The Tether advantages

Tethers orbit at a higher altitude (and slower velocity) than simple insertion into LEO. An SSTT that rendezvous with a tether only needs about half the velocity to orbit. The tether acts as one stage of a TSTO (two stage to orbit), with the SSTT as the second stage. To illustrate, the following paragraphs use typical numbers for our tether and methods from the past 30 years.

Normal launch from ground

In Low Earth Orbit (LEO) circular velocity is about 8 km/sec at . You loose around 2 km/sec from drag and climb. You get around 0.5 km/sec from the spin of the Earth. So a rocket has to provide a delta-V of about 9.5 km/sec. You need to circularize your orbit which means firing the engine again about 45 minutes after launch. This restart of the engine only needs to provide about 0.1 to 0.15 km/s, depending upon the altitude of the orbit.

Air Launch from 20 km to tether at 100 km altitude.

An SSTT needs a velocity of about 5 km/sec at the end of the tether. It looses about 0.5 km/sec from climbing from 20km to 100 km and air drag. It gets about 0.5 km/sec from the rotation of the Earth. There is no need to circularize the orbit as the tether has a big ballast mass and is in orbit. This means that an SSTT needs to provide a delta-V of about 5 km/sec.

How do we get 5.0 km/s?

The orbital velocity at 100 km is 7.85 km/sec. Orbital velocity at 600 km altitude is 7.56 km/s. If the center of mass of the tether is at 600 km high, we have saved 0.29 km/sec already.

In our final design the tether tip speed is 2.5 km/sec relative to the center of mass of the tether. So relative to the center of the earth the tip is moving about 5.06 km/sec (7.56 - 2.5). Between the two, the tip is 2.79 (2.5 + 0.29) km/sec below orbital speed at 100 km. (The above is tricky and worth reading twice.)

We get 0.5 km/sec from the rotational speed of the Earth and so only need 4.5 km/sec after altitude and drag loss. Starting from 20 km high we don't loose so much to drag. Our air launch will give us a running start, perhaps 0.2 km/s (at 450 MPH). Reduced air pressure enables a more efficient rocket engine. Our ISP can be higher, so our fuel efficiency is better.

What is the result?

An air launched SSTT needs around half the delta-V of a TSTO. We needed a two-stage rocket before, but we only need one stage rocket now. It is plausible to think of the SSTT as being the smaller second stage. The first stage could have been 5 to 10 times as large as the second stage, so we have saved the big expensive stage.

The tether replaces the first stage. Our SSTT is effectively the second stage of a two stage rocket system.

Mass production or RLV

Because we only go halfway to orbit, making a re-usable single stage vehicle is easy compared to an SSTO or TSTO.

If we use ELVs, a big savings is due to expected mass production or re-usability. Because we have a large number of small rockets, instead of the usual few big rockets, we can use assembly line methods.

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Copyright (c) 2002, 2003 by Vincent Cate. All rights reserved.