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Spaceship speed issues.

D

dxkj

Lifer
I think I remember reading recently that it would take 18 months or so to reach mars?

My question is this. (and mind you im not that bright 🙂

How fast can we go in space? If it takes 18 months, why can't we go 18 times as fast and have it only take 1 month?


If we can get up to 10k MPH with X amount of fuel, couldnt we get up to 180k MPH with 18x amount of fuel?
 
You have to remember space is almost a perfect vacuum, so any energy spent to go fast will also have to be spent to slow down.
 
Originally posted by: blahblah99
You have to remember space is almost a perfect vacuum, so any energy spent to go fast will also have to be spent to slow down.
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I understand that. That's why traveling far shouldnt be as much of an issue.... since once you get up to speed, you are set for X amount of time until you need to slow down.
 
i guess spaceship will get lost in space if go too fast.
plus those gravitational pull from planet....

we might lost in space easilly if go too fast? just my guess.
 
Originally posted by: dxkj
If we can get up to 10k MPH with X amount of fuel, couldnt we get up to 180k MPH with 18x amount of fuel?
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I can't remember the exact ratio, but it was something like 10lbs of fuel were required for every 1lb sent into orbit?

Now imagine the space shuttle with a liquid fuel tank ten times bigger and twenty solid rocket boosters instead of just two. It doesn't work out exactly this way (since the orbiter is not designed to travel to Mars, only orbit Earth), but it isn't as easy as just increasing the amount of fuel.

 
Originally posted by: LurchFrinky
Originally posted by: dxkj
If we can get up to 10k MPH with X amount of fuel, couldnt we get up to 180k MPH with 18x amount of fuel?
Click to expand...
I can't remember the exact ratio, but it was something like 10lbs of fuel were required for every 1lb sent into orbit?

Now imagine the space shuttle with a liquid fuel tank ten times bigger and twenty solid rocket boosters instead of just two. It doesn't work out exactly this way (since the orbiter is not designed to travel to Mars, only orbit Earth), but it isn't as easy as just increasing the amount of fuel.
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Thats why they could possibly bring up fuel, and store it in incremental amounts in space until they were ready.


Plus, solid rocket boosters.... those are used to get into orbit, overcoming gravity takes a lot. Once you were in orbit you wouldnt need rocket boosters... (unless you were trying to account for the added weight of the fuel)

Im not saying it would be easy, but if the time it takes is an issue, why not?


Also, less fuel would be needed to stop the ship than to start it (think of mass, a decently large amount would have been burned away already), and then, to stop it back at home wouldnt take much either.


 
So... would it be possible AND feasible to 'launch' any spacecraft sent to mars as opposed to it getting up to speed under it's own power? Of course it would still have fuel, for the trip home and such.
 
Don't forget, you'll need more fuel, but you'll also need to accelerate that fuel up to your speed. It's wrong to assume that if you need 100 lbs of fuel to get to x speed, then you'll need 200 lbs to get to 2x speed because you have the added mass of the extra fuel you'll need.

Furthermore, you'll need that much more fuel to slow yourself down again, so you need more fuel initially to get that fuel up to speed, and then more still to get THAT fuel up to speed...
 
The relation is not linear. The amount of energy you need to reach the speed "v" is E=mv^2/2, this means that for a x10 increase in speed you need x100 more energy(=fuel). And as Silverpig pointed out you also need to cary that extra fuel with you.


 
I hate it when that happens.

You reasonably, our rockets reach about the same velocity as their nozzle exhaust velocity due to diminish returns of your thrust with regards to payload. With our current fuels (LOX) the velocity is about 3800 m/s or 8500 miles per hour.

The distance to mars is about 36000000 miles if memory serves.

At an acceleration of 1.25 gravities, or 12.25 m/s^2 to keep the astronuts comfortable,

3800 m/s =a*t = 12.25 * t = 310 seconds or 5.17 minutes.

So, essentially, you can approximate going at full speed the whole time.

So that's 36mil / 8500 mph = 4235 hours or 176 days. 6 months.

Now that's pushing it hard. With extremely little payload and maximum effieciency. Add in all the non-fuel mass of the landing vehicle, the fuel for the trip HOME...... 18 months is right where it should be.

Now.... actually acheiving 3 times your exhaust velocity is impossible given fuel density and usage.
 
Max speed (as far as we know) is C, the speed of light.
Problem with reaching that speed would be the human occupants. Accelerating to just shy of the speed of light, and keeping the acceleration force at 1G would take nearly a year, and coming to a stop would take just as long. Doing what Enterprise does - 0 to warp 5 in a few seconds - wouldn't just turn you into "chunky salsa" as the one ST technical manual put it. You'd be blasted through a bulkhead at the speed of light, destroying the entire ship you were on. And you'd probably put out one hell of a shock wave. - we did fun equations in high school physics: what would happen if a golf ball were accelerated to X percent of the speed of light? I think at like .5C, a golf ball could wipe out New York City. Something human sized, at .9999C would probably...don't remember the equations anymore, but it'd probably take tge US off the map, at least.
 
Originally posted by: Jeff7
Max speed (as far as we know) is C, the speed of light.
Problem with reaching that speed would be the human occupants. Accelerating to just shy of the speed of light, and keeping the acceleration force at 1G would take nearly a year, and coming to a stop would take just as long. Doing what Enterprise does - 0 to warp 5 in a few seconds - wouldn't just turn you into "chunky salsa" as the one ST technical manual put it. You'd be blasted through a bulkhead at the speed of light, destroying the entire ship you were on. And you'd probably put out one hell of a shock wave. - we did fun equations in high school physics: what would happen if a golf ball were accelerated to X percent of the speed of light? I think at like .5C, a golf ball could wipe out New York City. Something human sized, at .9999C would probably...don't remember the equations anymore, but it'd probably take tge US off the map, at least.
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It works on TV, so I must believe that science is wrong. 😀

 
The thing doesn't travel in a straight line either does it? I thought it slingshot around a few planets/moons on its way, gathering convenient data... At any rate, while that wouldn't take more energy, you'd have to worry about centripital force on the entire ship. Maybe, since it's already pretty hard to get a ship to Mars, it's more stress then they're willing to deal with?

It's probably a bunch of reasons combined, like some of the good explanations that have already been posted and many we don't know about (any one here a rocket scientist? 🙂 sorry, I couldn't resist)
 
Originally posted by: kamper
The thing doesn't travel in a straight line either does it? I thought it slingshot around a few planets/moons on its way, gathering convenient data... At any rate, while that wouldn't take more energy, you'd have to worry about centripital force on the entire ship. Maybe, since it's already pretty hard to get a ship to Mars, it's more stress then they're willing to deal with?

It's probably a bunch of reasons combined, like some of the good explanations that have already been posted and many we don't know about (any one here a rocket scientist? 🙂 sorry, I couldn't resist)
Click to expand...

We are talking about a trip to Mars, Right? Look at a diagram of the solar system. There are no planets between earth and Mars! You are thinking of the deep space probes that have used the orbital motion of Jupiter and or Saturn to speed them towards deep space.

As stated above the, maximum attainable speed by a CHEMICAL engine such as we are currently using is the exhaust velocity of the propellent. Forget c, that is not now, and never will be (IMHO), on the technology road map.

There should be no room or plans for sending toursits (ie men) on such trips until we can repeatedly land and RETURN from Mars with cofidence and a high degree of repeatablity. Throwing huge amounts of resources into sending a couple of carefully choosen toursists to Mars would be a complete waste. There is so much good science that can be done without the impedimenta of men that it would be a total waste to even consider it (sending wo/men) at this point.
 
Another thing to remember in this discussion is that we're not just talking about getting out of earth's gravity well -- the ship must also work itself further out of the sun's gravity well. The most efficient way to do this is my essentially adjusting the spaceship's orbit around the sun by increasing its tangential speed to the sun, which causes it to move out into a more distant orbit that intersects with the orbit of Mars at a time when Mars will actually be there. This approach may take a long time, but it seems like the only practical way right now.
 
Originally posted by: RossGr
Originally posted by: kamper
The thing doesn't travel in a straight line either does it? I thought it slingshot around a few planets/moons on its way, gathering convenient data... At any rate, while that wouldn't take more energy, you'd have to worry about centripital force on the entire ship. Maybe, since it's already pretty hard to get a ship to Mars, it's more stress then they're willing to deal with?

It's probably a bunch of reasons combined, like some of the good explanations that have already been posted and many we don't know about (any one here a rocket scientist? 🙂 sorry, I couldn't resist)
Click to expand...

We are talking about a trip to Mars, Right? Look at a diagram of the solar system. There are no planets between earth and Mars! You are thinking of the deep space probes that have used the orbital motion of Jupiter and or Saturn to speed them towards deep space.
Click to expand...

There has been talk of the Mars trip involving a slingshot around Venus.

 
exactly what everyone else was saying. You reach a point where the more fuel you take, the slower your maximum speed will be. It is just does not make sence to increase fuel to go faster as you do reach a point where the fuel you take becomes the limiting factor in the speed you can reach.
 
It takes energy to move fuel, so doubling the amount of fuel available doesn't mean you can double your velocity.

R
 
also what happens when your spaceship going close to c collides with interstellar material remember ,F12=F21.Don't worry Star Trek already has that one solved as well.I don't see whats wrong with the trip taking 18 months,It only resembles the case of early european expansion.Trips then took years and were fraught with peril.That what makes it exciting?!
 
On a side note...Since traveling to other planets doesnt occur in a straight line, but in a tangental path...

I've often heard that when we launch something to mars, it orbits the sun in the same clockwise fashion as all the other planets. Gadually increasing its orbital distance. This causes the "thing" to fully orbit the sun 1.5 times until it catches mars.
Why would we not launch in the opposite direction against the flow of the planets and get there quicker in like .5 an orbit??

(in other words, there are about 2/3 less miles to travel and you can get there faster.)
 
Because when a sat is launched, it is going at the same speed as the earth. To reverse the orbit would require you to eventually reach a velocity of zero at some point. Unfortunately, the only stable orbit at v = 0 happens to be at the centre of the sun.
 
Someone above mentioned that you need fuel to slow down the rocket upon its approach to Mars. Actually, that's not quite true... You would need fuel to slow the decent to Mars surface, however, IIRC, the speed of Mars was actually 5000 mph faster than the speed of the Mars Lander. I've got the links to all those details on my desktop at school, they're someplace on the Nasa site... if anyone really wants them, and can't find them before Tues, pm me and I'll remember to post the links.
 
on another interesting side note. I have read somewhere that the star trek warp system may not be that far from what we'll have to do to travel to other galaxies. but instead of going at speeds past the speed of light we would effectivly bend space so that our destination is closer.
 
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