Spaceship speed issues.

[Hanny]

no matter what velocity the gasses are shot out the back of a rocket, it is still going to exert some force on the ship. the exhaust velocity isnt going to matter. the rocket is going to apply the same force regardless of how fast the ship is going, thus as long as you have fuel you can keep accelerating. you could throw bowling balls out the back of the spaceship and accelerate yourself. so yeah, besides the speed of light there isnt any speed limit to worry about.

 

[Armitage]

Ouch ... where's my copy of BMW when I need it. Ok where to start.

How about basic orbital mechanics?
All orbits are conic sections ... ie. they look like a slice cut from a cone ... any one of a circle (degenerate elipse), ellipse, parabola or hyperbola. The orbits of the planets around the sun are roughly circular ... close enough for this discussion.

For a circular orbit, the orbital velocity is V = sqrt(mu/r)

For a heliocentric orbit, mu=1.32715E11 km^3/sec^2

Average Orbital Radius:
Earth: 1.496E8 Km
Mars: 2.274E8 Km

Orbital Velocities:
Earth: 29.78 Km/s (Ve)
Mars: 24.16 Km/s (Vm)

So these are the bounding cases for the orbit transfer ... you start from tangent to earth's orbit at earth's speed, and you need to end up tangent to Mar's orbit at mars speed. And the path between them has to be a conic section. And of course, timing is everything, but this is simply controlled by your launch time.

The most efficient path is called a Hohman transfer. It is simply an elliptical orbit tangent to earth & mars orbits ... in this case, a perigee radius equal to the radius of the earths orbit, and an apogee radius equal to the radius of mars orbit. Skipping a bit of math we can find that the velocity of this orbit at perigee (earth orbit) is 32.71 Km/s (V1), and the velocity at apogee (mars orbit) is 21.52 Km/s (V2).

This means that the minimum delta velocity (dV) required for the Hohman manuevers to rendevous with mars is (V1 - Ve) + (Vm - V2) = (32.71 - 29.78) + (24.16 - 21.52) = 5.57 Km/s
Add to this the dV required to escape earth's sphere of influece ... the hyperbolic excess velocity ... approximately 11 Km/s You'll have some hyperbolic excess velocity on the mars end also, but the current missions are scrubbing this off through aerobraking.

We can also find the time required to make this trip ... half the orbital period of the rtransfer orbit. P = 2 PI sqrt(a^3/mu)

a is the semi-major axis ... orbital radius for circular orbits. For elliptical orbits, its half the length of the major axis or (Re + Rm)/2

P = 516.6 days
P/2 = 258.3 days approximately 8.6 months

So I'm not sure where the 18 month figure came from ... maybe referring to the roundtrip time?

This is the lower bound on dV required ... and therefore fuel. There is an infinate number of other possible transfer orbits. Some will get you there considerably faster, but at the cost of more gas. And carrying more gas isn't cheap. Imagine planning to drive from NY to LA without stopping for gas ... so you tow a trailer with a couple drums of gas ... say you get 40MPG, and its a 3000 mile trip. So you have to carry 75 gallons of fuel ... about 480 lbs IIRC ... but towing 480lbs of fuels, plus the weight of the trailer your car will only get 35MPG ... no problem, you now carry 85 gallons, about 550 lbs ... but your trailer was only rated for 500 lbs, so you get a heavier trailer plus the fuel, now you only get 32 MPG ... etc., etc. That's the kind of thing we're getting into here.

That's all for tonight ... past my bedtime. Tomorrow ... the Rocket Equation, specific impulse, and how much more gas it would take to get there faster

One other bit ... exhaust velocity is not a constraint on final speed. As Hanny suggested, you could stand out on the hull of a ship moving at 0.1c chucking rocks off the back by hand and still be accelerating your spacecraft.

 

[maluckey]

As mentioned above, provided that there is any constant thrust relative to the vessel itself, the ship would acelerate until it's straight line theoretical limit. The Ion drive is a good example of this. It has a very low (comparitively) thrust to weight ratio, but it will continue to thrust, and eventually reach it's straight line limits. It would be a good choice if they wished a hybrid propulsion setup to minimize weight and size of the vessel. The Ion drive is not particularly suited for initial, rapid acceleration, but as the sustained acceleration thruster, it would be great. It has already been used successfully, so it's not far-fetched to build a larger version to use for a Mars trip.

 

[Overkast]

A few years ago, I was watching a "popular science" type of show on the Discovery channel... it's was all about futuristic ideas and prototypes.

Well, one of the ideas was for an Earth-use commercial-passenger transport vehicle that got all of it's power and movement by charging ions in the atmoshpere. Basically it was thought to replace jets one day. It was really funny looking... no wings... it just had the body section where all the passengers sat, and 3 arms sprung out from the body attached to a gigantic ring that encircled the whole body.

Supposedly the way it worked was: The giant ring had devices built inside it to "suck" ions in through the front and "propel" them out the back by charging them + and - and this was how it moved... silently and without pollution.

Does anyone know what vehicle I'm talking about? I'm assuming someone else in here had to have seen this besides me.

Do you think it would work for a spacecraft, or are there no ions in space?

 

[Matthias99]

Ion drives work in space, although you have to carry something with you to ionize (space is a vacuum, remember). They've used them for some long-haul probes because the efficiency is extremely high and you can drive them off of solar power easily.

It's not tough to build a really small one of those things out of balsa wood and aluminum foil (it'll hover if you hook it up to about 120VDC power), but AFAIK, they don't scale up well.

 

[MadRat]

It would be possible to use particles in space to power a space ship. Its not that space is totally void, its that microscopic particles are sometimes moving in directions that would make any type of "particle scoop" contraindicative. Solar sails would use these tiny particles in space to catch a ride in much the same way as a wind sail uses the force of atmosphere moving close to the surface of our planet to traverse the waterways. Only it would be more like a sea chute can be used to ride oceanic currents. We know there are faster currents out there, like cosmic rays, but we would still have to find the right sail to use them...

Correct me if I'm wrong, but so far we're limited theoretically to around 110,000 MPH by ion engines. These are so far the only efficient way to travel long distances in space. The only other promising technology technique right now is to also use gravity of planetary objects as a slingshot, which someone already pointed out is more difficult since Mars is too close. We could exceed the limits of ionic technology if someone could figure how to harness photons and other electromagnetic energy further up in the spectrum, although to do it with any meaningful payload you run into the need for using an external power source which subsequently decreases exponentially in power as you move further away from the origin of the power source.

Chaos theory can probably map out an infinite number of super efficient pathways to Mars that are much closer than the current approach. If you have the much longer time periods to wait I'm sure it could be done on only a few percents of the amount of fuel currently being expended in the 18 month trips. Considering our short lifespans who really wants to wait??

 

[Matthias99]

Several people have mentioned being limited to the speed of a reaction-based engine's exhaust. This, AFAIK, is not true.

Conservation of force/momentum states that if you push something away from you (rocket exhaust, ions, bowling balls, whatever), it pushes back with the same amount of force (or, equivalently, keeps the net momentum of the system equal). It doesn't matter how fast you're already going. Any type of engine can continue accelerating you until you run out of fuel, although at *extremely* high speeds you have to take mass dilation into account.

 

[MadRat]

You can keep approaching the limit ever so slightly without ever touching the wall. The problem is that eventually you just need to give up and say I'm close enough and move on. Sure you could expend an almost infinite amount of fuel to go a tiny bit faster, but what's the point?

 

[kpb]

Not so much a technical issue but still an issue is the cost of getting the fuel up there. I read rescently the cost of getting something into space. I want to say it was like several hundred thousand dollars a pound. That makes just prelaunching more and more fuel rather impractical from a cost concern.

 

[MadRat]

In 1986 dollars Columbia was to revolutionize the space industry by bringing costs down to $13000/pound. In reality it was closer to $30000/pound. I'm willing to believe its close to $100k/pound today. But then think about the amount of energy necessary to lift each pound, compare what you pay per unit of energy (to heat your house in the winter) to what NASA pays, and it all really begins to look more and more like a bargain.

 

[drag]

The real solution I think would be nuclear rockets. (which they've had since the late 50's early sixties)

Lots and lots of energy in a very small package.

They have been able to make rockets using nuclear fuel in the past, but nobody uses it because the idea of having exploding-prone rockets flying around laden down with nuclear rods.

But if you set up a moon base then this is largely immaterial. The background radiation caused by the sun (one gigantic continous nuclear reaction) in our solar system will make anything we produce seem like a drop in the ocean of radiation.

Plus don't you use up something like 70% of your energy just to escape earth's gravity?

As long as you can make a safe way to transport it up their on conventional rockets (ship in blast proof/reusable titanium tubs or whatever), then it's not going to hurt anyone or anything on the planet no matter how f*k'ed up a accident happens on the moon.

And I don't think you would need exactly 2x the energy to speed up AND slow down. Since you can use you momentum to bleed off energy into the planet your going, too. You'd only have to slow yourself down enough to get caught in orbit, then slow down enough that you can use the atmosphere as a break and let the rest of your momentum bleed off ias thermal energy. Maybe 1.5x or 1.7x?

Hell if I know.


from here

The GCNR is a concept which was also experimentally investigated in the 1960s during the Rover program. The idea is to use a gaseous nuclear fuel instead of the solid graphite core used in NERVA. A gaseous fuel could attain tempertures of several tens of thousands of degrees which would allow specific impulses of 3000 to 5000 seconds to be considered. Such an engine would allow manned missions to Mars to be accomplished in a few months each way. Currently, research into the GCNR concept is underway at the Los Alamos National Laboratory under a program from the NASA Marshall Space Flight Center. Some of the research problems that are being studied are:

So flights to mars in a few months either way? Sounds good to me.

 

[DrPizza]

Again, you don't have to "slow down" when you reach Mars, other than slowing the decent to the surface, including any orbital velocity. Mars isn't a stationary target, the rocket isn't catching up to Mars and then slowing down to get into orbit around Mars. In actuality, as the rocket reaches Mars orbit, Mars is moving faster than the rocket and is catching up to the rocket.

 

[iam29a]

Well, you have to slow down or you would a) go into orbit around Mars, let alone have a controlled decent onto the Martian surface. Instead, you chance impacting with the surface, which has happened before, and also on the moon.

 

[Pulsar]

You need to seperate the two discussions. Yes, if you are talking about absolutely minimizing the time involved in getting to Mars, you will have to accerlate and then decelerate again to match Mar's orbital speed.

If you are using the current method of trying to minimize both fuel and time, then you can bleed off the extra velocity by bumping the atmosphere and placing yourself in an orbit of some type.

Of course, if you were going to LAND, you'd need to decelerate on the way down, be it by parachute or rockets.

 

[timmychen]

one thing that everyone is forgetting about is that as the space scraft is travelling, it needs enough time to send and recieve transmissions to and from earth INCLUDING steering it around space debris. From pieces of asteroids to chunks of comets left in space and other stuff, sizing from half a inch to the size of texas... imagine smacking into that at 180,000 MPH... if your car smacked into a tiny pebble at 180,000 MPH you would likely be dead.