guest blogger john hare
Momentum exchange tethers could be one of the enabling technologies of the next phase of space development, and almost certainly will be a centerpiece in the phase after that. The problem with them is the same as that of RLVs, turborockets, SSTOs and a host of other enablers, “How do you get there from here?”.
The sky-stalk or space elevator is not technically or financially feasible at this time. Rotovators are technically feasible, given a reasonable budget and development timeline. We can use something that helps early with less development cost if we can find it. I suggest an almost passive tether arraingement similar to a light net on an open fishing reel. The large fish hits the light net and barely notices, until the line paying out behind it with no drag on the reel starts to mass a size-able fraction of its’ own weight. As it slows down in its’ escape swim, it becomes slow enough for the drag brake on the reel to take effect, and eventually slow enough to stop it entirely and start reeling it in for dinner.
The advantage being sought by this catcher is early entry costs to market, not technical superiority over rotovators and other concepts. This concept, if feasible, could be tested with ground based equipment with the possibility of military markets for early variants. If it passes ground testing and validation, then it could be used to deorbit LEO debris in the second test phase. In this second phase, failure doesn’t cost that much compared to a full orbital system, and it could provide a useful service. In orbital phase three, a unit could be placed in orbit for revenue service with one launch.
In this concept, a very light net of very high strength material is in an eccentric LEO orbit with perigee just above serious drag altitude, and apogee based on energy expected to be imparted to the suborbital payload it catches. This very light net is attached to a very light coiled tether with a mild taper increasing away from the net. When a suborbital payload hits the very light net, it will experience a sharp jolt, then an acceleration as it pulls tether from the coil. From the other perspective, the tether from the coil experiences massive acceleration as it is pulled from the coil. If a one ton payload is uncoiling 100 kg of tether per second at 2,000 m/s, then it will experience 20 G acceleration. A second later relative velocities are 1,800 m/s and the same 100 kg only gives 18 G acceleration. This continues until all the tether is used or velocities have matched.
 If testing indicates that this catcher has a maximum safe payload capture velocity of 2,000 m/s, then catcher apogee is raised enough to balance payload and tether to minimum safe altitude. If payload and tether each mass one ton, then apogee is raised to give an orbital velocity of 8,600 m/s at perigee, while the suborbital ship must reach a velocity of 6,600 m/s for payload release. When the tether and payload match velocities, both will then be traveling at 7,600 m/s in orbit. Then the tether group can use high ISP engines to raise energy again for the next catch.
In the early use, when tether mass is similar to payload mass, the system only offloads 1,000 m/s from the launch vehicle. It is questionable whether that would be worth doing if that was all that could happen. There are a few reasons to think about it though.
If a newspace company is launching small payloads to orbit from pop up boosters, then the upper stage is very likely to be dense fuels. With dense fuels, that 1,000 m/s savings translates to a mass ratio of about 1.35. With a marginal payload capacity if any, that upper stage could put a heavier payload in the net equal to 35% of its’ dry mass. The difference between 5-10% payload and a 40-45% payload is huge in market terms. More important early in the game is the difference between -20% payloads and +15% payloads.
Second possible reason is offloading critical control functions to the point of delivery spacecraft. This eases the up front development burden on start up launch services. The delivered payload can be in a padded box instead of sophisticated final stage. This supposes that the capture system is part of a dedicated receiver complex. Or if the receiver system is a propellant depot, it could refuel the payload for additional revenue, while catching full tanks with any excess capacity.
Third is that by going to a tether catcher substantially larger than the caught payload, it is possible to offload 1,800-2,000 m/s from the suborbital vehicle. SpaceX could possibly set this up by using a Falcon I to place a system catching microsats from the smaller companies. Dropping the dense fuel mass ratio by 1.8 instead of 1.35 is a major enabler if it could be put in place. An dense fuel upper stage could get by with a mass ratio of 5 running from a mach 4 pop up to the catcher. That is nearly within the capabilities of several start ups now.
This system is scalable. If it works on the small, it can work on the large by adding tether segments as required. Bootstrapping this concept is a given if it works. The very smallest functional unit could start by catching further segments between revenue catches. Over a period of time, very small rocket vehicles could build this into a multi ton operation working with the largest craft that could handle the stress.Â
Calculating the actual performance of a system like this is limited by the acceleration that can be applied to the tether as it leaves the coil. The tether is being yanked from the coil at an incredible rate and will need calculation of actual limits. If it is much less that I suggest here, it might be good for a propellant less rendezvous only. If it is much larger, a compelling case could be made for putting it to early use.
Obvious improvements like using an electricity generating braking system to gain power while allowing extra performance from available dead mass, would need to wait on initial feasibility investigation.
johnhare
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If this system presupposes a high Isp spacecraft that deploys / retrieves / manuevers the tether arrangement, then why not place the retrieving spacecraft in a low eccentricity orbit, with a larger semi-major axis? That broadens the operational utility (you don’t have to worry so much about the location of the perigee), and reduces the velocity differential between the catcher and the payload.
One already has to develop tether based infrastructure, so why add the complexity of a 2 km/s closing rate “rendezvous”?
Without the 2 km/s rendezvous, the payload vehicle has to reach orbit on its’ own, same as a rotovator, except that the rotovator is inherently more useful. The advantage here is the ability to start at low investment and with short development times.
The high eccentricity is in the early phase when tethercatcher mass is similar to the payload received. As tether system mass increases, eccentricity is lowered to give more assist to the vehicle coming up.
I think I see where we are diverging. Your system envisions a much shorter tether to provide a quick transfer of momentum. The more traditional proposal of (near) circular orbit MEO spacecraft requires a long tether, whose earthside end dangles at LEO height, but is moving at suborbital (for LEO) speeds.
So we are trading one high risk problem (2 km/s intercept and capture) for a different high risk problem (multi-thousand km tether control and low speed intercept and capture).
I confess I am not convinced (but I can be convinced) that your proposal has schedule or investment risk advantage over the more traditional proposals.
One difference between this one and a traditional tether arraingement is that this one can be developed and tested on the ground with military projectiles, possibly even with a market for returning expensive PGMs undamaged. The rotovator can only be tested in space. This one can be further tested and developed in revenue service by deorbiting LEO debris.
When it does go into orbital service, it will be with a good understanding of its’ capabilities and limitations. The investment cost is also far lower than a rotovator which must enter service full size at the start, with little or no test history.
In operational use, this system presents a small target area to damage from or to other orbital objects most of the time. A rotovator presents many kilometers of hazard length at any time, and it sweeps a circle of that diameter constantly. Colision avoidance is far more of an issue with the traditional units than this one.
Extra thought. Does a fishing line get damaged more often when it is on the reel or when it is stretched completely out.
I had an exchange (email or usenet, can’t remember) years ago on the subject of using tethers for energy exchange on asteroids. Two points came up:
1) The fast rotators have very (<60 km, buildable today) short tether links to balance out their weak gravitational pull. So you can easily build a zero or +positive energy system for launching from the fast rotator class of asteroids.
2) You can actually apply an economic analysis to momentum exchange via tethers where you can take advantage of the asymmetry of the values of the delta-v change to the object – the cost of the delta-v change to the tether and peg a real price on it.
We called that idea “A Momentum Bank”. A run on that bank would push you out of a desirable orbit….
For asteroids a rotovator would make much more sense than the catcher I am suggesting here. With weak enough gravity, a ferris wheel or horizontal hub mount arraingement could work. It doesn’t seem worth it to have it in such a weak orbit.
There is another form I have been thinking about of late which consists of basically a tapered bow string with masses at each end with payload in the middle. One can then exchange energy between the kinetic energy of the payload and string and the gravitational potential energy of the end masses. This can be done in a direct or harmonic fashion if desired. It is basically the pendulum form of a rotovator. Tether mass ratios or catch/release speeds are not as good as for a rotovator (yet to properly model it), but some things are simpler.
First example, a mass at a higher orbit with another mass in a lower orbit both connected by a tether. The payload has a hook which connects with the tether (bow string) at mid length. One can actively winch energy in and out of the bow string oscillation at one end. Using an elliptic orbit one can also use tether cranking up and down to pump up the orbit (without expending any propellant).
Another example is the same system on say the moon. Two towers (or hills) with a string between them with two large suspended masses (or like force system) pulling it tight. Pull the string back and use it as a catapult. The non obvious advantage is that very high speeds can be reached using large low speed forces (it is effectively a gearing system) and only flexural elements – no bearings. Launch rates can be very high and payload catching/releasing much easier.