I know they are now planning a trip to Mars and it takes 8 months. But why cant they just accelerate half way there and decelerate the other half? because that takes too much fuel. ok then why cant they send the fuel to the space station so they can pick it up there after liftoff. and then of course they could send the fuel for the return trip on the slow boat and let it orbit mars till they need it.
and where did the 8 month estimate come from. why was that speed picked?
I believe the 8-month trajectory is called the Hoffman orbit. It’s the most fuel-efficient orbit - basically it’s an elliptical orbit around the sun that reaches the orbit of Mars on aphelion (the part of the orbit farthest from the sun) and reaches Earth at perihelion (closest to sun). Any less energy and your orbit doesn’t quite reach Mars.
You can of course go faster if you have more fuel available. For unmanned probes, the 8-month travel time is no problem at all, and there is no reason to spend more money to reduce transit time - especially with the NASA budget as tight as it is. I don’t think there is a firm plan for manned flight to Mars, so I don’t know what orbit we would use for a manned flight. Yes, it is possible to rendezvous with addtional fuel tanks and take it along. The question is, is the reduced transit time worth the cost of launching the extra fuel? I’d suspect that the supplies necessary for the 8-month flight is much cheaper and ligher than the fuel needed to speed up the flight.
Money man, Money. They could do exactly as you say and send fuel up ahead of time but it costs way too much. I’m not sure of the going rate but it is something like US$10,000 per pound to put stuff in space. To send up enough fuel for a quick trip to space would cost more than it would be worth.
A good site http://www.sciam.com/2000/0300issue/0300alpert.html
and a decent site http://mars.jpl.nasa.gov/.
NP: Misfits - American Psycho
please excuse my ignorance, but what is the current technology concerning engines used for space travel .
and what is on the horizon (not distant future but next 10
years)?
Isnt the high cost of putting a pound in orbit due to the overhead cost of NASA? I mean what are the direct cost of launching a payload? I am talking about an independent company/country using existing published technology and hardware. like those Yahoos a few years back that kept blowing up rockets on that small island.
what I am proposing here is that Nasa sticks to the creation of the hardware and leave the logistics of supplying the fuel to outside agents.
My wag is we will be lucky to see nuclear engines (where fuel is heated via nuclear fission reaction instead of burning it).
for unmanned probes I expect to see further use on ion and devepolment of solar sail (both real material and magnetic field sails), laser where the laser ablades material off the back of the ship. Not much more in 10 year time frame.
justinh said:
Manned space travel currently consists of low earth orbit. Propulsion is by chemical reaction fuel. Launch uses solid fuel boosters plus liquid hydrogen/liquid oxygen engines. On orbit activities use various combos of “hydrazine” - these are auto reacting compounds upon mixing and do not require addition of heat/a spark.
Unmanned probes mostly use similar systems, with recent experiments using ion drive (Deep Space 1) and this year an attempt for solar sail. Nuclear fission propulsion is possible but hasn’t been tried because of popular resistance to the idea.
Define direct costs. The fuel? Cost of manufacturing the payload itself? Cost of manufacturing the rocket (or rocket components and assembling)? Cost of installing payload into rocket? Cost of operating launch facilities? Cost of testing rocket equipment to ensure it will work? That assumes unmanned craft. Things being omitted as overhead include cost of building launch facilities, cost of maintaining a trained workforce, cost of storage and transportation of vehicles when not being used, cost of crew (training, salaries, resources during the flight). Plus design of the rocket.
Cost would depend on the system chosen. Fly on an Atlas, or a Titan, or your own shuttle? Use a Russian submarine to launch a converted ICBM? The costs vary accordingly. I can’t seem to find a link I read that compared some typical costs.
The high cost of putting mass in orbit is due in part to the fact that it is in fact expensive to safely put mass into orbit. Another factor is low demand, but the low demand is in part because of the expense. It’s not like anybody is being prohibited from launching their own space fleet.
One of the big costs is the vehicle itself. You use a rocket exactly once, and it’s gone. Imagine building a 747, flying it from New York to LA, and then throwing it away. It would cost a lot to ride on. Likewise, if you used Los Angeles Airport for eight flights a year, it would cost passengers quite a bit to keep it running. Same premise for rockets. Even the STS (Space Shuttle) isn’t completely reusable - the external tank is new for every flight (it burns up and the remnants land in the Indian Ocean). A completely reusable vehicle would cut costs a lot, but has other complications (like how to get the darn thing back down again), and is more expensive in other ways. Extra fuel is one big one. Another is, you almost have to have a crew on board, which costs in two ways - you have to pay the crew, and the vehicle has to be ‘man-rated’, i.e., safe to ride; you can afford to blow up a satellite, but look what happens when you blow up a schoolteacher.
Here are the figures for the STS (a.k.a. the Shuttle):
Marginal cost per STS flight - $45 million
Best case maximum payload - 24 990 kg (54 900 pounds)
Payload launch cost - $1 800 / kg ($820 / pound)
The marginal cost is the cost of adding or deleting a flight from the NASA schedule. This includes astronaut training, vehicle prep, fuel, etc. The actual cost is more like $200-300 million, which includes all the other costs such as salaries (astronauts’ and others’), site and launch pad upkeep, and sundries that I have no idea about. So a realistic estimate of what it ought to cost, assuming a price based on actual costs and a reasonable free-market markup, would be somewhere in the neighbourhood of $5,000-$6,000 per pound. I don’t know for sure though, I’m not a customer.
Let’s be clearer about the payload. This assumes you’re launching due east of Cape Canaveral (so a 28.5 degree inclination) to an altitude of 202 km (126 statute miles). Every extra km of altitude decreases your payload by 30 kg.
If you’re using Columbia, knock off another 3800 kg. Knock off even more if you want to orbit at a different inclination (call it 200 kg per degree, and more the higher you go). In order to be considerate, they cannot launch in orbits with inclinations exceeding 57 degrees (don’t want to crash into New York City if something goes wrong). If they launched from Vandenberg (I’m not sure they’re even capable of it any more), they can’t launch at an inclination less than 70 degrees; you probably don’t want to do that though, your payload drops to 10 000 kg or less. Oh, and subtract another 300 kg per person in excess of five. So it really does vary a bit.
Source for costs: Project Apollo mailing list
Source for payload: NASA and nauts.com (I checked the latter because the NASA page doesn’t have any data on Endeavour).
But, of course, even NASA has alternatives to the STS; they just aren’t as glamourous. Looking at NASA’s schedule, you can see that a lot of stuff is going up on Delta and Pegasus rockets, with a few Atlas, Taurus, Titan and Athenas mixed in.
If an independent company can launch your payload for cheaper, and as reliably, you can bet your bottom dollar that they will. You already have options - Arianespace for one, the Russians for another. And the Chinese are also getting into the act.
Anyway, the big problem with using conventional fuel to speed up the trip to Mars is that you run into diminishing returns very quickly. To haul the fuel to orbit, you have to carry even more fuel. And then you need more fuel to carry the extra weight of the fuel you need to carry the fuel… It’s a geometric progression, so there’s a very finite limit to how much fuel you can haul into orbit.
And once you get away from a Hohmann minimum-fuel transfer orbit and try to shave time off the trip, the fuel requirements start to go up enormously.
So if you want to get there much faster than a minumum-energy transfer orbit, you need to come up with a way to either increase propulsive efficiency of your fuel (by ejecting it from the back of the rocket faster), or by collecting fuel as you go (a Bussard Ramjet has a gigantic magnetic scoop on the front to pick up bits of interstellar hydrogen, but you need to be going really fast to make it work), or you need to hitch a ride on the solar wind with a sail, or use a laser beam to move your spacecraft.
The problem with the solar sail or laser propulsion is that it’s a one-way trip. Unlike a terrestrial sail, a solar sail can’t ‘tack’ against the wind. Basically, it’s just being moved by little charged particles hitting it and transferring its momentum. And since momentum is conserved, the ship is going to move in the same direction the original particle was.
Incidentally, a prototype solar sail was launched from a Russian submarine yesterday, but the experiment failed when the third stage of the rocket malfunctioned.
Sam Stone has some good points, but what are good points without a bad nitpicker?
Well, yes and no. There’s a finite amount you can haul on one vehicle, yes, for exactly the reason you described. On the other hand, you could send up STSes with 25000 kg of fuel each until the cows come home, and the amount in orbit would increase arithmetically. Yes, you’d burn millions of kilos of fuel to get it up there, but that’s not the point.
By how much, I wonder? Anyone got a link to a page describing how much delta-V it takes to get to our neighbours in X amount of time?
The Hohmann transfer orbit has the added advantage that one can use a reverse gravity-assist to help decelerate. The problem at Mars is, this would put you into a retrograde orbit, making it tougher to land or visit the satellites.
Still the cheapest way to beat the solar gravity well (if you can make them light enough and deploy them properly). By angling the sail, you can go across the wind, just not into it. If you really have to. Solar-sail yourself and all your propellant to Mars, and rocket back. Why not? Even without a solar sail, it appears the solar wind is strong enough to measurably affect the trajectories of the Voyager probes. If you could furl the solar sail at heliopause, and unfurl it again at Alpha Centauri’s heliopause, Presto! Free (if rather time-consuming) interstellar travel.
That’s a pity. From a submarine? Interesting…I’m talking through my hat here, but I imagine solar sail deployment (once you’re up there) would be a real be-atch.
To use a solar sail to get back you have to angle the sail to slow your orbital velocity around the sun - then the sun’s gravity will start to pull you in.