Although the notion of spinning a habitat capsule at the of a counterweight (or two identical capsules) seems simple in concept it is actually problematic in reality. One issue is secondary dynamics; this concept works when you think of the mass and countermass as being equal point masses but any variation in moments of inertia or shifting of mass introduces oscillations into the system that have to be damped out. Another is getting these masses into a condition in which they can be spun up without inducing other dynamics (since tethered systems depend upon having constant tension on an effectively inelastic tether). The third is that unlike having two counterrotating rings, a mass-tether system means having to propulsively induce rotational momentum into the system to ‘spin up’, and then propulsive remove the momentum to ‘spin down’ when you need to maneuver the individual components, which requires a large amount of additional propellant mass.
You can do it with an arbitrarily small propellant mass. Just extend the tether further beyond the capsules, and put the spin-up rockets at the end of the tethers.
That doesn’t help. Even if you assume a rigid member as the tether (rather than a flexible cable that is usually implied by that term) the total angular energy needed to achieve a specified rotation rate requires the same impulse regardless of how much ‘leverage’ you get with a longer moment arm.
You need energy. Energy is not the same thing as propellant. You could use a tiny amount of propellant to get the angular momentum, and then use solar or nuclear energy to reel in the tethers.
Using ion electric or nuclear thermal propulsion can certainly provide higher specific impulse and longer thrust durations than chemical rocket engines, but putting them outboard on (presumably rigid) ‘tethers’ or members doesn’t increase the impulse they can generate. Even though the moment arm increases the mechanical advantage (i.e. twice as distance requires half the force to generate the same torque but has to be imparted over the same angular displacement, and thus twice the arc length) the work that is required to get a mass rotating at some specified angular rate is the same regardless of how far away from the center of mass the force is applied:
There is no way to cheat mechanics by somehow imparting more kinetic energy to a rigid body with the same amount of applied impulse regardless of how you futz with the geometry of where the load is applied.
I didn’t say anything about ion or nuclear propulsion. You said that it takes a lot of propellant to spin something up. That is false, no matter what kind of propellant you use. It takes a lot of energy, sure, but energy can be very cheap.
I’m honestly having great difficulty following the point you are trying to make. Your post #174 suggested leaving the mass of the capsules in their operational position and extending the tethers out further with “spin-up rockets at the end of the tethers”, which doesn’t amplify the impulse provided. You now seem to be suggesting to extend the capsules out to the end of the tethers to some distance, applying some arbitrarily small amount of impulse, and then using the work imparted by pulling the masses inward against the centrifugal force to do additional work to achieve the desired rotational momentum and rotational energy. While this is conceptually possible, it would actually require making these tethers many, many, many times the length of their operational configure in order to amplify the impulse obtained from “a tiny amount of propellant” into sufficient angular momentum to generate a significant amount of centrifugal force, and any tether made of a real material is going to have some mass per unit length that will contribute substantially to the overall mass of the vehicle if they are really long.
And this is still notwithstanding all of the dynamics that such a system would have due to the elasticity of a real cable and the modal dynamics of the system that are induced every time there is some kind of change to mass properties or impulse. There are a large number of issues using some kind of tethered system that are not perfectly balanced rigid bodies that make this impractical for generating ‘artificial gravity’ for a crewed vehicle.
It’s not only conceptually possible; it’s the way that actual satellites are spun up, when for some reason that’s desired. And also very closely related to the way that they’re spun down, if for some reason they’ve acquired an undesired rotation.
Satellites and spacecraft are spun up to provide gyroscopic stabilization during deployment, because they are launched on a spin stabilized rocket (for instance, using the Payload Assist Module) where three axis control isn’t available, or to perform a “barbecue roll” to ensure even heating from solar irradiance. This is quite common where the complexity and mass of a reaction control system (RCS) is undesirable.
Smaller spacecraft and satellites sometimes employ a yo-yo despin mechanism, which can virtually eliminate rotation by transferring the spacecraft’s angular momentum into the outward radial motion a pair of counterweights attached to deployment lanyards which are unfurled and then released. This is a passive system (not requiring active control beyond initiation) of momentum transfer in which no propellants are used, and is typically employed on payloads for all-solid propellant ‘sounding’ and small orbital rockets with spin stabilized upper stages such as the Scout or Black Brant, although such a system was used on the Lunar injection stage of the Lunar Atmosphere and Dust Environment Explorer (LADEE). Such systems have the benefit of simplicity (no controls, reaction control system, propellant storage) and can easily be load balanced just by assuring that the counterweight masses and lanyard lengths are equal. It does produce orbital space debris (the counterweights and any restraint hardware that is not retained) and so has generally fallen out of use for spacecraft in LEO or MEO, and because the counterweights are expended inerts that have to be carried all the way into orbit is not really suited to heavier payloads.
In nearly 25 years of working in the space launch industry I have never never seen the kind of system you are describing (with rocket engines or motors mounted to cable deployed pods) used to either spin or despin a large spacecraft or satellite. If there is a need for despin or precise roll control, typically a regulated cold gas RCS is employed, or less frequently solid rocket engines like the Star 3 are mounted to the spacecraft to fire along a circumferential vector. For satellites and spacecraft that are expected to operate for many years, reaction wheels are utilized into the vehicle for attitude control so that control isn’t limited by a consumable and the mass of the spacecraft doesn’t change, although these are limited by the mass and maximum spin rate. Nor would such a system work in practice because the cables are not rigid and the pods would essentially wrap themselves around the vehicle unless they were actively tensioned by additional outward thrust, which would essentially be wasted impulse.
Got a cite for that? I can’t seem to find any references to actual operation satellites spun up by extending a tether. Closest thing I could find was the Young Engineer’s Satellite 2, but that’s not the same thing.
This. We are barely capable of getting people to/from orbiting spacecraft today in a predictable and routine way. We’ve gotten lucky a bunch of times, and unlucky a few times. Anyone handwaving away the difficulty and complexity of “going to Mars” is grossly oversimplifying things, and has probably been watching and reading too much science fiction.
SpaceX founder Elon Musk has said his Starship rocket will head to Mars by the end of next year, as the company investigates several recent explosions in flight tests.
Human landings could begin as early as 2029 if initial missions go well, though “2031 was more likely”, he added in a post on his social media platform X.
And I think most here agree that Elon should fly on that first trip–and be the first person to step foot on Mars!
I’d like to think that we had the tech and hardware in the 70s.
Looking at this diagram of the Saturn class of vehicles: 95db002f4ab9a0681ae3ac56640d3bdb.jpg (428×600)
Having the Saturn V sitting atop a supermassive First Stage, we could call it the Saturn 7.
Using a Skylab-type module for habitat during the year-long trip, topped with a Lunar Lander-type vehicle…equipped with a heat shield and parachutes for Mars re-entry.
What I’d like to think and what can actually work are two entirely different things.
