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Roeben wrote:Maybe I didn't make myself clear enough.

He didn't use the RTG for energy for the rover, the electricity he saved comes from not having to operate the built-in rover heater. The rover either couldn't tap enough energy out of the RTG to do anything at all as it was designed to only do the fuel conversion for the ascent stage, or the rover simply didn't have the right power plug.

Maybe in the movie. In the book, it specifically says that it decreased the charge time by using it as a power source.

[Contains swearing] I didn't say it let him drive for an hour, did I?

RedDwarfIV wrote:He used it for power in the book. Cut an hour off the rover's charge time IIRC.

Oh, my bad. That makes a lot of sense. I misinterpreted what you said, I guess.

hi hi

Considering that he was modifying everything he could find from its original purpose, I don't think the martian would have had too much trouble making an electrical adapter if the rover didn't have an appropriate power plug-in. Still, 100 watts isn't a whole lot of energy, and given the efficiency of most RTGs, somewhere between 1000 and 2000 watts of heat is respectable, but still could get chilly at night.

As for the comparison between RTG and solar panels, Mars has a maximum solar irradiance of about 590 watts per meter squared. Even with a phenomenal 20% efficient solar panel, he's only going to be getting at most 118 watts per square meter of solar panel in the most ideal conditions.

RedDwarfIV wrote:

Roeben wrote:Maybe I didn't make myself clear enough.

He didn't use the RTG for energy for the rover, the electricity he saved comes from not having to operate the built-in rover heater. The rover either couldn't tap enough energy out of the RTG to do anything at all as it was designed to only do the fuel conversion for the ascent stage, or the rover simply didn't have the right power plug.

Maybe in the movie. In the book, it specifically says that it decreased the charge time by using it as a power source.

[Contains swearing]

SpoilerShow

Using it as a heat source to avoid spending electrical power on heat would be the more effective approach...adding a hundred watts or so of electrical power vs. removing the need for up to a couple kilowatts of heating power. Having that power available for charging instead of being used for heating would have a much greater effect on charge time. Might need to remove some insulation to keep from overheating, though, considering the vehicle probably didn't have the power budget to spend kilowatts on heat to begin with.

He could have done both, though. A breathable-air-cooled RTG might even produce more power than one being cooled by small radiators and the barely-there Martian atmosphere. (Though Mars seems to have quite a bit more of an atmosphere in The Martian...)

Mjolnir wrote:He could have done both, though. A breathable-air-cooled RTG might even produce more power than one being cooled by small radiators and the barely-there Martian atmosphere. (Though Mars seems to have quite a bit more of an atmosphere in The Martian...)

He did. He used the electricity to charge the batteries, and submerged the RTG in a box of water, through which he ran the O2 regulator's air output (for the regulator to work, air had to be cooled to very low temperatures. Most of the regulator's power requirement was in heating the air back to room temperature.) in order to get warm air.

Mjolnir wrote: Using it as a heat source to avoid spending electrical power on heat would be the more effective approach... [...] Might need to remove some insulation to keep from overheating, though, considering the vehicle probably didn't have the power budget to spend kilowatts on heat to begin with.

He could have done both [...]

And that's exactly what happened in the book. Halved the load on the battery by removing the need for heater, but also tapped into the little bit of electricity output, and had to remove a whole lot of insulation to keep the cabin bearable.

hi hi

I'm still not sure what exactly the RTG was doing there in the first place. Why didn't they leave it in space? Also, I'm kind of curious about the rover's motor. The Tesla Roadster has a 215 kilowatt electric motor in it, for comparison.

Presumably it was there to convert the martian atmosphere into rocket fuel for the ascent stage that the rest of his crew escaped on early on in the story.

icekatze wrote:I'm still not sure what exactly the RTG was doing there in the first place.

I wondered that myself. It was odd that they introduced it without explaining what it was for.

Arioch wrote:

icekatze wrote:I'm still not sure what exactly the RTG was doing there in the first place.

I wondered that myself. It was odd that they introduced it without explaining what it was for.

Yeah, the movie doesn't do a good job of explaining it except to say "we moved it far away when we got here".

But in the book, yeah, it was used to generate fuel for the MAV.

RedDwarfIV wrote:

Arioch wrote:

icekatze wrote:I'm still not sure what exactly the RTG was doing there in the first place.

I wondered that myself. It was odd that they introduced it without explaining what it was for.

Yeah, the movie doesn't do a good job of explaining it except to say "we moved it far away when we got here".

But in the book, yeah, it was used to generate fuel for the MAV.

It powers the MAV while it creates fuel.

And now I'm getting the book and movie mixed up. That's what I get for buying and reading the book the day after watching the movie.

Does the rover flip in the movie?

alpha wrote:Does the rover flip in the movie?

No.

Huh. I just noticed, in the book he uses the second rover as a trailer. In the movie, there doesn't seem to be a second rover, the first rover just has a trailer already.

Probably seen elsewhere already, but SpaceX just stuck the landing of a falcon 9 first stage.



And delivered some communication satellites to orbit too a guess, but damn. They did it.

hi hi

Grats to Space X. They've joined a relatively small club, with the good ol' McDonnell Douglas DC-X, and the Blue Origin New Shepard.

icekatze wrote:hi hi

Grats to Space X. They've joined a relatively small club, with the good ol' McDonnell Douglas DC-X, and the Blue Origin New Shepard.

They did that a few years ago with the Grasshopper. They've started a new club with this...that's the first stage (and 9 of the 10 engines) of a rocket that just delivered 11 satellites to orbit. DC-X and New Shepard are dwarfed just by the payload this thing pushed to ~2 km/s at 80 km altitude before turning around and coming back.

Return to flight after a failure, with improvements in engine thrust, densified propellants, deployment of 11 satellites to LEO, an on-orbit restart of the second stage to demonstrate capabilities needed for geosynchronous launches...and first stage flyback and recovery at Landing Complex 1, all completely successful. Quite a Christmas for SpaceX.

Now all they need to do is a couple repeats with the same rocket stage.
Its good to see they accomplished it, but it is after all only the first step.

hi hi

I mean, it's very impressive that they landed the first stage, don't get me wrong. There are very few people who have done a successful vertical landing of a rocket. But plenty of rockets have delivered satellites into LEO, so that's really not a first.

I am, of course, very curious to see if they can put it back into orbit and save money with the recovery, or if maintenance costs will expand beyond initial projections like in recoverable projects of yesteryear. Definitely something to watch.

icekatze wrote:hi hi

I mean, it's very impressive that they landed the first stage, don't get me wrong. There are very few people who have done a successful vertical landing of a rocket. But plenty of rockets have delivered satellites into LEO, so that's really not a first.

I am, of course, very curious to see if they can put it back into orbit and save money with the recovery, or if maintenance costs will expand beyond initial projections like in recoverable projects of yesteryear. Definitely something to watch.

It's the first that the same vehicle has done both.

It's 90% of the overall launch vehicle, and unlike the Shuttle or the various other spaceplane boondoggles, it doesn't go to orbit. There's no exotic heat shielding tiles, it makes a much gentler reentry at just a few km/s. The added mass required for recovery makes a relatively small difference in the payload, and the added complexity amounts to landing legs, some small cold-gas thrusters, and some grid fins. If they want to pursue full reuse, recovering the second stage on its own is a much easier and less costly problem to solve. (Though they don't currently plan this with the Falcon 9, as the resulting payload capacity would be too limited.)

That's a very substantial amount of the overall launch cost that they don't have to pay on a subsequent launch, and they achieved it without anything that can be expected to cost a great deal...in fact, they're able to significantly undercut their competition while operating the Falcon 9 as an expendable launcher. Just recovering the engines means they can sustain a much higher launch rate for the same manufacturing rate...and the first stage has to be basically intact and operational in order to successfully land, so odds are they'll be able to reuse much more than that.

hi hi

Well, when you put it like that, it sounds basic and easy.