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What is 'Ask an Elf'?

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djgordy
962358.  Fri Jan 11, 2013 2:49 pm Reply with quote

The ideal would be something like they use in Fireball XL5.

www.youtube.com/watch?v=sBZN3c7O9wc

The initial rocket boosters are on a sled which, presumably, is collected and reused whilst the actual space ship is returns to Earth in one piece.

The trouble is that you still need a lot of fuel to push the ship into orbit and when that is spent you don't want to be carting empty fuel tanks around with you all the time. Of course, the creators of Fireball XL5 had a neat solution to this problem too, although it might not be practical in the real world.

They used strings.

 
PDR
962409.  Fri Jan 11, 2013 6:49 pm Reply with quote

Celebaelin wrote:
PDR wrote:
A pretty full answer.

That's pretty much what I thought - so this stuff* isn't only getting thrown away, it's being put into a state where it will only be re-usable over geological time if that. Except that is for the organic bits which will be 'recycled' by nature pdq.


Ah - I had assumed you were talking about natural stuff rather than man-made stuff. One thing that should be appreciated is that the actual AMOUNT of man-made stuff (the mass of metals, ceramics and plastics) which are shot out of the atmosphere to then return and "burn up" is very, very small.

The shear amount of effort/energy it takes to put something in "space" is so large that aside from a TINY number of manned flights, the typical "space object" weighs a few tens, or at most a couple of hundreds, of kilos. We dump more metal back into landfill in cooking foil every day than we put into orbit in a decade (a figure once given to me by a chap in our satalite division, but which I now can't find a cite for).

The manned vehicles are larger, but are tending to get more reusable components (like the space-shuttle's solid rocket boosters, which were all re-used after each mission). The space shuttle designs themselves were fairly re-usable as "space planes" but for reasons that are quite involved (which I can go into if people are really interested, but I don't want to bore people again unless requested) its replacement, which called the "Constellation Programme", is actually a pair of ballistic rockets rather than another "space-plane". Having said that, the degree of resuability of the Constellation system is actually higher than that of the shuttle system it replaces.

The real point is, of course, that most of the mass involved in space vehicles is fuel, and these exhaust products don't get re-used. There have been lots of studies into using ground-based accelerators (which can use energy rather than mass to achieve the required orbital speeds) but the numbers don't work at the moment. Getting to any substantial fraction of the required 18,000mph at ground level is pretty tricky (even with Lewis Hamilton at the controls) but in the thick air at ground level the problem of KEEPING that speed for long enough to get to orbit is even greater. That's why the rockts (and shuttle) all launch vertically and climb to a decent height before tipping over and accelerating horzintally once they've got to thinner air. So we're stuck with dragging the fuel with us for now.

PDR

 
mckeonj
962416.  Fri Jan 11, 2013 7:20 pm Reply with quote

http://wrongsideoftheart.com/wpcontent/gallery/stills/when_worlds_collide_113.jpg
Large image of model rocket launcher built for the movie "When Worlds Collide" (1951). The system was designed by Werner von Braun, designer of the German V2, who was "seconded" to the USA space program in 1945.
The aerodynamic 'V2' style rocket was mounted on a solid fuel rocket sled, which was initially catapulted by the gravity railway, and then flown to high altitude for transit to the other nearby planet, where it made an atmospheric landing. No return trip was required, as Earth had been destroyed.
It seemed to me at the time to be a feasible system.

 
bobwilson
962437.  Sat Jan 12, 2013 12:05 am Reply with quote

What is the economic definition of "work"?

 
PDR
962463.  Sat Jan 12, 2013 6:35 am Reply with quote

Let's assume we want to get to at least half the 18,000mph launch speed we need with a "sled along a track" system, and that the maximum acceleration we can stand is 5g. Let's ignore the detail that the fastest stable wheels ground vehicle made tio day achieved 760mph (barely). Lets also ignore the usse ogf supersonic shock waves (sonic booms) around the track area.


v^2 = u^2 + 2as

u= 0
v=0.5*(18,000)mph = ~4km/sec
a=5g =~50m/sec^2

So the required track length (s) to get to *half* the escape velocity would be:

s = v^2/2a = (16*10^6) /100 = 160km or 100 miles

Of course at this point you'd be doing 9,000mph at sea level, so the object itself would be getting fairly warm through shock (compression) heating.

You then have two choices - you can head into orbit "tangentally" (it has often been said of the old RAF Jaguars that they became airborne more due to curvature of the Earth than to aerodynamics - it's the same idea) but this leaves you in the thick air even longer, slows you down and continues with the heating issues.

The alternative would be (as shown in the film you referenced) to raise the ramp to the vertical. Let's assume we have the same 5g limit on vertical acceleration that we used for horizontal acceleration, and that we rotate to the vertical at the last minute to minimise the height of the track tower construction required. This means that the minimum turning radius we can tolerate would be:

a = v^2/r

a = 50m/sec^2
v=4km/sec

r = 16*10^6/50 = 320km

So to take the craft to a fully vertical position would require a 200-mile high tower, and that would be outside the atmosphere anyway.

OK, so let's look at it another way - if we ran a track vertically inside the highest thing we have to hand (Mt Everest - around 29,000feet, so lets call it 10km to keep the arithmetic simple) and ran along this track at the highest acceleration we can tolerate (5g), how fast would we be going at the end?

v^2 = u^2 + 2as

u=0
a=50m/sec^2
s=10,000m

v = sqrt(2*50*10,000) = 1000m/sec (call it 2,200mph)

This is barely over 10% of the speed we actually need, the effort required to get it is huge, and the resulting vehicle would STILL need to lug shedloads (technical term) of fuel with it for the rest of the boost phase.

That's why these ideas are more common in science fiction than in science fact!

PDR

 
Leith
962679.  Sat Jan 12, 2013 8:03 pm Reply with quote

Clearly you've not been following the commissioning of the new Space Catapult Centre* in Harwell, PDR.

* Ok, so it's more properly the Satellite Applications Catapult Centre, and is intended to launch business and technology innovations, but the former name seems to have stuck around these parts and is more fun to conjure with.

 
PDR
962714.  Sun Jan 13, 2013 5:19 am Reply with quote

:-)

PDR

 
Posital
968934.  Sun Feb 03, 2013 3:08 am Reply with quote

Q: What's the difference between deficit and debt?

A: Public debt is the total sum that the UK public sector owes. The deficit is the difference between the public sector's total receipts and its total expenditure over a given period.

Oh, thanks. Can't imagine how anyone can muddle them up. Unless they're "attempting to deliberately mislead the public about these statistics".

 
'yorz
968942.  Sun Feb 03, 2013 4:21 am Reply with quote

Are you your own elf, Posital?

 
Posital
968983.  Sun Feb 03, 2013 7:09 am Reply with quote

A bit of elf-help never went amiss.

 
mckeonj
969038.  Sun Feb 03, 2013 9:36 am Reply with quote

Try to avoid elf-abuse.

 
tetsabb
969191.  Sun Feb 03, 2013 7:00 pm Reply with quote

And don't forget elf 'n safety

 
Oceans Edge
969222.  Sun Feb 03, 2013 9:26 pm Reply with quote

if I didn't know better I'd say you're all elf-less

 
XimenaDelaney
1145593.  Tue Aug 18, 2015 4:12 am Reply with quote

Why not u go and check out ask community


Partly because we don't particularly appreciate people coming here for no reason other than to post commercial links to what might be considered as rival websites. (For which reason, the link that was in that sentence above has been removed.)

But mainly because people who ask their questions here do actually want to receive the right answer. QIM.

 

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