMS A FIREBALL TOO! But it still makes a
jet when burned in a rocket engine...
> You yourself said that the energy output through a scoutships engines
> equals 125 Terawatts (30kT per second is 1.254 Joules per seconds aka
> watts), and considering that this only uses 25 m3 of hydrogen *per
> hour* (2G) it seems to me that the exhaust is short on mass. I don't
> have the MT books here so I might be wrong about what liquid H2
> weighs, but it looks like the scout would use 480 grams of H2 per
> second. What kind of pressure will this flimsy jet inflict a few km
> from the ship? More or less than the air pressure?
Well, I don't have my calculator handy, but assuming that the
exhaust is a cone with length 5 times diameter (which is PROBABLY a
pretty crappy assumption - it probably is more like a cone with length
50-100 times diameter), then the cone is at or above atmospheric
pressure within 7000 meters or so of the nozzle. Using the 100x
assumption, the cone is above atmospheric pressure 20Km or so from the
nozzle.
Of course, actually, the exhaust will ALWAYS be at or above
atmospheric pressure, because atmospheric pressure will shape the
exhaust flare. Which will serve to make the jet longer and thinner,
rather than the reverse...
> (And what part of the exhaust will be in radiation? If the 480g/s is
>correct, and if the velocity you stated (20,000km/s) is correct, the
>kinetic energy will account for 22 of those 30kT that comes out of the
>engines. Will the other 8kT be radiation? Some of it in the form of
>soft xrays perhaps?)
Probably not. Probably hard gammas and neutrinos. But the radiation
will be stopped by the nozzle, or the ship will be useless in atmosphere
or out of it. So the radiation will go DOWN, and make the damage down
below worse...
>> DREAM ON, Bertil! Look at a REAL rocket launch! MUCH lower exhaust
>> velocity, MUCH lower pressures, and they DO _NOT_ form fireballs -
>> they form long tongues af flame.
> MUCH denser exhaust, MUCH less radiation.
I doubt the radiation has anything to do with the shape of the exhaust.
And the density is not relevant. What is relevant is the PRESSURE.
Which is MUCH MUCH MUCH higher in the fusion drive than the Saturn...
>> Quite frankly, I am pretty disgusted by the fact that everyone seems
>> willing to change the "laws of the universe" rather than face up to
>> the situation presented, and learn to live with it.
> I am pretty amazed that you lable speculation about how a fusion
> rocket might work in different environments as 'chaning the laws of
> the universe'. I have not seen a fusion rocket of the yields we are
> talking about here and neither have you. Have anyone of us done
> professional work on this? Like simulations or small scale experiments
> for example? No? Does anyone here know wether a fusion rocket with the
> thrustlevels in question EVEN WORKS IN AN ATMOSPHERE?!!
Bertil, once the plasma leaves the nozzle, it doesn't matter a flip
whether it was the result of fusion, chemical reaction, or someone
blowing real hard. All that matters is the kinetic energy of the
particles, and the size and shape of the nozzle. And all the fluid
dynamics *I* ever learned says that a jet that hot will do an enormous
amount of damage to everything below it. And it won't form a fireball,
unless all those fluid dynamics professors were incompetent inbeciles...
---Steve Higginbotham
"A reaction drive is a weapon, whose efficiency is in direct proportion
to its efficiency as a drive." ---The Kzinti Lesson (by Larry Niven).