View Full Version : Bad Astronomy in The Secret of Life by Paul McAuley

2002-Nov-08, 12:05 AM
I'm not gonna ruin the book by telling everyone what happens, but on pg 186 (in my copy), he mentions the use of an airship on Mars. It reads as follows:
Because there is no need for streamlining in Mars's think atmosphere, the airship's body consists of six fifty-meter-long tubes quilted with cells of hydrogen and bundled around a central spine like an air mattress rolled up lengthwise, with the pressure cabin and the cargo pods hung beneath. I-beam struts extend either side of the center of gravity, each bearing four turboprop motors.
emphasis mine

As most of you probably know by now, I'm a jet engine mechanic. I work on turboprop motors for a living, and Mars's atmosphere is waaay too thin to support the air usage of a turboprop motor.

Mars has a very thin atmosphere composed mostly of the tiny amount of remaining carbon dioxide (95.3%) plus nitrogen (2.7%), argon (1.6%) and traces of oxygen (0.15%) and water (0.03%). The average pressure on the surface of Mars is only about 7 millibars (less than 1% of Earth's), but it varies greatly with altitude from almost 9 millibars in the deepest basins to about 1 millibar at the top of Olympus Mons. (from http://www.seds.org/nineplanets/nineplanets/mars.html)

With such a thin atmosphere and a very small amount of usable oxygen, a turboprop motor would never start, and if it did, would constantly stall for lack of air. Simply put, the engine would starve to death.

Just thought I'd bring attention to this little oversight. (It's really a pretty good book.) *shrug* What can ya expect, I guess. Bad Astronomy found everywhere.

[Edited to fix punctuation]

"Some see the glass half full, some see it half empty, and some see it crawling with toxic alien parasites who want to devour your pancreas." - Sgt Aarhus, from the book Ascending by James Alan Gardner

<font size=-1>[ This Message was edited by: Jetmech0417 on 2002-11-07 19:09 ]</font>

2002-Nov-08, 02:15 AM
Let's see. I'll show my work so the mistakes will be easier to point out. Mars' atmosphere is mainly CO2 which is about 50% denser than N2, and at somewhere less than 7 mb. So 1 cu meter of Mars air would mass maybe 1% of earth's--about 10 grams. I didn't see any mention of the diameter of the tubes--assume 10 meters. So it displaces 6 x 25 x pi x 50 x .01 kg or about 250 kg. Mars gravity cancels out (the payload weighs less, but so does the air displaced). I think the gas envelope is going to eat up a fair chunk of that 250 kg.

You mention a passenger cabin and cargo pods--was the passenger bigger than a gerbil?

2002-Nov-08, 02:28 AM
Not sure what book you are talking about but.. You could have a giant envelope, while staying streamlined, and modules hung underneath with electri-prop engines. Considering the amount of available space you would have on top of your airship, you could have a huge array of solar cells to help power the engines.

The only problem I could think of for something like this would be if a sandstorm came up.. It would be one hell of a ride. /phpBB/images/smiles/icon_smile.gif Though the winds on Mars can be much higher than those on Earth, due to the thin atmosphere they would not have as much affect as Terrestrial winds of those speeds would. -Colt

2002-Nov-08, 01:29 PM
The passengers weren't much larger than gerbils.....that is, if humans aren't much larger than gerbils. There were 5 human passengers in the passenger pod. The amount carried in the cargo pods was never mentioned, though. The point I was trying to make, though, is that there is no way a turbofan engine could operate in Mars' thin atmosphere. 1) There's not enough oxygen to support the kind of combustion needed to power a turbofan engine, and 2) the air is simply too thin to produce much thrust if the engine did work.

2002-Nov-09, 12:48 AM
It could have been a ramjet or something, I highly doubt they would send 747 jet pods to Mars. /phpBB/images/smiles/icon_smile.gif -Colt

2002-Nov-09, 10:49 AM
No, he specifically said "turbofan". it's still a decent book, though. So far, that's the only bad science I've found in it. But, then again, there could be a crapload of it that I simply don't recognize.

2002-Nov-11, 09:01 PM
> The amount carried in the cargo pods was never mentioned, though. The point I was trying to make, though, is that there is no way a turbofan engine could operate in Mars' thin atmosphere <

Shows the difference in our orientation, doesn't it? The thing that struck you was the engines, the thing that got me was the lift.

Anyway, maybe there's a handwavy way out of the engine dilema. Suppose you stuck a turbine in a rocket exhaust, and had the turbine turn a BIG fan. This is much more of a turboprop than a turbofan i suppose. I don't know if there's a name for such a beast.

I don't see any way out of the lift problem other than making the gas bag bigger or the atmosphere thicker. Maybe if this was part way through a terraforming project.

2002-Nov-12, 12:43 AM
I can see what you're getting at, daver. it sounds like you know a bit about jet engines yourself. I, personally, wouldn't want to sit under a huge bag of hydrogen with engines that would require oxygen being literally dumped into them to achieve combustion. Or with a rocket, for that matter. As far as I know, rockets are powered by using hydrogen with oxygen as an oxidizer. But, what you described is akin to a turbofan, but I really have no idea what it'd be called. I've never heard of such a beast. But, the book specifically states that Mars was in no way being terraformed at the time. I guess I can't really complain. The book was written by a biologist. I don't know much about biology, but I would assume there wasn't much bad biology in the book, considering the guy has a PhD. It really wasn't a bad book, even if you're not into government "conspiricies".

2002-Nov-13, 12:47 AM
Could you provide more information on this book? How old it is, price. I have never heard of it before this thread but it sounds interesting. /phpBB/images/smiles/icon_smile.gif -Colt

2002-Nov-13, 02:04 PM
on 2002-11-12 at 19:47 Colt wrote:
Could you provide more information on this book? How old it is, price. I have never heard of it before this thread but it sounds interesting. -Colt
Let's see...the copyright date is 2001. I'm not sure how much it would cost in a book store, considering I got the book from a club called Science Fiction Book Club. You can find their website at http://www.sfbc.com . I don't remember if you have to be a registered member to access the site or not, though. It's been a loooong while since i was there. But, they sell almost nothing but hardcover books at 50-80% off of retail price. Well worth the money if you want to build a very nice book collection.