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.
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.
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