Claim
It is feasible to maintain the orbiting spaceport in LEO with horizontal "runways", despite their natural tendency to align with the gravity gradient.
Evidence
The "runways" of the orbiting spaceport are tracks built into the major chords of a long horizontal truss. The natural orientation for such a truss -- i.e., the minimum energy orientation -- would be vertical, aligned with Earth's gravity gradient. It requires active control to maintain the metastable equilibrium of the horizontal orientation.
As active control systems go, the system needed to maintain the spaceport's horizontal orientation in orbit is simple. Departures from the metastable equilibrium develop slowly, can be measured with high precision, and can be corrected quickly. The main difficulties involve assurance of reliability, management of backup modes, and integration with other spaceport subsystems.
There are three independent means available to correct drifts away from the metastable equilibrium point. In orbit, the spaceport rotates end over end at an average rate of exactly one revolution per orbit. The rate of revolution can be controlled over a small range with minimal expenditure of energy and no expenditure of reaction mass by varying the spaceport's rotational moment of inertia. That can be done by varying the distribution of mass along the truss, or by varying the length of the truss itself. The former can be done by shifting the positions of energy storage units or other movable masses. The latter cann be done through servo units operating at the junctions between truss segments. Servos between truss segments also enable controlled bending of the trust. That enables the truss to resist buckling under compressive loads.
The second means to correct drifts is through torque applied to gyroscopes or momentum wheels. That doesn't change the rotational moment of inertia of the spaceport itself, but it exchanges rotational inertia between the truss structure and the gyroscopes or momentum wheels distributed along its length.
The third means is via sets of steerable high impulse ion thrusters deployed along opposite sides of the truss. The thrusters are required in any case to maintain the spaceport's orbit after unbalanced catches and lauches have disrupted it. They are also needed for maintaining an optimal fixed orbital plane for the spaceport to accommodate shuttle launch windows. They can also be used for attitude control. However the high power draw of ion thrusters and the need to replenish reaction mass relegates them to use as backups if for some reason the first two m3qnw or control can't be used.
Reviews
The following reviews are limited in scope to the validity of the claim made above, and do not imply that the reviewer has taken a position regarding any other claim or the overall feasibility of a concept that is supported by this claim.
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Reputation: 0Verdict: SupportsI'm familiar with reaction wheels and magnetorquers from my time as a principal engineer on the Starlink project, although I didn't work directly on these sub-systems. I also have some multibody dynamics simulation experience.“The claim is likely valid and, with more in-depth engineering analysis, we'll have more insight into the cost and complexity of the orientation-keeping systems.”
I believe that this claim is likely valid. I certainly liked the originality of the idea to dynamically varying the structure's moment of inertia as a means of controlling its orientation around its axis of rotation.
Improvements to the supporting evidence could upgrade the claim from "likely valid" to "almost certainly technically and economically valid". For example:
- The concept could lock-in more on a preferred embodiment. For example, are the space port's tracks going to engineered with thousands of actuators that enable them to telescope for moment of inertia adjustment and also to self-straighten under compressive loads to prevent buckling? If so, that adds a lot of complexity which affects feasibility. This topic should be written up somewhere and then referenced here.
- For each of the suggested orientation control mechanisms, more engineering analysis is needed. For example, how much propellant would need to be shipped up to feed the ion thrusters and how much solar panel area would be needed to power them? Without a decent engineering estimate, there isn't enough information to assess the economic feasibility of each stabilization approach.
- At my last job, when referring to "gyroscopes or momentum wheels" for attitude control, we referred to them as "reaction wheels". Also, satellites use magnetorquers to unload accumulated momentum from the reaction wheels by coupling with Earth’s magnetic field.
- I think people will be suspicious about whether reaction wheels and magnetorquers can manage a structure that's many kilometers long. So, math is needed to estimate the size these components in a reference design.
- Analysis of the perturbations and disturbances is needed to generate requirements for the orientation-keeping mechanisms.
- Assuming the mass drivers use spinning screws, it would probably be a good idea to discuss whether there are massive gyroscopic forces in play that will complicate the analysis, or, if because of twin-counter-rotating screws, the gyroscopic forces all cancel out.
Another idea that might be worth considering is to architect the spaceport so that it's naturally stable. This would require placing much of the station's mass in two "anchors" one high above and the other far below the center of the runway. These anchors would then be connected to the runway with cables, like a cable-stayed bridge. The stays could transmit power between the anchors and runway's mass driver and would also help keep the runway from bending out of shape.
I think it would be a good idea to mention that the next step would be to develop a multibody dynamics simulation to validate the high-level analysis under realistic operating conditions.
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