All of this talk about the future of NASA, and with recent support going to research into nuclear rocket motors got me to thinking.
Extended periods of microgravity, as would be needed for extended trips, appears to be quite bad for a person. It can also be quite painful and lead to a long recuperation time necessary once the ship gets to its destination. It appears that compensating with short periods of hypergravity won’t do either.
The answer in science fiction has always been to spin the ship to create the 1g. But with a big nuclear motor, couldn’t you just accelerate the thing at 1g, then midway turn the ship around and decelerate it at 1g? . I understand the energy investment will be huge, but perhaps a nuclear motor could do it. Especially if the thing used fusion It would be fast and it would be uncomplicated.
Also, for someone familiar about the theoretics behind these nuclear motors that we are hearing about. Where do they jettison the spent fuel?
Let’s say you have a nuclear motor capable of ejecting reaction mass at 3x10[sup]7[/sup] m/s (0.1c).
Let’s say that the spaceship has a mass of 1 kg.
Roughly speaking (i.e., ingnoring special relativity), to obtain the 1 gravity acceleraltion you need about 10 N of force, which you’d get by ejecting about 3.3x10[sup]-7[/sup] kg/s at 0.1c.
At that rate, you’d run out of reaction mass after 850 hours. Of course, the spaceship gets lighter as you eject more mass, so you’d be able to keep running at 1 gravity for 1700 hours. Then the entire mass of the spaceship would be gone.
How do you figure? If you accelerate at 1g for 1700 hours, you’ll get out to over 1200 AU, well out of the solar system. You’ll also be going at 0.2c, so there’s an indication that our calculation should be relativistic, but when the result is three orders of magnitude greater than what we’d need to get to Mars, I think we’re set.
Well, the whole idea of the 1g was to preserve physical abilities over a long trip… 2.5 months is a while, but it’s been done before.
I’m also kinda concerned that if I get anywhere and find myself going .2c, I’m gonna wish I’d never gone.
Do you need the equivalent of earth gravity to maintain health?
Does anyone know if you could prevent bone loss etc. with the body experiencing just a fraction of 1g, or maybe even just an hour or so in a centrifuge a day, the rest in microgravity.
The current most plausible nuclear propulsion plans wouldn’t get you a full g, either. More like .001g . Most likely, you would just use a nuclear reactor to generate electrical power, and use that to power an ion drive like the one used on Deep Space 1. The key advantage of nuclear propulsion is not the amount of acceleration, but the fact that you can maintain it for a very long time. For a long enough trip (say, out to Jupiter or Saturn), this can make a very big difference.
In response to bunyip, Wired magazine just had an article about NASA experimenting with just that: using a centrifuge to create macrogravity to compensate for the physical side effects of microgravity. They scrapped the test one it became appearent that long-term exposure to macrogavity is ever worse to the human body. Check out the article here: http://www.wired.com/wired/archive/11.03/7g.html
The answer to the OP: if you can get to 1g with whatever your propulsion system is, then do it. SPOOFE also had the good idea of making the acceleration match the gravity of your destination, but only if you were planning on staying for awhile. If the trip were longer than the stay, you might as well keep up 1g and stay healthy. If your propulsion system can’t maintain 1g acceleration, then you’d best using some sort of centrifical force to mantain 1g. Even with Mars’ lessened gravity, after 18 months of 0g, most astronauts wouldn’t be albe to walk once they arrived on the red planet.
I think we could handle keeping the anstronauts’ gravity up. The big danger for interplanetary travel is radiation. I don’t think anyone is going further than the moon and back until we get past that hurdle.
Wouldn’t that mean that in the second half of the trip, the ship would be flying directly into the by-products of the nuclear propulsion system, instead of leaving them behind? Wouldn’t that require a lot of extra shielding (i.e. - mass) to deal with the radiation, which in turn would reduce the efficiency of the propulsions system?
The biggest problem, edwino – well, maybe not problem exactly – is that if you could pull a constant acceleration of 1g, you wouldn’t have to worry anyway because you could get anywhere in the solar system in only a few days.
Desmostylus’ example, however, shows a constant acceleration propulsion system would also make interstellar exploration possible. If you could accelerate a probe up to .2c or so on its way out of the solar system, there are a several dozen star systems that could be visited by unmanned probes within a human lifetime.
In reply to Northern Piper, if your ship has enough shielding to handle solar flares, you’ve got enough shielding to handle your propulsion system’ waste by-products.
It would take huge amounts of reaction mass, and an engine that has a velocity of a significant fraction of light.
We don’t have anything like that today.
But you don’t really need it. At 1g for half the trip, and 1g of braking, you could get to Mars in 2 days. At .1g, if my math is right, the trip to Mars would still only take 6.5 days. At .01g, Mars is still only about 20 days away.
At .01g, Jupiter is 58 days away. At .1g, Jupiter is 18 days away. At .1g, Saturn is 26 days away.
Clearly, if you can maintain even small amounts of acceleration continuously, the solar system is our backyard, and the trip would not take long enough for lack of gravity to have much physical effect on us.
(these numbers are somewhat simplified - I assumed straight line distances at closest approach to the Earth)
A note on the physiological effects of weightlessness:
Bears hibernate in winter for months with very little loss in muscle tone. A human with such a prolonged time of inactivity or minimal activity such as in space travel would experience severe wasting. I beleive research into how bears manage to stop their bodies from cabibalizing themselves is underway. Applying this research to humans somehow is a far better alternative to spinning the ship, from a cost perspective, once it becomes available. ( if ever).
Of course, if 1g acceleratons ever become feasable over long periods of time, that would be unnecesary.
NASA’s Prometheus project hopes to have a nuclear rocket ready for a Jupiter mission in 2010. That rocket would be capable of getting to Mars within 2 months. That’s probably fast enough that we could deal with the bone density problem.
Isn’t there a name for this particular maneuver? I know I’ve heard one for it before, but it was in a book, so maybe it wasn’t a ‘real’ name. It was also a pretty long time ago, so I don’t remember the full title or the author, but the title had ‘Archer’ in it somewhere.
The problem with spinning a ship is that from what I understand the coriolis effects cause real problems for the astronauts. So you’d need a very big structure spun relatively slowly. Otherwise, stuff happens like when you stand up your semicircular canals actually tilt and slosh around.
