space travel question

DWSTXS · Dec 3, 2009

I was wondering if this is true or not:

I was told that based upon our current technology, or maybe not based on the technology, but based upon the way we currently travel in space, that. . . a space ship, to the moon or mars etc, has no ability to simply turn around and come back to earth if there is a major problem.

I understand that it may be technologically possible to turn around, but that it's not feasible or very practical?

again: I'm talking about CURRENT day technology methods and practices used by NASA. not theoretical, and not 'if we had this technology' kind of thing.

Is this correct?

GeorgieB · Dec 3, 2009

I would say that currently, it is neither feasible nor practical. First, to get off the earth, a lot of fuel must be carried. A lot. To simply stop and turn around would mean carrying the fuel necessary to stop first (decellerating to zero relative, then accelerating to return).

To carry the fuel necessary to do all that means at least doubling the amount currently used...which has to be 80 to 90% of the total lift-off weight.

DWSTXS · Dec 3, 2009

GeorgieB said:
I would say that currently, it is neither feasible nor practical. First, to get off the earth, a lot of fuel must be carried. A lot. To simply stop and turn around would mean carrying the fuel necessary to stop first (decellerating to zero relative, then accelerating to return).

To carry the fuel necessary to do all that means at least doubling the amount currently used...which has to be 80 to 90% of the total lift-off weight.

Maybe i'm not understanding something here. Why, would one need to decellerate in order to return? Why not just keep going at the same speed, but turn and come around and head back?

GeorgieB · Dec 3, 2009

DWSTXS said:
Maybe i'm not understanding something here. Why, would one need to decellerate in order to return? Why not just keep going at the same speed, but turn and come around and head back?

How do you "come around and head back?" You would still need fuel to affect the turn, in effect decellerating that motion and then to accelerate to return. Travel in space is a whole lot different than here on earth.

Someone who can explain 3D vector motion might be better qualified to answer the question.

But, for descriptions of motions in space, you might read the James Campbell series "The Lost Fleet." Very good battle scenes, with relative motions explained.

DWSTXS · Dec 3, 2009

I think I understand what you're saying. I was just always assuming that one could make a slow turn and keep turning until one was pointed back in the direction you'd just come from and then come on back. But, I guess that the vehicles we'd use for travelling to mars and beyond wouldn't be like the space shuttle.

GeorgieB · Dec 3, 2009

Here's a great Web site with tons of info on space travel and maneuvering: http://www.projectrho.com/rocket/index.html

Tons of explanations.

GeorgieB · Dec 3, 2009

DWSTXS said:
I think I understand what you're saying. I was just always assuming that one could make a slow turn and keep turning until one was pointed back in the direction you'd just come from and then come on back. But, I guess that the vehicles we'd use for travelling to mars and beyond wouldn't be like the space shuttle.

The "slow turn" would be probably be a series of thrusts to change direction 180 degrees. Those, in itself would consume a lot of fuel (carried at lift-off time). There would still be the velocity AWAY from earth that would have to be cancelled. Coming about in space is nothing like coming about in a boat on the ocean where there is friction to cancel forward speed.

DWSTXS · Dec 3, 2009

Thanks GeorgieB. I appreciate the help, and the link.

Sevvy · Dec 3, 2009

Yeah, turning has to do with friction and having something to turn against. There's nothing to push off of or away from in space, so to turn would require a series of engines aimed and/or fired at the right times to turn the space ship around. That's why shuttles don't have steering wheels. ^__^

Pyrohawk · Dec 3, 2009

You can't simpy turn around in space. The shuttle actually carries very little fuel with it in to space. Relying on Newtons First Law, which states an object in motion remains in motion until an outside force acts upon it....to carry the shuttle most of its way. Once the shuttle reaches space and up to cruising speed, the engines are more or less shut off and the shuttle just coasts the entire distance. In space there is nothing to slow it down, no friction.

Since there is no friction in space.... You can't turn. There is nothing to turn against. Even if you somehow changed the ships orientation (like got it to point sideways or backwards somehow).....it would continue on in the direction it was headed. But to even change the ships orientation is impossible since there is nothing to use.....no air resistance even, so opening flaps or something wouldn't help!

In order to turn the ship you must fire up and use the engines. The shuttle relies on this constant motion to get where its going. Around the earth or the moon we can use their gravity to change our direction somewhat.... if your circling the earth, no engines are used just the earths gravity grabs the ship and puts it in to its orbit causing it to circle it. Same goes for the moon....if you've ever heard them say they are going to use gravity or orbit to slingshot themselves....this is what they mean. Like say they go to the moon, the moon then grabs them and they orbit around it, then when they are facing the direction they want to go like say...back to earth, they fire the engines to break the orbit and send themselves flying strait back to earth. Lots of geometry and exact timing is needed in case you hadn't gathered that already!

We just can't carry that much fuel with us, its to heavy and bulky. To carry enough to turn the ship around in mid space with nothing around to help like the moon or Mars' gravity is impractical. It would have to be a huge ship to carry enough fuel for stuff like that, and the bigger the ship the more trouble we have getting it "in" to space in the first place.

Sorry if I rambled....ok I know I rambled. Just trying to explain it without knowing exactly how to phrase it all. Hope that helped.

DWSTXS · Dec 3, 2009

great explanation Pyrohawk. I guess I just wasn't thinking with my science cap, plus I keep seeing, in my mind's eye, the space shuttle, and it's definitely not the type craft we would use to travel to Mars or beyond.

PeterL · Dec 3, 2009

There is the problem with fuel, and that would make it difficulot to have a ship take-off from Earth, go to the Moon, turn around, and return, because the ship might be without fuel when it tried to land on Earth. The turn around would be very easy, if one did it at the Moon. A closs orbit spin around the Moon would reverse direction without much fuel at all. The problem with doing that around the Moon is that the "close orbit" would be within a mile of some mountain tops.

The technology for a quick trip to the Moon and back does exist. Funding is deficient.

DWSTXS · Dec 3, 2009

PeterL said:
There is the problem with fuel, and that would make it difficulot to have a ship take-off from Earth, go to the Moon, turn around, and return, because the ship might be without fuel when it tried to land on Earth. The turn around would be very easy, if one did it at the Moon. A closs orbit spin around the Moon would reverse direction without much fuel at all. The problem with doing that around the Moon is that the "close orbit" would be within a mile of some mountain tops.

The technology for a quick trip to the Moon and back does exist. Funding is deficient.

and why does one need a lot of fuel to land back at earth? The apollo astronauts just splashed down. Is the fuel used to fire engines to slow down?

dclary · Dec 3, 2009

DWSTXS said:
Maybe i'm not understanding something here. Why, would one need to decellerate in order to return? Why not just keep going at the same speed, but turn and come around and head back?

It's a matter of vectors. At some point, you're decellerating... if you choose to do it in the process of turning, you're altering your course, possibly by several thousand miles, depending on your speed, as your trajectory changes during directional shift.

PeterL · Dec 3, 2009

DWSTXS said:
and why does one need a lot of fuel to land back at earth? The apollo astronauts just splashed down. Is the fuel used to fire engines to slow down?

A lot of fuel is a relative thing. The re-entry would require some fuel; they fire retro-rockets to slow the craft. The Apollo capsules used quite a bit of fuel lining up before they plunged through the atmosphere. The shuttle doesn't use much, but it takes a long, hot plunge through the atmosphere. Regardless of the type or design of a spacecraft, it has to slow down enough so that it won't just bounce along the top of the atmosphere, unless the heat shielding is amazingly good.

Pyrohawk · Dec 3, 2009

I'm not 100% sure on this...... I do know that teh shuttle has to fire the engines because they need to reorient themselves to safely land the ship. But I would assume that the Apollo would have needed to slow itself down to some degree as they were travelling at a speed of over 25,000 mph. Although safe re-entry speed is something like 17,000 mph so they wouldn't have to slow down very much.

However....in order to travel to places farther away than the moon, in a reasonable amount of time, we would need our ship to be able to travel at much faster speeds than the Apollo did....much faster. So they would have to slow down before re-entering the atmosphere.

efkelley · Dec 3, 2009

Apollo 13 has a lot of what you want to know in technicolor! ;-)

Similarly From the Earth to the Moon goes into a lot of detail, including some of the misconceptions that the NASA engineers had to overcome amongst themselves regarding space travel.

Pyro: Apollo did need fuel for those last-minute corrections to make your entry angle and speed meet that narrow margin between skipping off the atmosphere and plunging straight down in a burn-up. More information here: http://en.wikipedia.org/wiki/Re-entry

Pyrohawk · Dec 3, 2009

Gotcha. I said I figured it needed to use its engines for slowing down some. I just wasn't 100% sure so I didn't want to go in to details on a guess.

JimmyB27 · Dec 4, 2009

GeorgieB said:
How do you "come around and head back?" You would still need fuel to affect the turn, in effect decellerating that motion and then to accelerate to return. Travel in space is a whole lot different than here on earth.

Someone who can explain 3D vector motion might be better qualified to answer the question.

But, for descriptions of motions in space, you might read the James Campbell series "The Lost Fleet." Very good battle scenes, with relative motions explained.

You mean 'effect the turn'...</grammar nazi>

GeorgieB · Dec 4, 2009

JimmyB27 said:
You mean 'effect the turn'...</grammar nazi>

You know, I was gonna use the word "make" but decided instead to go whole hog in style.

Pthom · Dec 4, 2009

Spaceflight, as NASA and the Russians do it, is for the greater part an exercise in coasting. More precisely, the initial velocity of a ship obtained upon reaching orbit is just about all it gets, until someone decides to return the ship to Earth. This includes all projected trajectories to the Moon or Mars (or for robotic ships, to anywhere else we've sent them). As was mentioned upthread, modern spacecraft just don't have enough fuel carrying capacity to perform any but the most delicate maneuvers (like linking with the ISS, for example).

For all practical purposes, ALL the fuel used in getting the ship to orbit is in the launch vehicle, not in the spacecraft itself. (When the SST lands, its fuel tanks are empty primarily for safety reasons, but also because the ship is inherently unmaneuverable in the atmosphere.) This is a problem when arriving at a planetary destination, such as the Moon or Mars. You recall that the Lunar lander was an amazingly flimsy vehicle, with barely enough fuel to land, let alone take off again. Lunar gravity is 1/6 that of Earth. You may also recall that for the three latest (successful) robotic missions to Mars, the spacecraft landed surrounded by a bunch of big balloons. And they bounced. Lots.

And NO ONE has proposed that these vehicles return to Earth.

Now... If you have a nearby planet handy, or large moon, you might be able to steer your vessel close enough to it to use its gravity well to "slingshot" your way into a drastically different orbit (and, if done properly increase your velocity).

But if you're halfway to Mars and forgot your lunch, plan on going hungry. There ain't no turning around with currently available spaceflight technology.

DWSTXS · Dec 4, 2009

Pthom said:
Spaceflight, as NASA and the Russians do it, is for the greater part an exercise in coasting. More precisely, the initial velocity of a ship obtained upon reaching orbit is just about all it gets, until someone decides to return the ship to Earth. This includes all projected trajectories to the Moon or Mars (or for robotic ships, to anywhere else we've sent them). As was mentioned upthread, modern spacecraft just don't have enough fuel carrying capacity to perform any but the most delicate maneuvers (like linking with the ISS, for example).

For all practical purposes, ALL the fuel used in getting the ship to orbit is in the launch vehicle, not in the spacecraft itself. (When the SST lands, its fuel tanks are empty primarily for safety reasons, but also because the ship is inherently unmaneuverable in the atmosphere.) This is a problem when arriving at a planetary destination, such as the Moon or Mars. You recall that the Lunar lander was an amazingly flimsy vehicle, with barely enough fuel to land, let alone take off again. Lunar gravity is 1/6 that of Earth. You may also recall that for the three latest (successful) robotic missions to Mars, the spacecraft landed surrounded by a bunch of big balloons. And they bounced. Lots.

And NO ONE has proposed that these vehicles return to Earth.

Now... If you have a nearby planet handy, or large moon, you might be able to steer your vessel close enough to it to use its gravity well to "slingshot" your way into a drastically different orbit (and, if done properly increase your velocity).

But if you're halfway to Mars and forgot your lunch, plan on going hungry. There ain't no turning around with currently available spaceflight technology.

Okay, that said, how would our first Mars crew get back to earth?

Dommo · Dec 4, 2009

Project Orion. There I said it.

The solution to, and cause of all of our space problems.

Build a 100,000 ton ship. Power it with exploding nuclear weapons, and viola. You can get up to 10% C, land on any planet short of jupiter, and you can do it with technology that currently exists. The engineering of this one is actually pretty simple, but ironically the political aspect of this method makes it impossible.

MGraybosch · Dec 4, 2009

DWSTXS said:
I was told that based upon our current technology, or maybe not based on the technology, but based upon the way we currently travel in space, that. . . a space ship, to the moon or mars etc, has no ability to simply turn around and come back to earth if there is a major problem.

If you want to do an unassisted U-Turn in space, you'd need to carry a shitload of fuel. Velocity isn't just synonymous with speed, but a measure of speed in a particular direction. To change the direction in which your spacecraft is heading, you need fuel in order to change its velocity.

The more fuel you carry with you, the more massive your craft. The more massive your craft, the harder it is to reach escape velocity in the first place. This is why interplanetary flights are engineered to use gravity assists whenever possible.

DWSTXS · Dec 4, 2009

still, how do we get to Mars, and then get BACK?

we cant take enough fuel with us to land and then get all the way back. Right? Would we mfg fuel there, and use it to get us back?

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