ST401385@BROWNVM.BITNET (01/31/86)
There have recently been several postings relating to
electromagnetic launch from the Earth's surface.
Why not use an accelerator boost for a next-generation shuttle?
Rockets are not very energy efficient. Most of the energy of the
fuel goes to the exhaust. The other problem is that they have to carry
their reaction mass with them. Propulsion is much more efficient
if you can push against something else, like the earth.
In fact, propulsion energy efficiency approaches unity as the mass of
what you're pushing gets large compared to your mass.
Suppose we boost a shuttle from a linear accelerator attached
to the side of a mountain.
For an example, let's suppose this is Mount Kenya, a 5 Km
tall mountain on the equator (the most efficient place to launch
from, at least if you want equatorial orbits.)
A track slanting 45 degrees up the side of the mountain has a path
length of 7.4 km. Use an electromagnetic accelerator ("mass driver")
to boost it at one g. At the end of the track, the shuttle will be
moving at 380 m/sec, or mach 1.2 (speed of sound at 5 km is 320 m/sec).
Boost time is 39 seconds, and the acceleration on board (vector sum of
applied acceleration and gravity) is 1.85 g--not very stressful at all.
Certainly this is gentle compared to what most of the mass-driver
people design for, and with much smaller problems with how fast
power switching is required.
Now, 380 m/sec is not a large fraction of what is needed for orbit,
and the 5 mile altitude is small compared with orbit. However, keep in
mind that we are substituting cheap electrical energy for expensive
rocket propulsion at the most important part of flight, where we are
lifting not only the shuttle, but also a huge load of fuel.
I don't have good figures
for the shuttle weight and impulse, (how much of the fuel is needed to
get it to the first 380 m/sec?) but a rough calculation tells me that
this will save about 33%. You could use only one SRB, or
reduce the size of the ET to about a quarter of what it now is.
In fact, a tank that small would probably be better incorporated into the
shuttle (or into the SRB's): no throw-away parts.
How much energy would such a thing take? Lets see, if the vehicle
masses say 2000 metric tons; 150 billion joules. 40 megawatt hours?
That's tiny. Average launch power is 4 Gigawatts: high, but not
unreasonable.
Is this idea totally nuts, or would it work?
--Geoffrey A. Landis
Brown U.al@vger.UUCP ( Informatix) (02/02/86)
In article <8602010854.AA02457@s1-b.arpa>, ST401385@BROWNVM.BITNET writes: > > There have recently been several postings relating to > electromagnetic launch from the Earth's surface. > Why not use an accelerator boost for a next-generation shuttle? > Rockets are not very energy efficient. Most of the energy of the > fuel goes to the exhaust. Energy is a very small fraction of the cost of space flight. The vast majority of the cost is engineering salaries. Thus, minimizing engergy, while helpful, is not decisive. If you really want to reduce the cost of space flight, build an inexpensive, first class CAD workstation for space flight engineering. Right now the MacIntosh, with a couple thousand dollars worth of software and a hard disk, is a very good system for the aerospace engineer but it needs special purpose analysis software. Somebody ready to port NASTRAN to the Mac? > Suppose we boost a shuttle from a linear accelerator attached > to the side of a mountain. Apparently you haven't been on any tall mountains lately. The weather is awful, land slides are common, there's a lot of snow, sometimes they are volcanically active, and support facilities are usually minimal. Weather causes serious problems for the shuttle in Florida, think what a really first class series of mountain storms would do. > For an example, let's suppose this is Mount Kenya, a 5 Km > tall mountain on the equator (the most efficient place to launch > from, at least if you want equatorial orbits.) Africa is pretty unstable politically, Otrag (Ortag?) had to abandon launch attempts in Africa as the result of a revolution. In addition, you need a lot of support services to launch satellites, better stick to well developed industial countries, although Ariane seems to have done OK in South America. Also, a nearby seaport is extremely useful for many payloads. > Is this idea totally nuts, or would it work? The physics seems OK, although you need to get a lot more acceleration to really gain much. Shuttle launch maxes out at about 3g's so you could rework your figures, operational problems are overwhelming though.
ems@amdahl.UUCP (ems) (02/04/86)
In article <328@vger.UUCP>, al@vger.UUCP ( Informatix) writes: > > > For an example, let's suppose this is Mount Kenya, a 5 Km > > tall mountain on the equator (the most efficient place to launch > > from, at least if you want equatorial orbits.) > > Africa is pretty unstable politically, Otrag (Ortag?) had to abandon > launch attempts in Africa as the result of a revolution. In addition, > you need a lot of support services to launch satellites, better stick > to well developed industial countries, although Ariane seems to have > done OK in South America. Also, a nearby seaport is extremely useful > for many payloads. > A small point: French Guianna (or however it is spelt...) is an anomally in the world of colonies. It is legally a 'District' of France. To the french, districts are somewhat like states are to the US. French Guianna *is part of France*; and therefor part of a well developed industrial country. (Unless some political change has taken place in the dozen or so years since I learned of this state of affairs...) -- E. Michael Smith ...!{hplabs,ihnp4,amd,nsc}!amdahl!ems This is the obligatory disclaimer of everything.
space@ucbvax.UUCP (02/06/86)
This message is empty.
ST401385@BROWNVM.BITNET (02/18/86)
>... one can build towers that literally extend out of the atmosphere. >...T300/934 Graphite/Epoxy composite, used in airplanes, has a >density of 0.057lb/in^3 and a compressive strength of 215800 lb/in^2. >...the height of a column of graphite/epoxy that just barely can >support its own weight is 59.7 miles. If we taper the tower, we can >make it taller. You also want to work at less than theoretical >strength, but the order of magnitude is correct. Now a 60 mile tall tower: *that's* SF. It would have to be either very tapered, or else wire braced (like a radio tower) to avoid buckling. And if it is to support, say, 10 tons, it's gonna take a *lot* of graphite. But it's a neat idea. (still, why not build it on top of a mountain? On the equator? If you don't like Kenya, how 'bout Cotopaxi, in Equador (6 Km) or Huascaron, in Peru (7 Km), or even Mauna Loa, in Hawaii (not on the equator but at least near it, 4 km)?) Where did you get the compression strength of graphite, by the way? Is this the strength to failure by plastic deformation? Does it include the epoxy matrix? The electrically-assisted shuttle launch was an idea for a lead-in to true mass drivers. Most of the mass driver ideas I saw four or five years ago (I haven't kept up) required huge amounts of power in one or two seconds, to sustain hundreds of g's to get into orbit. This stretches out the power need to almost a minute, and lowers the g's to something that could launch almost any payload, including people. Gene O'Neill spent a year at MIT looking at mass drivers, and delivered a lecture talking about a "telephone pole launcher" to deliver packages about the size and shape of a telephone pole to orbit from Pike's peak. (or, at least almost to orbit: it obviously needs an apogee kick to make an orbit not intersecting the surface). I assume he published somewhere, but I don't know where. This would make a good device for getting kevlar into orbit if you want to make some sort of skyhook. --Geoffrey A. Landis Brown University. <Note to Evelyn C. Leeper: OK, I'll nominate Mark for a Hugo for best fan writer if you nominate *me* for best new writer (Campbell Award) >