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In The Martian (by Andy Weir) book, MAV fuel plant was able to make fuel using Martian atmosphere, but the process was slow. From Chapter 1:

The most important piece of the advance supplies, of course, was the MAV. The Mars ascent vehicle. That was how we would get back to Hermes after surface operations were complete. The MAV was soft-landed (as opposed to the balloon bounce-fest the other supplies had). Of course, it was in constant communication with Houston, and if there had been any problem with it, we would have passed by Mars and gone home without ever landing.

The MAV is pretty cool. Turns out, through a neat set of chemical reactions with the Martian atmosphere, for every kilogram of hydrogen you bring to Mars, you can make thirteen kilograms of fuel. It's a slow process, though. It takes twenty-four months to fill the tank. That's why they sent it long before we got here.

Just before Ares 3 crew landed, Martinez remotely landed MAV to Ares 4 site. Mark Watney reached Ares 4 site after about 1.5 years (i.e. less than 24 months). It means Ares 4 MAV didn't make fuel out of even all hydrogen on-board. Yet, NASA ordered Mark Watney to give MAV more hydrogen. From Chapter 25:

[13:07] HOUSTON: Congratulations from all of us here at Mission Control! Well done! What's your status?
[13:21] MAV: Thanks! No health or physical problems. The rover and trailer are getting pretty worn out, but still functional. Oxygenator and regulator both working fine. I didn't bring the water reclaimer. Just brought the water. Plenty of potatoes left. I'm good to last till 549.
[13:36] HOUSTON: Glad to hear it. Hermes is still on track for a Sol 549 flyby. As you know, the MAV will need to lose some weight to make the intercept. We're going to get you those procedures within the day. How much water do you have? What did you do with urine?
[13:50] MAV: I have 550 liters of remaining water. I've been dumping urine outside along the way.
[14:05] HOUSTON: Preserve all water. Don't do any more urine dumps. Store it somewhere. Turn the rover's radio on and leave it on. We can contact it through the MAV.

Later in Chapter 25:

Venkat flipped through the pages. "Care to summarize?"
"First, we're going to add fuel. The MAV makes its own fuel from the Martian atmosphere, but it's limited by how much hydrogen it has. It brought enough to make 19,397 kilograms of fuel, as it was designed to do. If we can give it more hydrogen, it can make more."
"How much more?"
"For every kilogram of hydrogen, it can make thirteen kilograms of fuel. Watney has five hundred and fifty liters of water. We'll have him electrolyze it to get sixty kilograms of hydrogen." Bruce reached over the desk and flipped a few pages, pointing to diagram. "The fuel plant can make seven hundred and eighty kilograms of fuel from that."

On Sol 529, Mark Watney did start the process of converting water into rocket fuel, by supplying fuel plant extra hydrogen. But, MAV took flight on Sol 549 i.e. after 20 Sols. First, MAV hadn't yet converted existing hydrogen to fuel and Mark added extra 60Kg of hydrogen. How was MAV fuel plant able to do hundreds of Sols of work within 20 Sols? Did I miss something? Or, is it a plot hole (I am less inclined to expect this because of scientific accuracies of the book)?

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    He removed a lot off stuff from the MAV less weight less fuel needed – Bigben59 Feb 9 '15 at 15:09
  • @Bigben59 See the quotes in the question. He also did make extra fuel which increased payload by 300 Kg. Without extra fuel, stuff removal wouldn't work. – Avenge The Fallen Feb 9 '15 at 15:12
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    @SS how else are you meant to interpret it? The original comment says "it takes 24 months to fill the tank" and yet its already finished 1.5 years in, 6 months before that 24 month period. So the reaction isn't as slow and steady as Watney first alludes to. When tested in 2011 in Mars conditions, an experimental Sabatier reaction engine (which is what is used here) worked at a rate of 1KG of fuel produced a day, and thats without the 20/30 years of development between now and the time period in the book. Remember that Watney isnt an expert on every system, he will have mostly had overviews. – Moo Feb 10 '15 at 9:22
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    @SS - the MAV is sent specifically when it is because the orbiting crew of the current mission uses remote control to land the MAV for the next mission. The other supplies are capable of bounce-landing and don't need the careful piloting of landing the MAV. – Scott Whitlock Feb 10 '15 at 17:26
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    Actually, on page 65 we see a different duration, along with a claim that it's not a hard number: "It takes eighteen months for the MAV to make its fuel, so it's the first thing NASA sends along. Sending it forty-eight months early gives it plenty of extra time in case fuel reactions to slower than expected." See here in google books – davidbak Feb 12 '16 at 0:40
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+50

MOAR Pirate Ninjas!

Lets recap. The MAV is sent on ahead to make rocket fuel. It does this by taking a "kilogram of hydrogen you bring to Mars, you can make thirteen kilograms of fuel." Let's dwell on that a second.

Either the MAV is directly converting energy into mass...or it is using martian chemicals to make up the other 12 kg of mass.

However we have already noted that Mars does not have a local chemist, stocked with canisters of chemicals.

Remember the MAV fuel plant? It collects CO2 from the Martian atmosphere. A 10-liter tank of compressed liquid CO2... That'll take less than a day to create.

However it is pretty clear without even doing simple calculations that the 0.1 Pirate-Ninjas that the MAV's original power plant (the RTG) has, is incapable of compressing that much CO2, from PURE CO2 at Standard Martian Temperature Pressure.

Therefore the main difference is that by hooking up the two Rovers worth of solar cells plus the battery plus the 2 RTGs, the MAV fuel plant would be able to get many more Pirate-Ninjas and much faster to complete its task of compressing 12kg of CO2.

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The book is inconsistent. On page 65 we see:

It takes eighteen months for the MAV to make its fuel, so it’s the first thing NASA sends along. Sending it forty-eight months early gives it plenty of extra time in case fuel reactions go slower than expected.

Here's a possible in-universe explanation: He says 24 months on Sol 6 (in his very first log entry). He says 18 months on Sol 63, by which time he's had a chance for a focused review of all the tech specs on his computer for all his equipment (in order to plan his survival). So on Sol 6 Mark just misremembered it. It's understandable, he was in pain and in shock:

I'm pretty much fucked. That's my considered opinion. Fucked.

(Not quite as good a first line as "Call me Ishmael" but not bad either.)

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