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#126 2020-02-17 20:56:09

SpaceNut
Administrator
From: New Hampshire
Registered: 2004-07-22
Posts: 19,881

Re: Running on Compressed Air?

The moxie process creates the possible fuel for land based crafts to be created and used early plus it requires no water mining for a trip with all that is required is to super cooling it to liquid. pg 50 shows the tracked verion of a vehicle using it.

http://www.niac.usra.edu/files/studies/ … 40Rice.pdf
ADVANCED SYSTEM CONCEPT FOR TOTAL ISRU BASED PROPULSION AND POWER SYSTEMS FOR UNMANNED AND MANNED EXPLORATION

The rover mission was defined as a 300 km one day trip. A system of gravel roads was assumed. Based on a trip distance of 300 km and a transit time of 10 hours, a velocity of 30 km/h was used. Turbine efficiency was assumed same for all fuels at 65% chemical potential to mechanical energy conversion efficiency.

For the initial iteration, the structural mass penalty for storing propellant was a 100 kg storage tank. The propellant mass penalty was ½ the mass of the propellant. For example, a 2000 kg rover that burns 200 kg of propellant and stores the byproducts has a mass of 2300 kg for the entire duration of the trip (with the tank mass penalty), while a 2000 kg venting rover has a mass of 2200 kg at the beginning of the trip and 2000 kg at the end. The mass used to calculate the propellant usage was therefore 2100 kg.

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#127 2020-09-22 19:38:37

SpaceNut
Administrator
From: New Hampshire
Registered: 2004-07-22
Posts: 19,881

Re: Running on Compressed Air?

Calliban wrote:

Addressing post 207.  Pneumatic transport, that is to say a vehicle pushed along by differential pressure, is definitely an option worth considering on Mars.  The interesting thing about the idea is that the vehicle need not carry its own power supply.  A rail based or wheeled vehicle propelled by differential pressure could simply be a set of carriages, equipped with brakes to control its speed.

On Earth, atmospheric railways have found niche applications, but were largely rendered obsolete following the application of electricity to railways.  However, small pneumatic pipelines carrying wheeled vehicles could be very useful for transporting bulk materials on both Earth and Mars.  A square conduit just 20cm wide, carrying wheeled vehicles at 10m/s, could move up to 1500 cubic metres per hour of freight or bulk materials between any two points.  The vehicle would have a loose seal at its front and back, to minimise gas leakage past it.

The required differential pressure needed to drive the vehicle would be a function of its mass (weight), the friction coefficient of the wheels and the cross-sectional area of the vehicle.

The minimum force needed to move a vehicle is given by: F = W x C, where W is weight in N, and C is coefficient of rolling resistance, typically 0.002 for steel wheels on steel rails.  A 1 tonne mass would weigh 3679N on Mars.  To move this mass in a rail vehicle will take a minimum force of 7.4N on Mars.  For a vehicle 20cm wide, this amounts to a required differential pressure of 184Pa per tonne.  This of course assumes a perfectly flat surface.

Hydraulic capsule pipelines are a related concept in which neutrally buoyant capsules of material are carried through a water pipe.  Typically, this carries capsules very slowly, about human walking speed.  The concept is one of the most energy efficient transportation methods known to man, but it has the limitations of low speed and the need to load and unload the capsules with whatever is being transported and handle them at both ends.  An added problem exists on Mars - low temperature.  Operating hydraulic pipelines on Mars would require either heating the pipes or using brine as a carrying fluid.  The first solution is an option if we have access to nuclear waste heat.  The low thermal conductivity of Martian regolith would make it an excellent insulator.  Using cold brine is problematic, because water at subzero temperatures becomes progressively more viscous, which pushes up pumping power.

One significant advantage of low speed pipelines on Mars is the extremely low vapour pressure of water at temperatures close to zero.  The pipelines would barely need to be pressurised.  They could be constructed from cast basalt, concrete, steel or polymer.  We would lay them between our colony and numerous mine sites, allowing colonists to access materials from hundreds or even thousands of kilometres away.  The water would carry the capsules along at speeds of no more than a few metres per second.

A pipe some 10cm in diameter, carrying capsules at a speed of 1m/s, could transport up to 250,000 tonnes of material per year.  If the pipe is made from steel with a wall thickness of 1mm, it would weigh 2.5 tonnes per km. A 1000 km long pipe would therefore weigh 2500 tonnes.  The pipe could therefore carry 100x its own weight in freight every year.  Useful, if we wanted access to resources from a large area to support the needs of a growing city.  We could for example, build a city under a polar ice filled crater.  We could grow food in an agricultural colony closer to the equator.  A pipeline would deliver water from the polar city to the agricultural colony during the spring and summer months.  During the autumn and winter, the same pipeline would be used to transport food north in capsules.

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