Best propulsion for a long range rover

JoshNH4H · 2014-03-11 00:07:54

The abstract does indeed sound interesting. Unfortunately I don't have access to that paper through the university. Regardless, methane, propane, or methanol is not a hard choice to make because all are known quantities. It is my opinion that methanol is still the best of the three because it can be stored at ambient temperature and pressure.

But that doesn't matter. Any of the three is very workable but we still lack an oxidizer.

Nitric acid is possible but not very desirable because it's corrosive and hard to make. Perchloric acid may actually be our best bet, insofar as its relatively possible to produce (it is an electrolytic process, which is unfortunate from an energy perspective, but I suppose we can make do), liquid, and has a good amount of oxygen per unit mass.

Quaoar · 2014-03-11 06:22:00

If it's so difficoult to store liquid oxigen, how can we preserve it for more than two years in the tanks of the Mars Ascent Vehicle?

Last edited by Quaoar (2014-03-11 06:43:43)

JoshNH4H · 2014-03-11 06:55:51

It's not that it's so tough, just that the equipment to do so it's heavy (if we're talking about a mission), hard to manufacture (if we're talking about a colony), and not necessary if we can find something else.

JoshNH4H · 2014-03-11 07:16:13

To be clear, for a mission I do support methane-oxygen fuel for the rover (with a recycle if necessary to use the exhaust as a dilutant). It's only in a colony, especially a self sufficient one or a nearly self sufficient one, that the economics of chemistry become important.

RobertDyck · 2014-03-11 07:22:58

Storing liquid oxygen in the Mars Ascent Vehicle, or Earth Return Vehicle, will require refrigeration. Mars has very thin atmosphere, so it will heat much more slowly than it would on Earth. Yet, it will boil. That oxygen gas has to be refrigerated to turn it back into liquid. That requires power, so again that requires the nuclear reactor. You don't want to carry an operating reactor on a vehicle that carries humans.

To keep mass down, reactors designed for space don't have radiation shielding. Uranium is safe to handle before it goes into a reactor. However, the fission fragments (nuclear waste) is very radioactive. Mars Direct deals with that by putting the reactor on a light truck, and parking it in the bottom of a crater some distance from the ERV. With a power cable trailing back.

GW Johnson · 2014-03-11 09:50:34

Methanol should be relatively easy to make on Mars, if you can make methane. In fact, here, that's the cheapest way to do it: from natural gas, not destructive distillation of wood or waste. Easy to store, too.

But (there's always at least one "but", in this case two): (1) as Josh says, we have no "good" oxidizer for it, and (2) methanol is very poisonous if you get careless with it. A lethal dose can be absorbed right through the skin. It's not the most energetic of the fuels, but it is useful.

The real problem is oxidizer. If you use oxygen, it's either compressed gas or a cryogenic liquid. Not much choice there, either way it's heavy, and the amount you need is large compared to the methanol (or the methane, or the propane, etc).

Now, using it: as fuel-oxygen, or as fuel-oxygen-diluent. The difference is around 500-1000 C in terms of flame temperature, at temperatures pretty close to 2000 C if you have diluent. That's tough to design for, even here. "Air" cooling on Mars would be quite difficult, because the "air" stream has too low a density to hold any heat for you. It's directly proportional to specific heat capacity, density, velocity of flow, and temperature difference. Density is the problem, at 0.7% of what we're used to here. That applies to direct air cooling, and to the final heat rejection radiator of a liquid-cooled system. Either way, on Mars, unlike here, you'll have more heat rejection potential from thermal radiation than you will convection to the local "air".

I still think the most straightforward solution for a chemically-powered rover is something like welding gas bottles of compressed gas reactants. If you have a tank of methanol for fuel, so much the better. But your oxidizer is likely to be gobs of bottled oxygen. Just swap the bottles out each trip. Unless somebody comes up with a better oxidizer idea.

Myself, I'd go for the fuel cell electric instead of a combustion heat engine, precisely because there is one whale of a lot less waste heat to reject under conditions that unfavorable at best. But that drives you right back to LOX-LH2 or compressed gaseous H2 and O2 as the most technologically-ready fuel cell reactants.

GW

Last edited by GW Johnson (2014-03-11 09:56:50)

Quaoar · 2014-03-11 09:58:08

RobertDyck wrote:

Storing liquid oxygen in the Mars Ascent Vehicle, or Earth Return Vehicle, will require refrigeration. Mars has very thin atmosphere, so it will heat much more slowly than it would on Earth. Yet, it will boil. That oxygen gas has to be refrigerated to turn it back into liquid. That requires power, so again that requires the nuclear reactor. You don't want to carry an operating reactor on a vehicle that carries humans.
To keep mass down, reactors designed for space don't have radiation shielding. Uranium is safe to handle before it goes into a reactor. However, the fission fragments (nuclear waste) is very radioactive. Mars Direct deals with that by putting the reactor on a light truck, and parking it in the bottom of a crater some distance from the ERV. With a power cable trailing back.

So, a Mars mission architecture that only relies on solar panels cannot use ISRU and has to bring a minimal MAV with storable propellant rocket, something like 3-4 tons of dry mass and 7.5-10 tons of N2O4-MMH?

How much power/kg is needed to keep LOX-LCH4 liquid with good multilayer insulated tanks?

Last edited by Quaoar (2014-03-11 10:01:53)

RobertDyck · 2014-03-11 13:36:52

Quaoar wrote:

So, a Mars mission architecture that only relies on solar panels cannot use ISRU and has to bring a minimal MAV with storable propellant rocket, something like 3-4 tons of dry mass and 7.5-10 tons of N2O4-MMH?
How much power/kg is needed to keep LOX-LCH4 liquid with good multilayer insulated tanks?

It's theoretically possible to maintain soft cryogenics on Mars, such as LOX and LCH4. But to produce propellants, liquefy them, and maintain them as liquid, you really need so much power that a nuclear reactor is required. I don't have any figures, but Mars Direct estimated 85kW for the ERV.

RobertDyck · 2014-03-11 13:44:15

GW Johnson wrote:

Myself, I'd go for the fuel cell electric instead of a combustion heat engine, precisely because there is one whale of a lot less waste heat to reject under conditions that unfavorable at best. But that drives you right back to LOX-LH2 or compressed gaseous H2 and O2 as the most technologically-ready fuel cell reactants.

So for compressed gas H2, carbon nanofibre batting in a composite tank, made of aluminized PCTFE film bladder within a graphite fibre cloth / epoxy shell? That will have some leakage of hydrogen through the aluminum layer, but just live with it. That limits fuel storage time. Keep LH2 at base and just fill the rover's fuel tanks before each sortee.

Quaoar · 2014-03-11 17:26:49

RobertDyck wrote:

It's theoretically possible to maintain soft cryogenics on Mars, such as LOX and LCH4. But to produce propellants, liquefy them, and maintain them as liquid, you really need so much power that a nuclear reactor is required. I don't have any figures, but Mars Direct estimated 85kW for the ERV.

But a semi-direct mission with an orbital ERV and a little MAV can relay on solar panel to produce 6-8 tons of ascent propellant or has to bring it from Earth?

JoshNH4H · 2014-03-11 17:42:08

To clarify, I doubt that 85 kW would be needed for cooling. I would guess that most of that goes to power generation and somewhere under 10 kW goes to cooling

SpaceNut · 2014-03-11 19:18:43

Other thoughts that I am reminded of is compressed gas powered engine as another alternative.

Compressed gas energy storage of which the topic in my post referenced the older threads with links that do not work....had to use google advance search to relocate them.
Hydrogen Car Powered by Expansion of Liquid H2 & Running on Compressed Air?

Saw the article The Car That Runs On Air

1) The Hybrid Air Car uses compressed nitrogen, which is held in a tank called the high-pressure accumulator.
2) A hydraulic pump and piston compress nitrogen in the accumulator. When the nitrogen is released (by pressing the accelerator), the pump runs in reverse. Acting now as a motor, it harnesses the energy of the moving hydraulic fluid to send power to the wheels.
3) After the hydraulic fluid passes through the motor, it flows to the low-pressure accumulator, where it is stored for later use.
4) A gasoline engine supplements the air power when accelerating or going up hills. This could be an 82-hp 1.2 L I3 for subcompacts and a 110-hp 1.6 L I4 for compacts.

RobertDyck · 2014-03-11 21:37:31

JoshNH4H wrote:

To clarify, I doubt that 85 kW would be needed for cooling. I would guess that most of that goes to power generation and somewhere under 10 kW goes to cooling

No. The SP-100 nuclear reactor, the one Mars Direct used, was designed to produce 100 kW electricity. To do that, the reactor produced 2,000 kW heat. One of the great improvements with SAFE-400 is was the power converter. But total power that ERV required was 85 kW electricity. Most of that was to produce propellant. Only a small fraction was for cooling.

JoshNH4H · 2014-03-11 21:39:36

The issue with compressed gas is energy density, and to be more precise volumetric energy density. It's simply not practical for it to hold as much energy as chemical fuels can. For example, Meth/LOX has an energy density of 13 MJ/kg. Its volumetric energy density is about 10 GJ/m^3. For comparison, an ideal gas with the same molecular weight as Hydrogen will have a specific energy of RT/mm (Ideal gas constant*temperature in Kelvin/molar mass in kilograms, ex. Hydrogen=.002). This equation means that even Hydrogen only gets about 1 MJ at 0 C*. However, volumetric energy density is another story. The volumetric energy density is equal to the pressure, so a pressure of 10 GPa would be needed to achieve a comparable energy density to methlox. This is not reasonable. Please note that this model assumes ideal behavior and expansion to infinite volume and zero pressure. Neither of these conditions could be met in reality.

*This is the principle behind the cold gas thruster. Experimentally, cold gas thrusters using Nitrogen achieve Isps of around 60 s (600 m/s). My equation predicts that the temperature required to achieve this would be 600 K, which is significantly hotter than cold gas thrusters normally fire at. I don't know where the energy is coming from, but they are operating at very high pressures so I wouldn't be surprised if there were significant deviation from ideal behavior.

RobertDyck · 2014-03-11 21:53:04

Quaoar wrote:

But a semi-direct mission with an orbital ERV and a little MAV can relay on solar panel to produce 6-8 tons of ascent propellant or has to bring it from Earth?

Semi-Direct brought return propellant all the way from Earth. But still required ISPP anyway; the MAV used ISPP to produce propellant for ascent to Mars orbit. So hauling return propellant from Earth dramatically increases cost without increasing safety. Really bad idea.

If free return is required, an express trajectory (6 month transit), can use Mars gravity to loop around the far side and head back to Earth. Only RCS thrusters are required to tweek trajectory. So no return propellant is required for "free return". That's why it's called "free". Apollo 13 did this with the Moon.

My mission plan uses an orbital ERV, but uses the MAV as the TEI stage. So requires full propellant load.

Quaoar · 2014-03-12 04:49:42

RobertDyck wrote:

Quaoar wrote:
But a semi-direct mission with an orbital ERV and a little MAV can relay on solar panel to produce 6-8 tons of ascent propellant or has to bring it from Earth?
Semi-Direct brought return propellant all the way from Earth. But still required ISPP anyway; the MAV used ISPP to produce propellant for ascent to Mars orbit. So hauling return propellant from Earth dramatically increases cost without increasing safety. Really bad idea.
If free return is required, an express trajectory (6 month transit), can use Mars gravity to loop around the far side and head back to Earth. Only RCS thrusters are required to tweek trajectory. So no return propellant is required for "free return". That's why it's called "free". Apollo 13 did this with the Moon.
My mission plan uses an orbital ERV, but uses the MAV as the TEI stage. So requires full propellant load.

The trouble is that is higly improbable that politicians will allow a mission plan with a nuclear reactor. So I guessed if it's possible to produce the ascent propellant for a little MAV only with solar panels.
For the ERV, we cannot use ISRU, but we can save a lot of propellant sending it unmanned in Earth-Moon L2 with a solar electric space tug: L2-MTO delta-V is less than 1 km/s. Using L2 as a starting point, plus aerocapture at Mars and Earth, the total mission delta-V will be not prohibitive.

RobertDyck · 2014-03-12 06:37:24

Quaoar wrote:

The trouble is that is higly improbable that politicians will allow a mission plan with a nuclear reactor. So I guessed if it's possible to produce the ascent propellant for a little MAV only with solar panels.
For the ERV, we cannot use ISRU, but we can save a lot of propellant sending it unmanned in Earth-Moon L2 with a solar electric space tug: L2-MTO delta-V is less than 1 km/s. Using L2 as a starting point, plus aerocapture at Mars and Earth, the total mission delta-V will be not prohibitive.

Again with the fear over nuclear. Congress has gotten over that irrational fear years ago. Curiosity does not use solar, it has a large RTG. That's Radioisotope Thermoelectric Generator. Cassini included several plutonium heat generators. Protesters panicked over that, but Cassini launched anyway. Spirit and Opportunity include 8 Radioisotope Heater Units each. These also use plutonium. It isn't plutonium-239, so it isn't fissile, but an isotope that will produce heat through decay.

Congress has gotten over it's irrational fear. It's time for you to do so as well.

Last edited by RobertDyck (2014-03-12 06:40:30)

Quaoar · 2014-03-12 06:57:48

RobertDyck wrote:

Congress has gotten over it's irrational fear. It's time for you to do so as well.

I dont fear nuclear: I fear people who fear nuclear.

I like this stuff.

Last edited by Quaoar (2014-03-12 07:03:55)

JoshNH4H · 2014-03-12 07:38:04

I don't think it's fair to say that congress has gotten over irrational fear. Yucca mountain has not been completed, after all. Congress nay have allowed a few kilograms of plutonium in an RTG to be used but they would be much more hesitant about a nuclear reactor.

RobertDyck · 2014-03-12 07:42:29

Hmm. Copernicus. That's the one with the rigid truss used for artificial gravity. Fairly short truss, but a truss. Big, heavy. At least not as heavy as some other designs. Rotation arm is rather short, so getting astronauts dizzy is a danger. But still interesting.
http://en.wikipedia.org/wiki/Space_Launch_System
http://ntrs.nasa.gov/archive/nasa/casi. … 003776.pdf

Found another document. This one shows the long rotation arm that was previously mentioned.
http://www.nasaspaceflight.com/_docs/NE … tories.ppt

All that would be nice. But we can go now with existing technology. The SAFE-400 nuclear reactor was developed in 2007. It's off-the-shelf.

Quaoar · 2014-03-12 10:18:10

RobertDyck wrote:

Hmm. Copernicus. That's the one with the rigid truss used for artificial gravity. Fairly short truss, but a truss. Big, heavy. At least not as heavy as some other designs. Rotation arm is rather short, so getting astronauts dizzy is a danger. But still interesting.
http://en.wikipedia.org/wiki/Space_Launch_System
http://ntrs.nasa.gov/archive/nasa/casi. … 003776.pdf
Found another document. This one shows the long rotation arm that was previously mentioned.
http://www.nasaspaceflight.com/_docs/NE … tories.ppt

The best compromise is one gee with 56 m radius and 4 RPM, but astronauts can adapt even at shorter radius and faster rotation (i.e. 35 m and 5 RPM and probably up to 20 m and 7 RPM) if RPM increment is gradual. Probably some drugs like scopolamine will be necessary in the first days, but it's still better than six months in microgravity.
Even a low cost 4.5 m radius internal centrifugue, that can be easly fitted in a 10 m diameter SLS launced module, may be an interesting tool of study adaptation at high RPM rate, body modification in low gravity environment and may be useful to optimize training protocols.

RobertDyck wrote:

All that would be nice. But we can go now with existing technology. The SAFE-400 nuclear reactor was developed in 2007. It's off-the-shelf.

The NTR of Copernicus are the old Rover-Pewee rockets developed tested and qualified in Los Alamos, plus a Stirling electric generator. I think there are no problems on rebuilding it if we want. On Mars we can go well also with chemical rockets, but if we want to go beyond, we need nuclear.

GW Johnson · 2014-03-12 11:00:16

You could use bottled compressed gas storage on the rover without swapping out welding gas bottles. Just build the gas tanks into the rover, and fill them at your gas plant when you drop by to fill up. That reduces manual labor in a spacesuit, at the expense of higher pressures and storage volume required of your stationary gas plant. This is true for both plain compressed gases, and for pressurized liquids like propane. Just different storage pressures.

GW

Last edited by GW Johnson (2014-03-12 11:01:23)

RobertDyck · 2014-03-12 11:41:11

Quaoar wrote:

The best compromise is one gee with 56 m radius and 4 RPM, but astronauts can adapt even at shorter radius and faster rotation (i.e. 35 m and 5 RPM and probably up to 20 m and 7 RPM) if RPM increment is gradual. Probably some drugs like scopolamine will be necessary in the first days, but it's still better than six months in microgravity.
Even a low cost 4.5 m radius internal centrifugue, that can be easly fitted in a 10 m diameter SLS launced module, may be an interesting tool of study adaptation at high RPM rate, body modification in low gravity environment and may be useful to optimize training protocols.

Um, no. NASA found humans can tolerate a maximum of 6 RPM. Engineers no not to push anything to the max, especially if you're dealing with the variability of human individuals. So you want to reduce it a touch. After all, you don't want individuals unable to move. Just a small sharp movement could push it over the limit, inducing nausea, disorientation, dizziness. Not good if the astronaut is working on something critical. So 4 RPM is the practical limit. You don't want them drugged up on Gravol 24/7.

Quaoar wrote:

The NTR of Copernicus are the old Rover-Pewee rockets developed tested and qualified in Los Alamos, plus a Stirling electric generator. I think there are no problems on rebuilding it if we want. On Mars we can go well also with chemical rockets, but if we want to go beyond, we need nuclear.

And you're the one who raised the issue of people who fear nuclear. Nuclear thermal rockets have to be tested. In the late 1960s and early 1970s, that was done at Jackass Flats, where nuclear bombs were tested. Today the public would panic at the thought of exhaust from a nuclear engine in open air. A solid core nuclear thermal rocket should contain fission fragments, so no radioactive exhaust, but that's if everything works right. The reason for testing is to determine if it does work, to identify problems and fix them. What if fission fragments leak into exhaust? That would panic the public.

By the way, one way that SAFE-400 was so much better than SP-100 was use of a Brayton cycle electric generator.

Last edited by RobertDyck (2014-03-12 11:42:09)

JoshNH4H · 2014-03-12 12:16:54

RobertDyck wrote:

Um, no. NASA found humans can tolerate a maximum of 6 RPM.

Actually, the research in this area is generally contradictory and it's not really possible to obtain a maximum value for acceptable RPM with any degree of certainty.

Quaoar · 2014-03-12 13:56:04

JoshNH4H wrote:

RobertDyck wrote:
Um, no. NASA found humans can tolerate a maximum of 6 RPM.
Actually, the research in this area is generally contradictory and it's not really possible to obtain a maximum value for acceptable RPM with any degree of certainty.

We have very few data and we absolutely need an artificial gravity module in space to test and verify the RMP limits, the ability to adapt at high rotation rate and the long term effects of a reduced gravity environment.
At this time, the only well prooved evidence is that microgravity is very unhealty and it has to be avoided in a more than two years Mars mission, because it would be very dangerous for an astronaut to break the thighbone during a surfare excursion. So, if you compare the risk of some drizzle douring trasfer for an high rotation rate to the risk of a femour fracture or a vertebral collapse douring exploration, the second is more and more mission critical.

Last edited by Quaoar (2014-03-12 14:03:11)

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