[X] My Assistant

[maycm]

From the following "Space" article LINK

Spirit’s solar arrays currently produce about 650 watt hours while Opportunity’s generate up to 700 watt hours, Laubach said. A minimum of 300 watt hours is required for Spirit to function, though Opportunity can operate on slightly less, she added.

What is the reason for the lower power requirements of Oppy?

I thought the two rovers were identical.

[djellison]

Ahh - you're assuming 100% efficiency in the use of petrol.

Typically, an efficient Petrol engine is 30%, Diesel perhaps as much as 40% or more

So you'd need not 72 ml/sol, but more like 220 ml/sol

Doug

[abalone]

Ahh - you're assuming 100% efficiency in the use of petrol.

Typically, an efficient Petrol engine is 30%, Diesel perhaps as much as 40% or more

So you'd need not 72 ml/sol, but more like 220 ml/sol

Doug

It was only intended as a comparison. If you use the petrol in a fuel cell you could get 70% eficiency. Maybe petrol was a poor choice, have worked it out for liquid hydrogen and its about 10.5g/day or about 8kg for the entire mission if we assume a fuel cell efficency of 80%. And before you ask, hey it not my fault there is no oxygen on Mars.

I only did the calculations to illustrate the small amount of energy that we are dealing with here

[helvick]

It was only intended as a comparison. If you use the petrol in a fuel cell you could get 70% eficiency. Maybe petrol was a poor choice, have worked it out for liquid hydrogen and its about 10.5g/day or about 8kg for the entire mission if we assume a fuel cell efficency of 80%. And before you ask, hey it not my fault there is no oxygen on Mars.
I only did the calculations to illustrate the small amount of energy that we are dealing with here

You'll still need an additional 8x the mass of O2 for your fuel cell ~ 370kg in total. I've got a number somewhere that has a specific power for Fuel cells of ~ 0.03kw /kg. So you'd need about 100kg of fuel cell and maybe 100kgor more of cryo storage for the fuel. It would be kind of bulky.

If you forget LH2\LO2 and go for C12H26 (Kerosene)\H2O2 as your oxidiser then you'd have a 10:1 oxidiser:fuel mass ratio. All you really needed for MER was a 90day mission so you could have safely budgeted for only 1kg of fuel, 10kg of oxidiser and throw in about 10kg max for a reliable and efficient 140 watt self contained diesel generator.

[Bob Shaw]

You'll still need an additional 8x the mass of O2 for your fuel cell ~ 370kg in total. I've got a number somewhere that has a specific power for Fuel cells of ~ 0.03kw /kg. So you'd need about 100kg of fuel cell and maybe 100kgor more of cryo storage for the fuel. It would be kind of bulky.

If you forget LH2\LO2 and go for C12H26 (Kerosene)\H2O2 as your oxidiser then you'd have a 10:1 oxidiser:fuel mass ratio. All you really needed for MER was a 90day mission so you could have safely budgeted for only 1kg of fuel, 10kg of oxidiser and throw in about 10kg max for a reliable and efficient 140 watt self contained diesel generator.

Ah, of course, that makes more sense - so, for a mission requiring 'bursts' of mechanical energy, might not a hybrid diesel/solar electric vehicle make a lot of sense? Think of it as a winter warmer... ...and I bet you could have a much smaller engine - look at the engines in R/C models, UAVs etc.

Bob Shaw

[stevesliva]

Ah, of course, that makes more sense - so, for a mission requiring 'bursts' of mechanical energy, might not a hybrid diesel/solar electric vehicle make a lot of sense? Think of it as a winter warmer... ...and I bet you could have a much smaller engine - look at the engines in R/C models, UAVs etc.

How about a solar concentrator powering a stirling engine turning a flywheel?

[Bob Shaw]

How about a solar concentrator powering a stirling engine turning a flywheel?

Yes, of course - and a hydrocarbon/oxygen engine as previously described has a major efficiency problem in the sense that it produces heat rather than mechanical energy. But what do we need inside a Mars rover, but *heat*? And as for the exhaust gasses, you store 'em. And then you use them to blow your solar concentrator/panels/etc free of dust...

The bottom line is that an exclusive approach to power generation actually *reduces* your options rather than expanding your mission!

Bob Shaw