Design Reference Mission 5.0

GCNRevenger · 2008-03-20 17:12:53

If the cable failed, nothing really bad would happen, you'd just lose your artificial gravity and need to make a minor course correction.

As far as mid-course burns, do we really need to do those until the vehicle nears its destination? The last week or two of the trip could be spent zero-G without much trouble.

And as far as orienting a communications antenna or a solar flare shield, its possible to use the RCS systems to control which direction the axis of rotation points, either at Earth for communications or the Sun for solar flare shielding. Complicated yes, but not overly so.

Tether-driven artificial gravity is a realistic option, it trades a major human health concern(s) that is/are hard to fix for technical ones that are much easier to deal with.

It is true that it won't work for DRM-5 very well, but it would work for the older DRM-3.

cIclops · 2008-03-21 01:50:16

As far as mid-course burns, do we really need to do those until the vehicle nears its destination?

Probably. Phoenix for example has its TCM-2 at L+60 days and TCM-3 at E-45 days on its 10 month cruise. MER had three cruise phase corrections during its faster Type I trajectory that's closer to a manned mission.

Swoosh · 2008-04-01 05:06:51

As far as mid-course burns, do we really need to do those until the vehicle nears its destination? The last week or two of the trip could be spent zero-G without much trouble.

Mid course burns can be achieved during tethered flight. For velocity changes in the plane of rotation, thruster bursts at the appropriate tangential vector each rotation would do the trick. And for changes perpendicular to the plane of rotation, a steady gentle thrust in the direction required would do. This could all be micromanaged by computer, presumably.

cIclops · 2008-04-01 06:58:12

And what effect would there be on the other part of the spacecraft that is being rotated at the end of the tether?

RobertDyck · 2008-04-01 17:55:37

In Mars Direct, the thing on the other end of the tether is the spent TMI stage. A used rocket stage with empty fuel tanks, so it doesn't matter. Not unless you want to retain a little fuel to coordinate its thrusters with hab thrusters during manoeuvres.

cIclops · 2008-04-02 02:19:13

The tether is unstable, any firing of thrusters at one end will induce oscillations in it. The tether will have to be reeled in before course adjustments are made, then reeled back out again unless there's a clever way to control its dynamic behavior.

Swoosh · 2008-04-02 06:53:48

You could control oscillations the same way you control coning motions during launch, or for that matter just about any of the new fighter jets these days that are inherently unstable - with computers making micro-adjustments many times a second.

cIclops · 2008-04-02 07:16:22

Launchers and fighter jets are rigid structures under continuous propulsion, small adjustments will keep them on course. In space this rotating unstable structure is in a state of continuous motion, it will respond to every impulse by oscillating, there's no damping mechanism such as an atmosphere.

RobertDyck · 2008-04-02 08:35:42

So instead of connecting the tether via rigid attachment, connect it via shock absorbers. The same kind of hydraulic shock absorbers on your car. That will dampen any oscillations. In fact, you only need shock absorbers at one end of the tether.

Climbing rope is designed to be springy. It can extend it's length somewhat like an elastic band, so when a climber falls the rope won't stop him with a sudden jerk, but rather a softer elastic deceleration. You would want the same thing for the tether. Wouldn't a woven rope with elastic fibres dampen its oscillations? I believe they use a combination of Kevlar and spandex to do this, woven together. Tech chord is similar, but uses high tech material so it is lighter and thinner than climbing rope. US military Special Forces use tech cord to rappel from a helicopter.

Elastic tether mounted with shock absorbers at one end should dampen oscillations quite nicely.

cIclops · 2008-04-03 04:17:55

Shock absorbers will not control oscillations perpendicular to the tether. The tether would be more stable if it could be stiffened after deployment, however this would require more mass and probably more complexity.

RobertDyck · 2008-04-03 11:53:11

Um, no. Stiff is not good. The idea is not to produce a rigid truss structure, the idea is a light weight tether. Rigid = heavy, tether = light. You need to reduce launch weight to make it affordable. Light weight structures are flexible, they don't break from lateral stress, they give. You need a tether than can flex with lateral forces rather than shear.

Commodore · 2008-04-03 13:20:50

Elastic items teld to lose elasticity after a while.

cIclops · 2008-04-03 14:22:38

You need a tether than can flex with lateral forces rather than shear.

And a flexible tether will oscillate ...

Swoosh · 2008-04-03 16:11:51

Would it? I'm not so sure. There will be 10,000kg tension on this cable after all. Your thrust is only going to be a tiny fraction of that. And even if it does oscillate, how do you know it can't be controlled with micro-adjustments? Has this ever been tested?

Commodore · 2008-04-03 16:25:39

Would it? I'm not so sure. There will be 10,000kg tension on this cable after all. Your thrust is only going to be a tiny fraction of that. And even if it does oscillate, how do you know it can't be controlled with micro-adjustments? Has this ever been tested?

Gemini 11 toyed with a tether attached to a Agena target vehicle.

RobertDyck · 2008-04-04 00:00:33

Would it? I'm not so sure. There will be 10,000kg tension on this cable after all. Your thrust is only going to be a tiny fraction of that. And even if it does oscillate, how do you know it can't be controlled with micro-adjustments? Has this ever been tested?
Gemini 11 toyed with a tether attached to a Agena target vehicle.

They didn't spin, at least not significantly. The page you linked says "Conrad had problems keeping the tether taut, but was able to generate a modicum of "artificial gravity."" Stabilizing a tether requires more than a "modicum" of centifigual force.

A tether was also tried on Shuttle mission STS-75. But they tried to deploy a 20.7 km straight tether to generate electricity by dropping a satellite into Earth's ionosphere. It got as to 19 km before the tether broke. The broken end of the tether was burnt, they collected more current than they expected, more than the conductor could handle.

STS-46 had tried a tethered satellite, but the tether jammed after just 860 feet. The later STS-75 fixed that problem.

This mission won't connect the tether by hand like Gemini, and it won't rotate slowly with a "modicum" of acceleration, it won't attempt at 20.7 km long tether, and it won't contuct electricity. It will also use modern flight controls.

cIclops · 2008-04-04 00:08:45

There will be 10,000kg tension on this cable after all. Your thrust is only going to be a tiny fraction of that. And even if it does oscillate, how do you know it can't be controlled with micro-adjustments? Has this ever been tested?

The tension is longitudinal in the tether. Course corrections in the tethered stack will induce transverse oscillations, using thrusters to damp the oscillations will cause further course changes. The simplest solutions would be to reel the tether in before changing course or stiffen the tether. Other than the experiments described above, no testing has been done. This is why it's important to understand the dynamics before designing the MTV. Furthermore it's even more important to test if this rotation will counteract the effects of zero gravity on the crew.

Swoosh · 2008-04-04 00:19:27

Furthermore it's even more important to test if this rotation will counteract the effects of zero gravity on the crew.

How can it not? The body requires a modicum of force acting on it to remain healthy. Whether that force is from gravity or centripetal acceleration should have no bearing.

As for whether 1/3g is enough, well why not test it out in the process of going to Mars? Humans have survived longer periods at ZERO gee before. No point subjecting human subjects to months of boredom in space just to see what happens.

Meanwhile, should this topic be moved to another thread, since DRM 5.0 doesn't even employ a tether?

idiom · 2008-04-04 04:53:01

Won't the Centripetal force dampen oscillations in a flexible tether? The Neutral end will tend to find a stable rotation around the Hab as long as the Hab doesn't over-correct.

Time to go play in Mat-lab... :?

RobertDyck · 2008-04-04 07:42:15

As Robert Zubrin pointed out in "The Case for Mars", course corrections in the plane of rotation can be made by waiting until the hab is aligned with the direction of the correction then firing thrusters away from the center of rotation. That always pulls the spent stage, no tangential forces. The trick then is to align the plane of rotation with the spacecraft's plane of orbit about the sun. The Earth and Mars are both orbit in the plane of the ecliptic, so a transfer orbit will be in that same plane. Course corrections will also be in the same plane. Course corrections tend to be due to solar wind pushing the spacecraft, so that drift does not leave the plane. It all works.

Upon arrival at Mars you will have to cut the spent stage free so you can make final course corrections in 2 dimensions, and make corrections more easily (not waiting for rotation to align). So perhaps 3 days in zero-G before aerocapture into orbit. That won't cause bone loss or muscle atrophy.

cIclops · 2008-04-05 12:19:57

As Robert Zubrin pointed out in "The Case for Mars", course corrections in the plane of rotation can be made by waiting until the hab is aligned with the direction of the correction then firing thrusters away from the center of rotation. That always pulls the spent stage, no tangential forces. The trick then is to align the plane of rotation with the spacecraft's plane of orbit about the sun. The Earth and Mars are both orbit in the plane of the ecliptic, so a transfer orbit will be in that same plane. Course corrections will also be in the same plane. Course corrections will also be in the same plane. Course corrections tend to be due to solar wind pushing the spacecraft, so that drift does not leave the plane.

Such alignment will be momentary so propulsive manoeuvres will have to be made with very short pulses - a very inefficient way to use fuel unless low thrust, low Isp thrusters are used.

Earth and Mars have different inclinations to the Solar equator. The solar wind also continuously varies in its angle to the equatorial plane, so its effect on the spacecraft direction will also vary.

RedStreak · 2008-04-05 13:05:00

What about having the tether rotating perpendicular to the path? That way the engines could have a direct alignment and considering the crewed portion has the fuel versus an empty spent stage it'd be closer to the barycenter/axis of the setup.

My guess is NASA bureacrats will insist on using something more like an exercise treadmill or a rotating bed setup if this tether argument proves too costly or if it turns out those deep space issues can be mitigated with something simpiler.

RobertDyck · 2008-04-05 21:45:33

There are lots of alternatives. For example, if you just use existing zero-G exercise equipment with astronauts exercising 2 hours per day, 7 days per week, they should be as strong as Shannon Lucent after her Mir mission.

Another alternative: implant electrodes into bone marrow of all long bones, with thin wires connecting to a small implanted controller. That controller would have an implanted jack, using the new technology that uses textured metal to simulate deer antler, causing skin to grow right into it. That leaves a jack on the outside of skin, something you can plug a power source into. If the jack is in the belly button, it wouldn't be too intrusive. Electrodes with a very mild electric current are documented to cause bone growth. A battery pack carried on a belt can cause continuous stimulation the entire waking day. Combine that with vitamin supplements of calcium with vitamin D.

Lack of calcium in the blood causes osteoclasts to dissolve bone. Plenty of calcium prevents this acceleration of bone loss. Calcium alone will not absorb into the blood stream, you need to eat vitamin D with it to digest it. Note: milk has calcium and vitamin D mixed. I'm told astronauts now are given pills with calcium and vitamin D mixed. Fine, continue.

Various experiments have been done with powerful magnets held in a cast. They do induce some current in the bones which stimulates nerves that in turn stimulate osteoblasts to grow bone. However, the effect is very small, the magnets big and heavy, and they have to be held in exactly the correct position all day for weeks, hence the cast. Implanted electrodes have been shown to be much more effective, it's just that many doctors don't want to implant anything. If astronauts are willing to accept the implant, it'll work.

RedStreak · 2008-04-05 23:14:39

An implant sounds a bit extreme, and I don't think using deer antlers is enough of a workable arguement. Most attempts at 'cyborg' implants usually have to deal with the problem of the body rejecting the implant as well as infection problems - and an astronaut with an infected bone marrow (or a slew of other issues) is as bad as or worse than one with weakened marrow...all several million miles away from Earth.

RobertDyck · 2008-04-06 05:49:40

Bionic Dog Gets Carbon-Fiber Paw
20 July 2007

The world's first bionic dog, a Belgian shepherd named Storm, now has a carbon-fiber prosthetic paw. The dog underwent an amputation earlier this year to remove a tumor; his new paw should have him up and around soon. Veterinarian surgeon Noel Fitzpatrick of Farnam, Surrey (U.K.) hopes that the technique will also be of possible benefit to humans:
"The technology is not just the first time that the implant type has been used outside the human finger," Fitzpatrick said. "Because it has been implanted into the radius of the forearm of the dog, it will act as a model for human amputees in the future and provides hope for people without feet or hands."
The implant, made of titanium alloy, is cemented to the main bone of Storm's foreleg (see diagram). It is hoped that the dog's skin could eventually bond with the implant. A plug-in prosthetic paw made of laminated carbon fiber is inserted into the implant.

This has been researched for years, and the doctor in the article states it has been used before for a human finger. This prevents infection because skin grows into the textured surface of the metal. Do you get infections under your fingernails? This technology is just as secure.

As for rejection, the controller would be coated in the same material used to coat pacemakers. Wires would be as thin as a thread, strung under the skin via long needles. Conductive coating on the electrodes would be the same material used for electrodes of a pacemaker. Rather well researched technology.

On the other hand, you wouldn't be the only one squeemish about implants. I expect large scale settlement of the Red Planet will be done with reusable Earth orbit to Mars orbit ships that have rotating sections for artifical gravity. The trick is to get the initial exploration mission to happen.

I see 3 options to zero gravity:
• zero-G, use excercise equipment
• rotating with a tether
• implants

I really don't think any sort of a rigid truss for rotation will ever be feasible. The additional weight will increase cost to make it prohibitive.

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