tag:blogger.com,1999:blog-3784553273521533552.post6606065876353891782..comments2023-05-05T01:47:38.139+10:00Comments on Casey ex Australia: The Martian: A Technical CommentaryCasey Handmerhttp://www.blogger.com/profile/10894614796135316267noreply@blogger.comBlogger12125tag:blogger.com,1999:blog-3784553273521533552.post-12165304712211789492017-04-05T13:28:58.092+10:002017-04-05T13:28:58.092+10:00Well, the vapor pressure of propane at room temper...Well, the vapor pressure of propane at room temperature is about 8 bar. Butane is even lower - in fact butane stoves stop working at temperatures that some people still find suitable for camping! So those gases are stored as liquids under modest pressure.<br /><br />Oxygen's critical point is at -119 C and 50 bar. I guarantee it's not stored as a liquid in space suits. So if LOX is put into the space suit tanks at all, it would have to be in carefully metered amounts to avoid overpressurizing when it boils. But I strongly suspect it's GOX all the way.Chris Phoenixhttps://www.blogger.com/profile/13368959552769808835noreply@blogger.comtag:blogger.com,1999:blog-3784553273521533552.post-66508855446115225372017-04-05T09:33:31.299+10:002017-04-05T09:33:31.299+10:00I think it would have to be filling in the liquid ...I think it would have to be filling in the liquid phase. That's how eg butane torches, LPG (BBQ) gas bottles, etc are refilled. Lots of steam!Casey Handmerhttps://www.blogger.com/profile/10894614796135316267noreply@blogger.comtag:blogger.com,1999:blog-3784553273521533552.post-65709756995401188262017-04-05T04:38:53.174+10:002017-04-05T04:38:53.174+10:00Glad to contribute! I just thought of another fun ...Glad to contribute! I just thought of another fun point: some things are GOX compatible but not LOX compatible. I think we lost a couple of X planes because someone specified a treated leather gasket in a LOX line.Chris Phoenixhttps://www.blogger.com/profile/13368959552769808835noreply@blogger.comtag:blogger.com,1999:blog-3784553273521533552.post-58426169140961697422017-04-05T03:19:21.041+10:002017-04-05T03:19:21.041+10:00Great point, thanks!Great point, thanks!Casey Handmerhttps://www.blogger.com/profile/10894614796135316267noreply@blogger.comtag:blogger.com,1999:blog-3784553273521533552.post-54855267904402197562017-04-05T02:30:03.202+10:002017-04-05T02:30:03.202+10:00On fast air refill for the suit: Surely the suit t...On fast air refill for the suit: Surely the suit tanks would contain pressurized GOX, not LOX. Filling a compressed-gas tank fast is a good way to get on the wrong side of PV=nRT and melt/burn (or at least weaken) a hole through the tank. <br /><br />Two examples: SCUBA tanks take a long time (hours?) to fill, _and_ they're water-immersed. And Armadillo Aerospace lost a helium tank that way (despite helium's good thermal properties).<br /><br />Filling a GOX tank from a LOX tank could go much faster, but you'd have to be very careful not to put in too much LOX. And until it warmed up, you'd have very cold gas and maybe even liquid getting into systems designed for relatively warm gas.Chris Phoenixhttps://www.blogger.com/profile/13368959552769808835noreply@blogger.comtag:blogger.com,1999:blog-3784553273521533552.post-3569513511336654912015-11-16T07:51:28.756+11:002015-11-16T07:51:28.756+11:00You're exactly right. A full blog dealing with...You're exactly right. A full blog dealing with the movie would be longer than this, but fortunately they got most of the macro-detail right. <br />In this case, most of the on-screen gauges and numbers were about right (Hab pressure of 12.1 psi is fine), but the oxygen at 5% makes no sense. As soon as the pp02 falls below about 150mBar you fall asleep real fast.Casey Handmerhttps://www.blogger.com/profile/10894614796135316267noreply@blogger.comtag:blogger.com,1999:blog-3784553273521533552.post-20237737787143750432015-11-16T07:47:17.145+11:002015-11-16T07:47:17.145+11:00Thanks! This is a great article!
In the film, the...Thanks! This is a great article!<br /><br />In the film, the "percent oxygen" is shown at various times. Once (if I remember correctly), the percent oxygen in Watney's suit drops to 5%. Mars' atmospheric pressure is about 150x smaller than Earth's (and, presumably, Watney's suit's) pressure, so when his suit is breached, all the gases in his suit should dissipate at the same rate. This would mean that his oxygen percent would remain at 20-21%, at least until the suit gets down to Mars pressure and gas diffusion through the breached suit begins to occur. Unless I'm missing something, shouldn't his oxygen *pressure* (or perhaps partial pressure) have been the quantity falling, not the "percent?"<br />Anonymoushttps://www.blogger.com/profile/06191180873703626259noreply@blogger.comtag:blogger.com,1999:blog-3784553273521533552.post-62009114806463137092015-10-27T04:42:57.045+11:002015-10-27T04:42:57.045+11:00Thanks for your input!
The Apollo LM had much more...Thanks for your input!<br />The Apollo LM had much more delta-V than the MAV would need, because EDL on Mars can employ aero-braking. <br /><br />I didn't talk about the movie MAV design, which makes no sense on several levels, but ... it's just a movie.<br /><br />I think an undershot puts you into a lower orbit on Mars, which has a higher velocity, but I haven't done the math. Casey Handmerhttps://www.blogger.com/profile/10894614796135316267noreply@blogger.comtag:blogger.com,1999:blog-3784553273521533552.post-3104485901958816152015-10-26T08:14:30.323+11:002015-10-26T08:14:30.323+11:00Nice work! Some comments of my own.
• In Chapter ...Nice work! Some comments of my own.<br /><br />• <i>In Chapter 7, on Sol 63, Watney says that he has "over 100 square meters of the most expensive solar paneling ever made" with "an astounding 10.2 percent efficiency".</i><br />Back in Ch.2 he said, "Two hundred square meters of solar cells, ..."<br />Not strictly inconsistent -- 200 <i>is</i> "over 100" -- but still....<br /><br />But 100 m2 of growing plants seems problematic. Insolation on Mars is half that on Earth, even if the panels are 40% efficient, then there's the efficiency of his lighting inside the Hab, and how many light fixtures does he have anyway?<br /><br />But for traveling, he's limited by the rovers battery capacity. More efficient panels would mean he needs fewer of them. Well, and he could eliminate 'air days' entirely.<br /><br />• <i>In Chapter 26, Johanssen states that the Ares 4 MAV mooring clamps are released about 5 seconds before launch. US rocket designs tend to clamp the rocket to the launch pad until full launch thrust is achieved, ... Nevertheless, to what is the MAV clamped? There is no launch gantry or pad here. The descent stage is all parachutes and empty fuel tanks at this point.</i><br />In Weir's design, the landing stage seems to be fairly substantial. "Though the bottom half (the landing stage) is still here. No reason to take that back up when weight is the enemy. It includes the landing gear, the fuel plant, and anything else NASA figured it wouldn’t need for the trip back up to orbit." <br /><br />I had pictured is as something like the Apollo LM, unlike the movie version. I guess the clamps are there to keep the ascent and descent sections together during the flight from Earth and landing, and then during exposure to storms for several years waiting for the expected crew. Anyway, the clamps don't hold the MAV down against the engines' full thrust; it sits there for several seconds after the clamps are released before "The MAV launched with incredible force."<br /><br />• <i>Weir states that it has two stages, though one stage is perfectly adequate for the relatively low delta-V required to reach Low Mars Orbit (4.1km/s). </i><br />And the movie takes it further, with a detachable capsule in top of the second stage.<br /><br />• <i>On Sol 505, Bruce says to Venkat that Hermes is flying by Mars at 5.8km/s. Mars escape velocity is only 5.5km/s </i><br />Mars's escape velocity is only 5.0 km/s. I wonder if the 5.8 figure is the delta-v needed, and like the 4.1 km/s needed to reach low orbit, allows for gravity losses, atmospheric drag, whatever.<br /><br />• <i>During the intercept procedure, Ares 3 crew have to think fast to find additional sources of delta-V to move the Hermes close enough to catch Watney as he flies by. The distances and velocities mentioned during this passage in Chapter 26 are correctly calculated and almost entirely realistic. </i><br />I have some troubles with the intercept. Since the MAV undershot the rendezvous, shouldn't it be behind <i>Hermes</i> and falling further behind, rather than being overtaken? <br /><br />Also, with only an 11 m/s difference in velocity, it seems like a 31 m/s delta-v budget from the attitude thrusters would be enough without the extra boost from the 'Iron Man' maneuver; it'd just take longer.<br /><br />NASA's look at the Purnell maneuver: https://www.insidescience.org/content/inside-spaceflight-martian/3251<br />And Andy Weir's original: http://galactanet.com/martian/hermes.mp4Billhttps://www.blogger.com/profile/08749459207189576328noreply@blogger.comtag:blogger.com,1999:blog-3784553273521533552.post-22720385508331634482015-10-20T03:02:44.282+11:002015-10-20T03:02:44.282+11:00I think compressed air would be a great way to dus...I think compressed air would be a great way to dust cells, it's probably much less scratchy than a broom, and easier to do in a space suit.<br /><br />Toluene - I doubt it. But definitely some way to write outside. For the film, big markers are a dramatic choice. Easier to see, more motion in their writing.Casey Handmerhttps://www.blogger.com/profile/10894614796135316267noreply@blogger.comtag:blogger.com,1999:blog-3784553273521533552.post-17262581304844184332015-10-20T02:06:08.872+11:002015-10-20T02:06:08.872+11:00"On Sol 529, Watney discusses using the inter..."On Sol 529, Watney discusses using the interchangeable systems to pump hydrogen down in the Ares 4 MAV airlock - an impressive engineering achievement given hydrogen's general misanthropy. "<br /><br />Hydrogen's general misanthropy. I love this succint wording.Random Blankhttps://www.blogger.com/profile/14018083785388257158noreply@blogger.comtag:blogger.com,1999:blog-3784553273521533552.post-26680950362563677802015-10-20T00:46:19.720+11:002015-10-20T00:46:19.720+11:00Question to the geeks to wrote this (and I love it...Question to the geeks to wrote this (and I love it) - per the movie, would Nasa really have SCUBA tanks in Mars to brush off dust from solar cells? And would Nasa have black markers on board which generally contain toluene? There is no Plan Bhttps://www.blogger.com/profile/13583282770002097268noreply@blogger.com