The Mars Underground

[Torquemadus]

This is a link to the updated edition of the full length documentary The Mars Underground. It explains Robert Zubrin's Mars Direct mission plan.

With in space refuelling expected in the next KSP update, this seems quite relevant!

[ndiver]

Thanks for the link

I'm reading right now The Case for Mars, and it's interesting to see who are the people involved in this project, but also to visualize with animations the key features of the mission.

[AngelLestat]

I saw it 2 years ago, good documentary, I guess the mission is too risky, but I have a great respect for zubrin as scientist. He publish amazing papers and I saw him in many debates and symposiums.

[Nibb31]

Zubrin is a bit delusional. Although his ideas are popular and make spectacular pictures, nobody in the space industry really takes his obsession with Mars seriously.

[maccollo]

I never understood how it would be possible to transport hydrogen to Mars without it boiling away during transit. Still it seems to me that ISRU is definitely the way to go if a manned mars mission ever happens. If you just bring the methane from Earth and generate the oxygen on Mars you remove 77% of the propellant mass, and thus you get an effective specific impulse of 1600 at high thrust.

[SargeRho]

You can store it as Ammonia, which can be split into Hydrogen and Nitrogen with appropriate catalysts I think.

[maccollo]

Yes, by binding the hydrogen you can transport it without boil off.

However now you are also carrying around the binder, in this case nitrogen. Since ammonia is has a molecular mass of 17 you are carrying around 5.6 kg of nitrogen for every kg of hydrogen, and so your effective specific impulse is about 1170. Carbon already performs this role with methane, and has a higher fraction of hydrogen by mass, and you don't have to convert it to anything.

Edited January 1, 2015 by maccollo

[Bill Phil]

Nitrogen is still useful. You could use it to make the life support have a non-pure oxygen atmosphere.

[maccollo]

Nitrogen is still useful. You could use it to make the life support have a non-pure oxygen atmosphere.

That's true, but you only need it to replace that which might trickle away. I don't think you will need 30 tonnes of nitrogen for a 18 month stay.

Also, since the ISRU plant is already extrating the CO2 from the atmosphere, the process of getting nitrogen from the atmosphere is already half done.

What's left after the CO2 has been removed is equal parts nitrogen, argon, and 0.1% carbon monoxide. The last step would just be to use a catalyst like platinum to react the CO

with some of the excessive amounts of oxygen that is generated, to turn it into a safe-to-breathe concentrations of CO2.

Edited January 1, 2015 by maccollo

[Albert VDS]

IIRC the hydrogen would be in gel form.

[tater]

Zubrin is a bit delusional. Although his ideas are popular and make spectacular pictures, nobody in the space industry really takes his obsession with Mars seriously.

Except maybe Elon Musk.

The players that exist by sucking at the public teat exclusively want whatever invokes the largest possible cash transfer from the taxpayers to, well, them. It was my understanding that the current standard thought on a US Mars mission is pretty close to Mars Direct, anyway, sending a few craft ahead vs complicated on-orbit assembly.

[Skyler4856]

You can store it as Ammonia, which can be split into Hydrogen and Nitrogen with appropriate catalysts I think.

Or as hydrogen peroxide, the left over O2 could be used for medical, scientific, or engineering purposes....

[maccollo]

IIRC the hydrogen would be in gel form.

At first I thought it wouldn't make a difference, but then I found this http://www.grc.nasa.gov/WWW/Fuels-And-Space-Propellants/GELLED.htm

Apparently the boil off rate can be decreased by just mixing liquid hydrogen with liquid methane, although it doesn't say by how much. Let's stipulate that a 50% mixture would be enough to prevent boil off on the 6 month transit, and you then took the hydrogen and turned it into methane on Mars. You would have two and a half times more methane that if you brought only methane. Thus the effective ISP would increase to...

...almost 3200 seconds. So now the MAV is effectively at the performance level of electric propulsion, but at the TWR of chemical.

Edited January 1, 2015 by maccollo

[Bill Phil]

Wait. What do you mean? How can it be as efficient as electric?

[Kulebron]

It's ISP is ten times (3200 / 320) bigger than that of chemical, and almost equals to ion engines.

[Nibb31]

Except maybe Elon Musk.

The players that exist by sucking at the public teat exclusively want whatever invokes the largest possible cash transfer from the taxpayers to, well, them. It was my understanding that the current standard thought on a US Mars mission is pretty close to Mars Direct, anyway, sending a few craft ahead vs complicated on-orbit assembly.

Elon Musk's approach to Mars is very different from Zubrins, and even more delusional. What he has done with SpaceX is admirable, and he is a visionary, but self-sustaining colonies on Mars in this century with thousands of emigrants buying tickets to live there? No way.

NASA's DRM approach uses orbital assembly of multiple vehicles, and precursor cargo landings, which is similar to Zubrin's.

However, Zubrin's Mars Direct is more minimalistic and a bit reckless. It relies on ISRU, which is great as a long term goal, but not mature enough to risk astronauts lives on. It also implies tether-based artificial gravity, which adds unnecessary complexity.

Personally, I don't get the point of a manned expedition on Mars at this stage. The technology is simply not mature enough, and it will take decades to develop and bring up to a usable TRL. During those decades, we would be doing nothing.

I would rather see NASA concentrate on doing stuff that it can afford, with goals that are reachable today. If that means sticking around LEO and the Moon, then so be it. It might not be as spectacular as Mars, but I'd really want to see some actual achievements rather than another 30 years of powerpoints and youtube videos.

Edited January 2, 2015 by Nibb31

[maccollo]

Because you are turning 1 tonne of half-hydrogen half-methane into 2.5 tonnes of methane and 8.6 tonnes of oxygen. That's an 11 to 1 mass leverage. I typed in the wrong number at some point btw, the effective ISP would be about 4070, not 3200

Thrust equals massflow * exhaust velocity, It's a linear relationship. If the flow rate is reduced by half the ISP must double to give the same thrust. Since we only had to load around 9% of the actuall fuel tanks on the original stack when it took off from Earth that means the effective fuel flow is 9%, thus our effective ISP goes up by a factor of 11.

Edited January 2, 2015 by maccollo

[ndiver]

@Nibb31: that's normal, NASA DRM is based on the Mars Semi-Direct variant, also proposed by Zubrin to include few points requested by the NASA.

As explained by Zubrin, the only expeditions (to the poles, Australia, America) that worked in the past were the ones using local resources. That's why his project is minimalist. But it's safe, as there are always emergency solutions to nearly all the steps of the mission. For the ISRU, that's whatever a necessary step for the first manned missions in the solar system, otherwise, you need battleships, that bring complexity and risk.

[Bill Phil]

Because you are turning 1 tonne of half-hydrogen half-methane into 2.5 tonnes of methane and 8.6 tonnes of oxygen. That's an 11 to 1 mass leverage. I typed in the wrong number at some point btw, the effective ISP would be about 4070, not 3200

Thrust equals massflow * exhaust velocity, It's a linear relationship. If the flow rate is reduced by half the ISP must double to give the same thrust. Since we only had to load around 9% of the actuall fuel tanks on the original stack when it took off from Earth that means the effective fuel flow is 9%, thus our effective ISP goes up by a factor of 11.

Problem. Thrust in an engine is dependent on many factors. Throat area, chamber pressure, nozzle geometry, flow rate, etc. Although that is a good ball park estimate.

The thrust, in Newtons, is equal to the momentum of the engine and the exhaust.

What you're saying still doesn't make much sense... Flow rate is the same, just the amount of mass total is less/more. A total mass increase means more time to get the increased mass through the engine. The flow rate ratio stays the same.

Plus, the oxygen isn't in the methane-hydrogen mix. So it's 2.5*.

[maccollo]

Wait... silly me. I just realized it only works that way if you don't have to carry the hidden mass with you, like an air-breathing engine. The actual effective ISP in this case would be an inverse deltaV equation, and the effective ISP would depend on the delta V one wishes to attain.

I think I got an idea for how to predict the relationship between the deltaV of the fully fueled ship and the reduction in effective ISP. For now I will just lazy it.

The effective ISP for a 370 second actuall ISP with a propellant mass leverage of 11 to 1 is as follows:

2500 seconds at 3.8 km/s, or low Mars orbit.

1500 seconds at 8 km/s, or Mars surface to trans Earth injection.

Edited January 2, 2015 by maccollo

[Bill Phil]

Is that mass ratio the full/empty? Earlier you said it was some other thing with that ratio.

Wait. What numbers are you using in the equation? And by effective ISP do you mean effective exhaust velocity?

Edited January 2, 2015 by Bill Phil

[maccollo]

Is that mass ratio the full/empty? Earlier you said it was some other thing with that ratio.

The mass ratio is the fuel you bring vs the fuel you have at takeoff by the use of ISRU.

The efficiency of the ISRU depends on that propellant leverage, and also the actual fuel fraction at takeoff, as in what kind of ISP the rocket would need it launched with the fuel that it had when it landed.

So if we landed an MAV with 1.27 tonnes of methane hydrogen, which then turns into 14 tonnes of methalox, what ISP would be necessary to push a 7 tonne payload to 4 km/s if we assume it actually took of with just 1.27 tonnes of propellant.

Edited January 2, 2015 by maccollo

[Bill Phil]

The ISP would be the ISP of the engines. And the mass ratio should be the ratio of the the craft.

And what is Effective ISP anyways?

[maccollo]

The ISP would be the ISP of the engines. And the mass ratio should be the ratio of the the craft.

And what is Effective ISP anyways?

Effective exhaust velocity/g0, though I realize I used that term incorrectly.... But I believe can be excused for doing so. After all Zubrin is an aerospace engineer.

http://youtu.be/sZGwLhZW09k?t=25m2s

Anyway, effective ISP is obviously can't be used to describe it since "thrust = Ve * massflow" doesn't work. There probably isn't a term for it. However the point is simply to describe the effectiveness of using ISRU to generate propellant.

[Bill Phil]

So it's normal specific impulse. And it is one weird way to measure the effectiveness if ISRU. One that sounds wrong. Sone other number. Maybe the ratio you described earlier. Mass advantage that ISRU provides.