Development Costs

There at several parts to this concept and we address them each below.

SSTT rocket

At 4000 Kg payload suborbital rocket is smaller than most orbital rockets. This makes it cheaper to develop. Each one that you build for testing does not cost so much to build. Since the rocket only needs to do about half the delta-V, the weight requirements are not nearly so tight for an SSTT as with SSTO.

Because the requirements are not that hard, there is more chance that a private company that would be able to develop a rocket to do this if it was part of their business plan. The only question is "How reusable is it really?" Do you need to replace a heat shield after each launch? Do the engines really only last 50 launches? But even with maintenance it should still be far cheaper to operate than an expendable launch vehicle (ELV).

The closest thing to the SSTT rocket may be the X-34 reusable rocket that was a NASA project valued at $85.7 mil. It seems this project was canceled when NASA wanted to change the design after it was already signed off without paying any more for the changes. This rocket is 18,000 lbs empty, and generally much bigger than the SSTT rocket proposed here. It may be that a small missile is closer to what I am thinking of.

NASA has Cost Estimating Guidelines and an online cost estimator. If I use this online estimator and put in "Launch vehicle stage", "750 lbs", and "Quantity 15", I get $240 mil.

Just to put an initial number out I am going to estimate $200 mil for development.

Note that, even with an ELV, the air launch/tether is still interesting.

Tether

It has to be launched into space on existing rockets. If the tether and other non-ballast equipment comes to 30,000 lbs and we have to pay $2000/lb then we are about $60 mil to launch it.

The tether is mostly a big rope and some ballast, which seems like it should be cheaper to develop per pound than the average satellite. We need some solar cells, ion-thrusters, and probably an separate small rocket. However, it has not been done before, so it might not be cheaper per pound to develop. A rule of thumb is that satellites cost about as much to make as they do to launch. If we use this rule then we could estimate the cost of the tether system at $60 mil.

So making and launching the tether could be about $120 mil total.

Ballast

For the tether to work right we need a substantial ballast. Bringing it from Earth would cost $2,000/lb. We eventually want 120 tons, so this would be $500 million if launched at todays prices. One interesting idea proposed by Henry P. Cate Jr. in Junk Man's Ladder: A Path to the Moon is to collect old space junk for the ballast. This should be cheaper than lifting it from Earth. Another idea is to get a Space Shuttle External Tank. Another, idea is getting some very small asteroids, though this is probably harder than collecting junk already in LEO.

Another idea is to just start out with a small ballast and send up small payloads to add to the ballast. As the ballast gets bigger the payloads can get bigger. This would still be much less expensive than regular launches, especially toward the end when you could use full payloads.

Harpoon

The harpoon firing computer needs to know how far away the target is and what the accelerations are on the target and itself, and optically aim the harpoon. There is some development here, say $20 mil.

Space Tug

The most similar thing to our space tug is the Demonstration of Autonomous Rendezvous Technology (DART) that cost $47 million including launch costs. So I will estimate $35 mil.

Solar Cells

For the initial launch, we assume lightweight, expensive solar cells at 100 w/kg. For the power hungary tether, we assume heavier (40 W/kg) but cheaper solar cells. The tether to move 4 ton payloads needs 6 megawatts of photovoltaics (PV). If we can get PV at $5/watt, this much power will cost $30 mil.

Total

Given the above estimates we are about $500 mil in development. With NASA's $15 billion/year budget they spend this much money every 2 weeks.