Why No Space Stations with Artificial (centrifugal) Gravity?

Factual Questions
81

Your characterization of my previous post as “bullshit” argues differently. Which is curious, because as I have posted, the NTRS has literally hundreds of papers on the difficulties of Mars descent and landing modes, many of which start out addressing the landing of a heavy craft (>1 ton) as “uniquely challenging”, “represents a significant technical problem,” et cetera. I’ve worked two different crewed Mars program adjunct studies (validations and modifications of NASA DRM 3.0 and 4.0) as well as a number of other studies for hypersonic flight and reentry/recovery modes, and the entry and descent has always risen as one of the top technical challenges that can’t be resolved simply by making the system larger or more robust (unlike protection from radiation, power systems, or meeting overall mission reliability criteria). Nor is this claim unique to me or the studies I’ve worked on; there is a general acknowledgement in the aerospace community that in terms of interplanetary missions, Mars is the most difficult solid body to land upon because of the number of regimes a lander will encounter and the uncertainties associated with aerodynamics in a thin but not inconsequential atmosphere. See the various discussions of the “Six Minutes of Terror” in the descent and landing of the Mars Science Laboratory mission. (Mercury is the most difficult to reach in terms of momentum, and the Kuiper Belt Objects by duration and distance traveled, but both are almost trivial for a craft to land upon; Venus is challenging for a vehicle to survive for any significant duration due to the pressure and corrosiveness but a craft will practically float to the surface even with minimal buoyancy control.)

The ability of computational codes (e.g. RANS and LES CFD, PIC, and DSMC methods) to predict flow behavior and stability at complex shock boundary and high density plasma for a supersonic retropropulsion mode is still nascent. The data from Falcon 9v1.1 Stage 1 reentry is somewhat useful, but not representative of the dynamic pressure and angle of attack conditions that would be seen in a Mars hypersonic reentry trajectory. The only all retropropulsive reentry architecture I’ve seen had an EDL vehicle initial mass of 265 T for a landed mass of 40 T, which was twice the mass of any other architecture and required a nuclear thermal space vehicle in order to achieve Mars injection (see here and here for recent architecture studies).

So while it is true the atmosphere provides the ability to use aerocapture to slow the space vehicle, and aerodynamic braking to slow the landing vehicle during descent, it also presents significant challenges and uncertainties versus landing on an essentially non-atmospheric body. Again, this is not some claim that I am uniquely making; it is a commonly made observation in the aerospace industry among engineers and aerofluid analysts who have worked on Mars descent modes.

Stranger

82

Coming at the idea of “centrifugal gravity” (especially for non-orbital, space-faring missions) a bit differently; why not spin the entire craft perpendicular to its velocity?

Granted, this would require a vastly different design for the ship, but with decks on either end, each deck would be a larger fraction of gravity as it approached zero-Gs toward the center.

83

Not quite correct. Since you are rotating with the outer shell of the centrifuge, you have momentum in a tangential direction; if you jump so that both feet are off the ground at once, you retain this tangential momentum, and you will follow a tangent until you meet the ‘floor’ again.

From your own rotating frame of reference it will appear to you that you jump up into the air, then fall straight back to the floor as if gravity has pulled you there.

84

Since this is the Straight Dope, I should admit that the last post was a bit of an oversimplification. Because of the spin there are effects from the Coriolis force that change the characteristics of any ballistic movement significantly. This page discusses those effects in some detail;
http://www.artificial-gravity.com/Dissertation/5_7.htm
in particular take a look at the drawing of a fountain at the bottom of the page, showing the trajectory of a stream of water in a rotating habitat.

85

I really don’t like getting into this, but let’s be clear: a claim can be bullshit without the person being a bullshitter. I wouldn’t be conversing with you if I thought you were. But everyone can be wrong from time to time. Hell, I probably say bullshit all the time and don’t even know it.

Mercury is a fantastic example. As you say, landing is not too bad: 3.06 km/s from low orbit to the surface by my data. But from an Earth->Mercury direct transfer you need an extra 6.31 km/s for capture and 1.22 km/s for circularization. That’s 10.6 km/s of delta V; considerably more than you need to launch from Earth to LEO! So any probe that had to land on Mercury from direct Earth transfer would have to be of similar size, optimization and sophistication as an Earth launch vehicle (and that’s ignoring even getting on that trajectory in the first place).

Well, fortunately in the case of Mercury one can use gravity assists, like the MESSENGER probe did. But it took 6.5 years. That’s not an option for Mars since the appropriate bodies aren’t there, and even if there were it would take too much time (particularly for a manned mission).

Mars gives us an atmosphere, though. Not enough to make it easy, but easier than if there were no atmosphere at all. If there weren’t, we’d need to lose that 6.4 km/s some other way. And that means a big, complicated rocket, and certainly no way to get a ~25% mass fraction even for small probes (nuclear engines notwithstanding).

Sure. I’m not saying that we know everything we need to trivially build such a EDL mechanism. But there is at least some hard data. The SpaceX data does happen to be at an altitude which roughly corresponds to Martian surface conditions.

That’s a monster! As you know, scaling laws hurt greatly here, and the larger the vehicle the more difficult things get. In fact, I’m quite happy to agree with the original statement when it comes to very large vehicles–as the ballistic coefficient gets too high, the utility of aerobraking goes down, and you end up with something not much better than landing in a vacuum–except that you still need a heat shield and all the other stuff. It’s all loss and no gain.

Clearly, if one wants the benefits of aerobraking (and to some extent aerocapture), there are size limits, or at least density limits (landing a large but mostly unfueled ship could be ok).

At any rate, thanks for the links; I’ll read through them.

There is certainly simplicity in landing in a vacuum. But there’s no way around the laws of physics, and those laws say that if Mars were a vacuum, your 300 s Isp storable propellant engines would only get you a 1:9 mass ratio–at best. On Mars, you can do better, but it’s work.

I’m quite aware you aren’t alone in making the claim–that’s why I called it “bullshit”, which by my reckoning is a claim that’s widely repeated, superficially plausible (and perhaps containing elements of truth), but not generally true. It’s a nice sound bite that doesn’t quite convey reality. The Martian atmosphere is not the worst of all worlds; a vacuum Mars would be worse. Even limiting ourselves to existing worlds, Ganymede and Io present very significant difficulties (~5 km/s from low orbit to surface).

86

Correcting myself a bit: I misread my delta V chart; low orbit to surface for those moons isn’t that high (it’s more like 2 km/s). Capture velocities are more like 5 km/s, but those can be improved with gravity assists (not sure to what degree, though). Among the bodies with no atmosphere, Mercury has the highest requirements from low orbit to surface at ~3 km/s.

87

Still, if there ever is going to be a real thriving colony of humans in space it will probably be a basically a giant spinning tin can with houses, farms, and fields stuck to insides.