Some years ago I was knocked off my chair when I was soldering a flash for our camera. The flash was powered by two AA batteries. How could such a dinky power source pack such a wallop?
It was because of the flash's capacitors. A capacitor will build up a charge over time and then release the accumulated charge suddenly. In this case the flash would deliver a very bright and brief flash of light when the camera shutter was open.
The orbital tether as a capacitor for momentum.
An ion engine can have an exhaust velocity of ~30 kilometers per second. That is nearly eight times that of the best chemical exhaust, around 4 kilometers per second. That means a much smaller exponent in the rocket equation. When we're taking exponents, scaling by 1/8 can make a huge difference in delta v delivered per kilogram of propellent.
The problem is the ion engine's dinky thrust. A chemical rocket can slam you back in your seat with 4 or 5 g's. But an ion engine's delicate push is barely perceptible, like the push of a feather. It takes a long time to build up delta V which makes it difficult to enjoy an Oberth benefit. It can also mean a trip lasting months for a trip that would take hours via a chemical rock
But a tether with an ion engine can take months or weeks between catches or throws to build up momentum. So while it takes a long to build up momentum, it can release or impart it suddenly with a Catch or a throw.
So a tether can impart a brief and powerful change in momentum even with the ion rocket's barely perceptible thrust. It is like a capacitor but for momentum instead of electricity.
More tether stuff
It's been a long time since I did a post on tethers. So I'm going to toss in some other random bits that have accumulated in my head over the years.
Musk and Carmack on orbital tethers.
Back in 2016, shortly after SpaceX had landed a booster on an earth platform, John Carmack tweeted:
I was delighted to see this exchange. I've been hanging around spaceflight forums since the 90s and Carmack has long been a big name in new space. Carmack and Armadillo Aerospace were X-Prize winners in 2006. I don't think I need to review what Musk has been doing.
Carmack, Fear and Dread
Carmack and ID software made a very successful computer game set on the Martian moon Phobos. The names of the Martian moons (Phobos and Deimos) means Fear and Dread. Which is very appropriate for the computer game Doom.
I love the idea of using the Martian moons as settings for science fiction stories. I believe they will be great assets in humanity's effort to settle the solar system. I've done a number of blog posts on Phobos and Deimos:
But if I'm trying to sell Phobos and Deimos maybe I shouldn't be mentioning Doom. Oh well.
ZRVTO between Phobos and Deimos
Thinking about elevators anchored on Phobos and Deimos it occurred to me there would be a Zero Relative Velocity Transfer Orbit (ZRVTO) between the moons' tethers.
The transfer orbit's velocity at periapsis matches the speed of Phobos' tether top. Velocity at apoapsis matches the foot of the Deimos tether. Thus passengers and cargo could be exchanged between the moons using very little propellent.
I hope this idea will eventually be used.
ZRVTOs in other settings
There can be ZRVTOs in other settings. The tether anchor masses need to be tide locked in circular, coplanar orbits. Which describes a lot of the moons of Jupiter, Saturn, Uranus and Neptune. I look at this in Mini Solar Systems.
In Trans Cislunar Railroad I look at ZRVTOs between possible tethers in earth orbit:
So my system of tethers is actually about 5,000 kilometers longer than LEO tether capable of slinging payloads to the moon. That's a little disappointing. But tether to payload mass is less than 4.
"My cost estimate was for near-future transport to LEO."Contemporary civil airlines' operating costs are on the order of triple the cost of fuel. (Equal shares: fuel, airframe depreciation and maintenance, and crew/ground support costs.)"If you want to do it for less than triple the fuel cost, you need to beat the standards of a viciously competitive industry that's been trying to pare costs for around a century."SpaceX currently cite the cost of fuel for a Falcon 9 as ~$200,000. So my BOTE would get us to $600,000 for a ~20 ton payload. I gather they're currently quoting about $60M, so there' someroom for improvement."