by Robert G. Williscroft, 2015

The following is the foreword I wrote for '''Slingshot'''. R.G. changed some statements (for stability reasons, the iron rotor pipe is not "flexible", but there are sliding joints to relieve axial stress) while he trimmed it to two pages (and moved me from Beaverton/Portland Oregon to California) - ah well, editors, what can you say?

Launching to space requires a little altitude and a lot of velocity, 7.4 kilometers per second or more (30 times faster than a 747), starting at the equator. Today, we do this with rockets: huge stacks of fuel, wrapped in thin-walled aluminum fuel tanks feeding rocket motors in back and pushing small payloads in front, through thick atmosphere and clinging gravity. Most launch energy becomes a trail of hot smoke, very little ends up in the payload. Many skilled people are needed to build and launch rockets. Their salaries, divided by low launch rates, are why rockets cost so much. Is there another way?

Arthur C. Clarke's 1979 science fiction novel The Fountains of Paradise (inspired by Jerome Pearson's 1975 Acta Astronautica paper) introduced us to the space elevator, a 100 thousand kilometer superstrong cable extending vertically from the equator far into space, supported by centrifugal acceleration. Step into an elevator car on the ground, and step out in orbit a few days later. Inspiring! Mind opening! And probably impossible, for a long list of depressing reasons that colleagues are still battling decades later. Some of us said "THAT won't work. But what if...?" Two years and a dozen what-ifs later, I discovered the Launch Loop, which will throw, not lift, payload into orbit.

Imagine a stream of water out of a firehose. Without air friction, the stream might make a parabolic arc 20 meters high. Faster, and the arc goes higher and farther. A stream moving 7.3 kilometers per second would come down on the other side of the planet, and a stream moving 11 kilometers per second would keep going into interplanetary space. Wrap the stream in a frictionless hose, and...THAT won't work either. But what if...?

The Launch Loop: Replace the water with iron pipe, 5 centimeters outer diameter, 3 tonnes per kilometer, moving at 14 kilometers per second. Bend it to the curvature of the earth with a stationary magnetic track, 7 tonnes per kilometer, 2,000 kilometers long, at 80 kilometers altitude. Turn it around at the ends with powerful magnets, and complete the loop.

On the eastbound section, 5-tonne payloads ride on magnets designed for high drag, which accelerates payloads at 3 gees. Payloads exit the east end of the track between 7,600 and 11,000 meters per second, to equatorial low earth orbits, the moon, or interplanetary space. The rotor stores 450 gigawatt hours - enough energy to power the US electric grid for an hour. Launching a payload weighing 5 tonnes to low earth orbit consumes 180 megawatt hours, about 15 thousand dollars worth of electricity, or \$1.40 per pound of payload. Passengers will still need vehicles and air, but freight can be launched on wooden shipping pallets. This small Launch Loop can launch 2,000 5-tonne payloads to orbit per day. Heavier Launch Loops can launch thousands of standard 30-tonne intermodal shipping containers per hour, and also store peak power for the global electrical grid. Space travel can be as cheap as ocean cargo travel.

The whole world launches 400 tonnes to orbit per year. Launch loops will be assembly lines to add orbital velocity to megatons of payload. They will make economic and practical sense after global launch demand increases 100 fold, and grow in number and size from there. My current focus, Server Sky (http://server-sky.com), will create that launch market, using space solar power to do the world's data center computing.

In the early 1980s, there were other Fountains what-if-ers. I published in an American Astronautical Society Newsletter and other journals, and presented at many conferences. Much of that is preserved at http://launchloop.com. In England, physicist Paul Birch wrote about orbital rings in the Journal of the British Interplanetary Society. Ken Brakke, a math professor in Pennsylvania, published his version of orbital rings.

Ken, Paul, and I met at one of the Space Studies Institute conferences at Princeton. We spent three days developing nomenclature, doing math, finding errors and fixes. I met Robert Williscroft in the mid-1990s in Philadelphia while I was on a trip to the East Coast. He had contacted me about a novel he was outlining - Slingshot. We spend a day together, becoming acquainted and have kept in touch since then. Slingshot in its current form is a result of our brainstorming during that visit.

For years afterwards, Paul and I swapped ideas. Under Jerome Pearson's leadership, and with our friend John Knapman (http://spacecable.org.uk), we submitted grant proposals until Paul's untimely passing in 2012. We were friends, never competitors, though Paul was much better at reciting Tennyson. I hope one of us will be the first launch loop astronaut. Conflict makes great stories, but friendship makes great lives, so I now pass page control to my friend R. G. ...

Notes:

1) Firehose - 275 psi max line pressure -> 1.9M Pa. Assuming losses and safety factors, perhaps 500K Pa for "lift", 1000 kg/m * 10 m/s2 -> 10 K Pa per meter vertically.

2) Transfer orbit from 80 km altitude to 300 km altitude, a=6458km, e=0.01675, v0=7791.4 m/s, perigee velocity is 7922 m/s, rotational speed is 471 m/s at altitude, 7451 m/s velocity injection. Removing 7451 kg-m/s from a 14000 m/s rotor subtracts 104.3 MJ per kilogram, restoration at 80% electrical to mechanical efficiency requires 36 KWhrs per kilogram. At 8.33 cents per kWhr wholesale rates, that is \$3 per kilogram, or \$1.40 per pound. To make the numbers come out cleanly to 12 GW-hrs, that is 333.33 tonnes, or 67 5 tonne payloads.

3) Estimated 380 tonnes from 90 launches in 2014, Gunter's space page and others. For defense satellites, launch vehicle and orbit estimate.

4) MKS vs. FPS - Although some English readers in 2015 may only understand foot/pound/seconds units, far more English readers only understand MKS. Including readers in India (the most most populous English-speaking country in the world) and english-as-a-second-language readers, there are literally billions of potential MKS readers, and a few million people at most who do NOT understand MKS but do understand numbers as more than magic placeholders for "more than I can count on fingers and toes."

4a) BTW - The United States is 4% of the world's population, and minority consumer of fiction. "Slingshot" could be marketed to a much larger audience than US science fiction fans - be aware of that foreign market. Most of "launch loop fandom" is in the UK and Europe, some is in India and Japan. And if you do this right, Slingshot sales will keep growing in these markets, especially as people realize that it is the only plausible way out of the "Rocket Trap" that the Nazis put us in.