Proposal for a crewed tether-centrifuge mission

[Starman4308]

Hello,

I've been thinking a bit on what I think could be a good mission that can be carried out with mostly existing or near-future equipment. While I think it's at least the basis for a solid mission, I'm short on some details, including some of the necessary physics.

 

Overall mission description: study the effect of simulated low gravity (between microgravity and full Earth gravity) on astronauts and potential Moon/Mars equipment using a rotating pair of spacecraft attached by tethers. This, I suspect, will be a much lower-mass option than a true centrifuge module.

Custom equipment necessary: counterweight module, habitat and service module, tethers.

Near-future equipment requested: Commercial crew delivery program spacecraft (CST-100 Starliner, Crew Dragon). Backup in Soyuz. Possibly near-future heavy launch vehicles.

Orbit: LEO? Must be at sufficient altitude that, if the tether gets cut at the unluckiest possible moment, habitat section perigee is still in LEO.

 

Launches Necessary:

Launch 1: Tether counterweight. Can optionally be replaced by a copy of the other sections, at obvious increase in cost.

Launch 2: Habitat/Service Module. Will rendezvous and latch to tether counterweight. Rotation will not be initiated at this point. Will have 3-4 docking ports in addition to the counterweight latch.

Launch 3: Cargo spacecraft: Cygnus, Dragon, Dream Chaser. Will dock to habitat, containing consumables, scientific equipment, and miscellaneous supplies.

Launch 4: Crewed spacecraft: CST-100 Starliner, Crew Dragon, Soyuz. Contents: biological payload astronauts to conduct the mission. Will dock to the habitat.

Launch On Demand: Rescue spacecraft, probably the same as the crewed spacecraft. Hopefully unnecessary.

 

Upon docking to the assembly, the crew will first verify functionality of the habitat, then verify good docking and contents of the cargo spacecraft. If necessary, supplies can be transferred by EVA.

Once the habitat-cargo-crew assembly is verified, the counterweight and habitat will tether to each other. My guess is the physical tethers would be on the counterweight module (which is otherwise dead mass). An EVA will be conducted to verify tether attachment, with any malfunctioning tethers either replaced or fixed by the astronaut(s) on EVA.

Only once all this is verified will the counterweight and habitat separate to mission distance and begin rotation. Crew will likely be in the crew vehicle and in flight suits for an abundance of caution. Propellant for the separation and rotation can be carried on the counterweight, which again will mostly just be dead mass.

Next: Science!

Upon exhaustion of the crew transfer vehicle's endurance or the cargo vessel's supplies, rotation will be stopped, tethers withdrawn, and crew will return to Earth. Re-use of the mission would probably require another cargo spacecraft, another crewed spacecraft, and another LOD rescue spacecraft prepared.

In event of habitat depressurization or other failure, crew will evacuate to the spacecraft and return to Earth. In event that cargo vehicle failure is only detected at mission start, either crew can return to Earth, or a readied cargo spacecraft can be sent. In event that the crew transfer vehicle fails, that's what the Launch On Demand spacecraft is for. In event of unplanned tether separation, crew is to very quickly return to the spacecraft and return to Earth.

 

Simulated gravity will probably be pegged at lunar gravity (0.17G) initially, though the station should be designed for up to 0.5G (in excess of 0.38G Mars gravity), and down to... 0.05G? Maybe?

If we use a 200 meter tether, and I didn't mess up the math, that would mean a range of linear velocities from 7 m/sec to 22.15 m/sec, with rotation rates of 4 degrees/second to 12.7 degrees/second. Assuming our tether is made out of Kevlar, each side of the tether is 50000 kg (probably an overestimate), the tethers have 4x the necessary tensile strength, and I still didn't screw up the math, we would need about 160 kg worth of tether material, with a total cross-sectional area of 5.4 cm².

If the rotation rate is too fast, tether mass goes up linearly with tether length, while rotation rate to maintain simulated gravity goes up proportional to the (-1/2) power.

 

Launch vehicles:

Counterweight and Habitat: ???. The mass and volume of these, I am really uncertain on, other than a suspicion they would require a heavy lift vehicle: Falcon Heavy, Delta IV Heavy, New Glenn, Proton, or Ariane 5. The counterweight's volume is small (mostly ballast), so it should fit in the existing Falcon 9/Heavy fairing, although a custom payload fairing adapter may be required. The habitat and service is likely to be larger in volume, particularly if a rigid habitat is chosen; I find it unlikely this would fly on the volume-limited Falcon 9/Heavy.

Cargo spacecraft: I'm going to mix it up and suggest Cygnus launch on F9 and Dragon on Antares. I'm sure Orbital ATK and SpaceX would be very happy with this arrangement.

Crewed spacecraft: Pairs exist for all of these. Atlas V 552 for CST-100 Starliner, Falcon 9 for Crew Dragon, Soyuz for Soyuz.

 

One other element that I've kept in mind: the current commercial crew transport services are designed for the ISS, but the ISS is an aging station, and I have no confidence whatsoever that it will be promptly replaced, leaving us with crewed spacecraft and nowhere to send them. This would be a mission utilizing mostly either existing hardware or near-future hardware to fill in a gap in knowledge that cannot be practically addressed aboard the current ISS.

EDIT: Naturally, I only think of a significant problem after I post: I'm not sure modern docking ports are designed to survive 0.05g accelerations, nevermind 0.5g! Crew and cargo spacecraft may have to dock at the center of the tether at a special non-rotating attachment point, and that ruins the nice contingency plan of "if the hab fails, just board the spacecraft!".

Edited January 18, 2018 by Starman4308

[Ultimate Steve]

I agree, a low gravity medium term mission is probably necessary at some point. However, it can be done for cheaper, maybe even in one launch.

Say you took a very lightly modified Falcon 9/Dragon combo (Starliner might also work) so that once in orbit, stage two and the Dragon would be connected with a tether. The Dragon's RCS could be used to spin it up. The tether would have to be specially attached to the Dragon/S2, hence the slightly modified (as you said, docking ports probably can't handle the forces) part. Crew Dragon isn't huge, but it's still pretty large considering it's a space capsule. A crew of three might work for medium term. the extra oxygen can fit in the trunk.

Granted, it would be pretty cramped and you wouldn't be staying for six months, but it would only require one mission if done right.

I do like the idea of a multi launch artificial gravity complex, though. Some day...

[Nibb31]

16 hours ago, Starman4308 said:

Custom equipment necessary: counterweight module, habitat and service module, tethers.

Near-future equipment requested: Commercial crew delivery program spacecraft (CST-100 Starliner, Crew Dragon). Backup in Soyuz. Possibly near-future heavy launch vehicles.

Orbit: LEO? Must be at sufficient altitude that, if the tether gets cut at the unluckiest possible moment, habitat section perigee is still in LEO.

This, and 5 launches makes it a non-starter.

Huge development budget for not enough ROI. You could build a small moon base for a similar budget.

[tater]

The ideal experiment would be to do a long term mammal study in lunar and then martian gravity.

No need for crew. 2 launches. Both are (soon to be discontinued) Dragon capsules. One has a berthing adapter/tether in the trunk, with 1 berthing adapter facing out, one in. Dragon A docks to adapter in the truck of Dragon B. Dragon B separates the adapter in its trunk, and pulls away. It flips, and berths to the other side of the tether adapter. Now 2 Dragons, nose to nose, with a tether spool between them. Use RCS to spin the pair, then let out the spool. In each Dragon are science experiments, ideally live mammals with automated feeding, etc. Long term habitation at the desired gravity is tested, including gestation and growth of babies.

Edited January 18, 2018 by tater

[DerekL1963]

19 hours ago, Starman4308 said:

I've been thinking a bit on what I think could be a good mission that can be carried out with mostly existing or near-future equipment.

 

19 hours ago, Starman4308 said:

Custom equipment necessary: counterweight module, habitat and service module, tethers.

The idea of using existing equipment is a good one, but cancelled out by having all the important bits be new-build.   And in the case of the tethers, not only new build but requiring an extensive and expensive R&D program.  Tethers are still...  extraordinarily poorly understood to put it very charitably.

[peadar1987]

Counterweight could be a spend upper stage, I think that's what was proposed for Mars direct. It's going almost to orbit anyway, might as well give that mass a bit of an extra kick instead of needing an extra launch.

Resupply could be carried out with a "climber" that gets attached to the central point of the tether, then crawls along the tether to the hab with supplies. That's a little tricky to achieve, but my gut says easier than docking Interstellar style with a rapidly rotating station.

[HebaruSan]

1 hour ago, peadar1987 said:

a "climber" that gets attached to the central point of the tether, then crawls along the tether to the hab with supplies

Or, at the start of the mission, the counterweight is supplies. At the end of the mission, the counterweight is waste. Your climber ferries them back and forth.

[Starman4308]

1 hour ago, peadar1987 said:

Counterweight could be a spend upper stage, I think that's what was proposed for Mars direct. It's going almost to orbit anyway, might as well give that mass a bit of an extra kick instead of needing an extra launch.

Resupply could be carried out with a "climber" that gets attached to the central point of the tether, then crawls along the tether to the hab with supplies. That's a little tricky to achieve, but my gut says easier than docking Interstellar style with a rapidly rotating station.

A lot of the idea was based around "dock in zero-G, then begin rotation". A good thought that, shortly after posting, hit the brick wall of "but current docking mechanisms can't withstand being attached to something accelerating at anything more than a very gentle pace".

5 hours ago, Nibb31 said:

This, and 5 launches makes it a non-starter.

Huge development budget for not enough ROI. You could build a small moon base for a similar budget.

Are you factoring in the cost of developing the near-future vehicles (commercial crew delivery, HLVs) in that budget? I'm having a hard time justifying how the cost of a relatively small amount of custom equipment could match the cost of a lunar base, that requires the crew, the habitation module, the supplies, basically everything but the tether and counterweight, not in LEO but all the way on the lunar surface, something that takes an additional ~5.5 km/sec.

There's not only the raw dV requirement for a lunar base, but the fact that you have to develop a new manned lunar landing vehicle, a stack to get it there, any necessary lunar cargo vehicles, etc.

Granted, I've probably overestimated how ready for use orbital tethers would be: that could be a significant project on its own, in part for a reason I hadn't considered at first: the energy stored in a tether when it snaps. A tether hit by an MMOD hazard could have to be somehow stopped, which probably means the tether would have to be built in segments with robust baseplates, and it's definitely more complicated than I first thought.

I'm still pretty amazed by the claim that this would be more expensive than a lunar base.

[DerekL1963]

41 minutes ago, Starman4308 said:

I'm still pretty amazed by the claim that this would be more expensive than a lunar base.

Nobody made that claim.
 

43 minutes ago, Starman4308 said:

I'm having a hard time justifying how the cost of a relatively small amount of custom equipment could match the cost of a lunar base

You're not grasping that it's not the amount of custom equipment - it's the nature of the custom equipment.  The habitation module is basically a free flying space station, with all the complexity and cost that implies.  Not to mention the costs involved in developing and flight qualifying the tether system before committing human lives and and expensive hardware to it.

 

47 minutes ago, Starman4308 said:

Granted, I've probably overestimated how ready for use orbital tethers would be

"Overestimated" doesn't even begin to describe the situation.  Tethers on this scale are basically TRL 1, maybe 2 at best.  Nobody is going to commit money until you're at TRL 5-6.

[Starman4308]

Just now, DerekL1963 said:

Nobody made that claim.

Do you have Nibb31 on ignore? He made that exact claim, or at least something close to it, that it would be a "similar budget".

That is what I have an issue with at this point. I've given up on the tethered space ship proposal being economically feasible without a lot of fortuitous developments, but I am still amazed by the assertion that it would be on the same scale as a lunar base, with all the complexity and custom equipment that implies.

[DerekL1963]

9 minutes ago, Starman4308 said:

Do you have Nibb31 on ignore? He made that exact claim, or at least something close to it, that it would be a "similar budget".

He said "similar", you said "more expensive than".  Words mean things.
 

11 minutes ago, Starman4308 said:

I am still amazed by the assertion that it would be on the same scale as a lunar base, with all the complexity and custom equipment that implies.

That's because you don't grasp the complexity and custom equipment your concept requires.   Something that I've mentioned twice now and that you seem to have missed or ignored.

[Starman4308]

7 minutes ago, DerekL1963 said:

That's because you don't grasp the complexity and custom equipment your concept requires.   Something that I've mentioned twice now and that you seem to have missed or ignored.

You win. EDIT: You made your argument, you convinced me, I'm just not very happy with how things got misinterpreted.

I'll call in a mod and have this thread locked, because I am getting very angry at how things are being said here and I do not trust myself to keep a civil tone any further.

Edited January 19, 2018 by Starman4308

[Dman979]

Thread locked at OP request.