PDA

View Full Version : Martian Escape Velocity

AlexMoose
2004-Jan-28, 05:58 PM
What is the Escape Velocity on the Red Planet? how would this effect any manned missions there?

ToSeek
2004-Jan-28, 06:01 PM
Martian escape velocity is 5.02 kilometers per second, which is about 45% of Earth's escape velocity. The reduced escape velocity would have at least the following effects:

- It's harder to get into orbit because you have to slow down more.
- It's easier to get out of orbit because you don't have to speed up as much.

The net effect for a round-trip mission is probably a wash.

Dancar
2004-Jan-28, 06:49 PM
As I think about it, the most difficult obstical to having a manned mission to Mars is how the astronauts are supposed to get off of Mars.

Think about it: If you were to land on Venus or other unihabited planet with the gravety of Earth, you'd need to bring a complete Saturn or Space Shuttle type vehical loaded with fuel to leave again.

While Mars would require less fuel than Earth to reach escape velocity, it would still be considerabley more than the Apollo astronauts had to take to the Moon. And you'd have to land your fuel-laden Rocket safely on Mars first before you could use it to leave again.

Dancar

ToSeek
2004-Jan-28, 09:24 PM
Unless the gravity of the object is too little to matter, no matter where you'd go you'd probably use some variant of the Lunar Orbit Rendezvous technique used in Project Apollo: the spacecraft that takes you back home remains in orbit, while what takes you down to the surface is only capable of getting you down and back up again. That's a huge savings in fuel and a huge reduction in complexity.

daver
2004-Jan-28, 11:45 PM
What is the Escape Velocity on the Red Planet? how would this effect any manned missions there?
As has been posted, escape velocity from Mars is essentially 5 km/sec, as opposed to 11 km/sec from earth. In addition, Mars' atmosphere is much thinner than Earth's (for most intents, it's a vacuum). Which means that (1) a rocket has to achieve a lower velocity to escape (duhh, but bear with me), (2) it loses less velocity to atmospheric resistance, and (3) its exhaust bell can be optimized to essentially vacuum pressure. All of which are working in the plus direction, making it much easier to blast off from Mars than from Earth.

So, for an Earth escape, a rocket might need a delta V of perhaps 13, whereas for a Mars escape, the rocket might need a delta V of maybe 6 (I'm pulling the Mars number out of the air). So, a single-stage H2/O2 rocket on earth might need a mass ratio of 22 (a mass ratio of 22 means that the total weight of the rocket (payload + structure + engines + fuel) must be at least 22 times the dry weight (payload + structure + fuel)) to achieve escape velocity, a single stage H2/O2 would need a mass ratio of a bit more than 4. Which means that single stage to escape on Mars would be trivial; even using a poor fuel like CO/O2, single stage to orbit is easier on Mars than using the best fuel combination on earth.

This opens up a lot of options for Mars exploration. Your descent vehicle can easily rendezvous with anything in Mars orbit, or even rendezvous with a cycler shuttling back and forth between Earth and Mars. If you first land several refuelling stations at various parts around Mars, the descent vehicle can hop from depot to depot, allowing the astronauts to investigate a variety of terrain during their (15 month or so) stay.

JonClarke
2004-Jan-29, 03:44 AM
You might want to use MOR to return to earth, you might want to return direct. Either way it makes more sense to manufacture the propellant on the surface of Mars using local resources than lugging it all the ay from earth. This results in a passive reduction in landed mass and the mass that needs to be boosted from earth.

It takes less energy to escape from Mars than it does to reach earth orbit. LEO is more accessible from mars that it is from the earth's surface.

Jon

AstroSmurf
2004-Jan-29, 01:42 PM
How difficult is it to generate CO/O2 if you roll out some solar cells? Could it be done within a practical time-frame?

daver
2004-Jan-29, 07:00 PM
How difficult is it to generate CO/O2 if you roll out some solar cells? Could it be done within a practical time-frame?
I don't know. The Martian atmosphere is perhaps 1/100 as dense as the terrestrial atmosphere, but what atmosphere is there is almost pure CO2 (this sounds like there's more CO2 in a cubic meter of the Martian atmosphere than in Earth's, which doesn't sound right at all). Anyway, pretend for now that that makes 10 grams of CO2 per cubic meter of atmosphere, so you'd need to process 1e5 cubic meters to get a ton. You'd probably need on the order of 100 tons to fuel your lander.

That's about as far as I can take this. Hmm, maybe I can throw a bit of chemistry at it. I'll be pretty explicit in my calculations because it's been decades since I learned this and I'll almost certainly blow it somewhere.

Heat of formation of CO2 is -393.5 kJ/mol, of CO is -110.5 kJ/mol, so to separate CO and O2 from CO2 requires 283 kJ/mol. One mol of CO2 is 44 grams, so we're talking roughly 6.4 kJ/gram or 6.4e9 joules/ton or 6.4e11 joules/100 tons.

OK. Solar cells. Figure an insolation of perhaps 3 kWh/sq meter/day, which means we might be about to get on the order of .5 kWh, or 2e6 joules/sq meter/day. So 100 sq meters would generate 2e8 joules, which would take perhaps 3e3 days. We'd like to reduce that by a couple orders of magnitude, but that's getting into a pretty hefty solar array.

I'm not sure how to crack CO2 into CO and O2. I imagine it involves pressurizing the CO2 until it liquifies and then electrolizing that.

Dancar
2004-Jan-29, 08:58 PM
There's a rumor/joke going around that Haliburten already has a deal to start drilling for oil on Mars once Bush's manned mission arrives. (You think you'd need to establish the past presense of life before you go prospecting for fossil fuels).

Unless that works out, I suspect that any source that could supply enough energy to manufare enough fuel from available materials to reach Martian orbit could simply be used by itself to return to orbit.

Dancar

JonClarke
2004-Jan-29, 11:46 PM
Funny you should mention CO-O2 In Situ Propellant Production from atmospheric CO2. This link http://www.aa.washington.edu/research/ISRU/ARES/ares.htm has two different mission architectures showing just this type of ISPP.

Other ISPP options exist.

Jon

daver
2004-Jan-30, 01:09 AM
There's a rumor/joke going around that Haliburten already has a deal to start drilling for oil on Mars once Bush's manned mission arrives. (You think you'd need to establish the past presense of life before you go prospecting for fossil fuels).

I guess I'm too boring--the joke didn't strike me as funny, just stupid (it's what, four orders of magnitude from making sense?)

Unless that works out, I suspect that any source that could supply enough energy to manufare enough fuel from available materials to reach Martian orbit could simply be used by itself to return to orbit.

Dancar
Look at the solar example--a few kW is woefully insufficient to get anything into orbit (unless you have a tether), but you can store the output of the panels over months or years in the form of chemical energy and burn it in a few minutes to get you into orbit.

JonClarke
2004-Jan-30, 01:43 AM
Companies like Halliburton have some of the best expertise in drilling, which is an important technology for the study of Mars. They certainly have been working with Ames over developing drilling technologies for use on Mars.

http://www.petroleumnews.com/pnarch/010228-49.html

Good idea.

Jon

Jpax2003
2004-Jan-30, 06:26 AM
I remember reading something (http://science.nasa.gov/headlines/y2003/03dec_membranes.htm) about membranes being used to pull certain chemicals from the atmosphere while blocking others. This would allow for hoarding of reagents needed to make a rocket fuel. It doesn't need electricity from solar panels.

Better yet, this membrane technology could be used to vent CO2 from the mars habitat module while keeping O2 inside without using valuable amperage from the power system for active scrubbing. Or not.

daver
2004-Jan-30, 05:36 PM
This would allow for hoarding of reagents needed to make a rocket fuel. It doesn't need electricity from solar panels.

I'm not sure how useful it will be--the Martian atmosphere is already 95% CO2. It might help to separate out the nitrogen, but there are other ways to do this.

Better yet, this membrane technology could be used to vent CO2 from the mars habitat module while keeping O2 inside without using valuable amperage from the power system for active scrubbing. Or not.
That's not a particularly good idea--you need to get the O2 back somehow, so you're going to have to crack the CO2 someplace. And that needs energy. Here, they might be useful in concentrating the CO2.

Jpax2003
2004-Jan-31, 02:59 AM
This would allow for hoarding of reagents needed to make a rocket fuel. It doesn't need electricity from solar panels.

I'm not sure how useful it will be--the Martian atmosphere is already 95% CO2. It might help to separate out the nitrogen, but there are other ways to do this.

Better yet, this membrane technology could be used to vent CO2 from the mars habitat module while keeping O2 inside without using valuable amperage from the power system for active scrubbing. Or not.
That's not a particularly good idea--you need to get the O2 back somehow, so you're going to have to crack the CO2 someplace. And that needs energy. Here, they might be useful in concentrating the CO2.
The referenced article answered most of those questions. According to the article 95% CO2 is not pure enough to use to make Methane with hydrogen. What are the other ways to purify CO2 from the mars atmosphere without using electricity?

While you're on mars does it matter if you get O2 from the atmosphere or the habitat? Submariners can tell you that it's more important to remove CO2 than it is to add O2. Anyways, You don't have to vent the membraned CO2 to a hard vacuum. You can vent it to a soft vacuum instead and then recycle it internally. Yes, the recycling process would probably use power, but the article was refering to fuel creation primarily.

daver
2004-Feb-02, 06:11 PM
The referenced article answered most of those questions. According to the article 95% CO2 is not pure enough to use to make Methane with hydrogen. What are the other ways to purify CO2 from the mars atmosphere without using electricity?

My mistake, I should have read the article. This is the scheme where they send a tank of liquid H2 to Mars and react it with the CO2 in the atmosphere to produce CH4, isn't it? In that case, reducing the amount of electricity required makes sense. If you're directly cracking CO2 to CO and O2, you're already using copious amounts of electricity--reducing the amount used slightly doesn't make much sense.

The obvious way to purify CO2 from N2 and H2O and whatnot is through difference in boiling points--compress the air to a few atmospheres, freeze out the H20, and then condense out the CO2. The N2 will remain gaseous. There may be reasons this won't work, but your fuel processing plant is going to need access to cryogenic temperatures anyway.

While you're on mars does it matter if you get O2 from the atmosphere or the habitat? Submariners can tell you that it's more important to remove CO2 than it is to add O2.

Submariners have access to a more convenient source of oxygen than a Mars base does.

If you have a mechanism to extract O2 from the martian atmosphere already, it might make sense to use a membrane to vent to the outside. You will certainly have to extract some oxygen from the atmosphere, so it might make sense to go ahead and extract it all, and not worry too much about recycling it.

My rough calculations put the partial pressure of CO2 in the martian atmosphere at roughly the partial pressure of CO2 in a terrestrial atmosphere. That would be a handy coincidence if it turns out that i didn't make a mistake.

JonClarke
2004-Feb-03, 12:08 AM
The Ares explore link I posted uses electrolysis to produce CO and O2 from CO2. Other ISPP proposals use hydrogen from earth to manufacture methane and water from martian CO2 by the Sabatier reaction, and then electrolyse the water to produce oxygen and use the excess hydrogen so released for further feed stock. As I said there are several ways to approach ISPP. Alternatively you could extract water from the Martian atmosphere (also discussed in the Ares explore paper) and electrolyse that, either to make H2-O2 propellant (although storage is an issue), or to provide feedstock for the Sabatier reaction.

Jon