Ok people, lets stop and have a talk. Assume that this thing runs 100 people to Mars on a trip lasting 80 days. Assume each person is 200 pounds. People weighing less than that we can assume bring personal gear with them to meet that number. Most male astronauts, at least in the US, weight in the 190-230 range female astronauts will weigh about 3/4 that on average. Let's use 200 to keep what is about to be rough math simple. The weight of the people, not the food, water, spacecraft, only the people will be 20,000 pounds. That is about the weight of a current large satellite. That is 10 tons. Next item. Water. Every human on this ship will need roughly 1.5 gallons of water a day. In space, due to calcium loss in zero G, you need even more water to prevent kidney stones. But let's keep the math low-ball and simple. The water needed for 100 people on an 80 day, one-way trip to mars is 12,000 gallons. 1 Gallon of water is 8.36 pounds, meaning that the water for this crew, AND ONLY THE WATER will weigh in with a mass of 100,320 pounds. The cargo limit of the US Space Shuttle, as an aside, was 60,000 max weight to LEO. So what is the volume of this water? Roughly speaking, a spherical container of radius 16.5 feet (diamater of 33 feet or almost exactly 11 meters) will hold this water. That was a lot smaller than I was expecting. Now, we have a total mass to orbit of 120,000 pounds, or about the lifting capability of the Saturn V rocket. We have not talked tanks, air, food, the weight of the airframe itself, the weight of the fuel, engines anything. Humans need roughly 3-4 pounds of food a day, or 32,000 pounds for our crew here. Cut that in 1/2 due to dehydration etc as you won't take that with you; you will use the water. so 16,000 in food alone, not counting packaging, storage, refrigeration, cooking etc. In the end double this for the space craft itself. Now you are talking about a payload in the 250,000 pound range. That is double the to orbit mass of a Falcon 9 Heavy. And there are some design oddities here that make me raise an eyebrow. Why reusable to the surface of mars, and not something that you end up keeping in the mars environment. An Aldrin Cycler makes more sense to save every bit of mass you can. Yet... he build a space program. He built the Falcon 9. He returned a booster to land to reuse. He is supplying the ISS. Musk has a history of overpromising, so let's take a step back from the train to hypeville and get a few Falcon Heavy cargos in orbit first. The dragon to Mars mission needs to happen, a lunar landing needs to happen, then I will start to get on the hype train. But man, his art team is on-fucking-point, ain't it? Edit to fix numbers cause IRsmrt
He does talk at length about staging materials in orbit in "thousands of flights" to set up a constant resupply for both the spacecrafts and the Martian inhabitants. And, again, as he says at the very beginning, his whole intent here is to make a trip to Mars seem possible. To get people more comfortable with the idea, so that it simply becomes a series of problems to solve toward a visualized goal, rather than a long litany of problems that - once solved - could allow us to mount a trip to Mars. He's not going to do it alone. He's not going to do it without solving some very big problems. But a reusable barge with big fucking rockets on the back that goes to space, turns around, and locks itself back into position back on the pad, where it refuels, restocks, and flies again... that's a pretty fine start to solving almost all of the weight problems. And he has practical solutions to all of those issues at least in alpha testing, if not beta. Add to that the intent to refuel for the return trip using native elements on Mars, and a powered landing craft, and ... well ... it seems pretty Moon-shot (which worked) to me!
Yea. The water weight and volume shocked me as I was expecting those numbers to be much higher. If I was doing this myself, however, this is what I would do. Launch the 100 man vessel. Then launch the "refuler" and dock it. Then launch a propulsion unit to dock with the other two units, go to mars and use the unmanned portions as a space depot of sorts in martian orbit. Land the people, do you thing and with enough of the "empties, you can either refuel them or salvage them for parts. I'll go out on a limb here and assume you read Zubin's plan correct? Being able to dump 300 tons into LEO, and recycle the booster is going to create a massive demand for space based services IMO. I hope it all happens and we can cheer it along, but I am no on the Hype train... yet. Let's get a few Falcon 9 Heavy launches under our belts and get manned resupply to the ISS, maybe even a manned Lunar landing. Then I will start to get excited.
Hypothetical question (possibly also to am_Unition?): do you know of any benefits for placing similar 'refueller' at places like L4 and L5 Lagrangian points in Earth-Sun system? I know that what you wrote above is still a long way from any form of completion, but aside of sci-fi I'm rarely hearing about even as much as speculations about these locations.Launch the 100 man vessel. Then launch the "refuler" and dock it. Then launch a propulsion unit to dock with the other two units, go to mars and use the unmanned portions as a space depot of sorts in martian orbit. Land the people, do you thing and with enough of the "empties, you can either refuel them or salvage them for parts.
L4 and L5 have no benefits for an earth-Mars transfer orbit. We are talking about Hohmann transfer orbits as the most efficient fuel-wise trip to get to anywhere in the solar system.do you know of any benefits for placing similar 'refueller' at places like L4 and L5 Lagrangian points in Earth-Sun system?
Yeah, francopoli is right. You'd waste a fair bit of fuel decelerating to park at L4 or L5, and/or lengthen the total time of the trip unnecessarily. This on top of the fact that the station-keeping required to keep anything there would be fairly intensive. L1, L2, and L3 are "saddle" shaped (in gravitational potential) equilibriums, unstable in one dimension (radially), but L4 and L5 are unstable both radially and tangentially to the orbit. I can't think of anything that'd be gained by using Lagrange points to get to Mars, but it was a good exercise :).
Nailed it. The first one there is always the "crazy" one. Then, 18 months later, there are 25 companies doing the exact same thing, and you have a market. A virtual location in orbit where people can place things to go to Mars is a very tasty location to me. Everything from cubesats to Amazon Fresh shipments can sit there and be picked up by SpaceX for the continuous stream of missions going back and forth to Mars. (And the Moon.) I see some sort of a hub thingie floating in orbit. It has solar panels, power, and thousands of "mount points" where companies can attach their thingies to await pickup by SpaceX. Containers of water. Electronics. Food. Each of them just a simple box with a magnet on one side that can maneuver up to the Hub, attach, and await instructions to detach and maneuver into the waiting ship. FedEx hub in space, man. ...create a massive demand for space based services IMO...
Doesn't it make more sense to have the shipyards in space? I envision a future where we have two different types of vehicles, gravity well tugs, and interplanetary vessels. That way you're not dependent on one type of frame to do all the heavy lifting AND be efficient as an interplanetary transport.
There are some interesting numbers in the last part of the Wait But Why post on SpaceX. I do agree with goob - there are hard problems ahead, but they are not unsolvable.
Actually, that's a lot bigger than what I was expecting. After calculating it myself I got the following: t = 80 days (flight duration) n = 1.5 gallon/(person * day) (water requirement per person per day) ~= 5.7 litres / (person * day) Radius calculated from: That would place the diameter 4.4 meters or about 14'4" if I didn't just make a fool of myself by messing up the unit conversion. (EDIT: Turns out that I did, at least in a way. I wrote my radius approximation in feet and inches instead of diameter. It was originally 7'8") However, in a bit of a dick move, I have to say that a sphere of a diameter of 11 meters (just to be clear, I know that's a double of radius and used 5.5 meters for calculation) should contain roughly 700 m³ (or 700000 litres or about 185000 gallons)… so at the very worst both of us made some mistake. ;)That was a lot smaller than I was expecting.
P = 100 person (crew) V = P * t * n = 12000 gallons ~= 45600 litres = 45.6 m³ V = (4 * pi * r³) / 3 <=> r = cbrt((3 * V) / (4 * pi)) ; cbrt standing for cubic root r = cbrt((3 * 45.6) / (4 * 3.14)) = cbrt(136.8/12.56) = cbrt(10.89) ~= 2.2 meters
100 people times 2 gallons of H2O a day times 80 days is 16,000 gallons. A gallon is 8.36 pounds. Giving a total mass of 133,760 pounds, which I convert as 16,000gal to 60567 litres. This would be 60.6 cubic Meters. And fuck the Imperial System. I'm being rushed right now, but for some reason we are doing the same math and getting different numbers. Either way, this volume is way less than I expected it to be.
I went with the lazy solution and asked the Holy Wolfram Alpha to bestow the solution upon us ;). My solution is closer… for a given definition of a gallon. ;) Either way, I agree with this: It's not as unbearable amount as intuition suggest. I've posted that mainly because something felt fishy, but that's not the reason to split hairs. Amen! I'm less tense about solving and explaining some completely unintuitive special relativity problem than when I'm being asked to convert some ounces per cubic foot to metric myself.And fuck the Imperial System.
I've been enjoying this conversation a lot but waiting for someone to point out that the water can be recovered and recycled. This is already common practice, though I couldn't find a number for recovery rate on the ISS. I did see a NASA project that aims to improve the rate to 94%.
I'm assuming that they will be using water not only for keeping the people alive but also radiation shielding. And aquaculture. And fuel. etc. This is worse case scenario. Still, I anticipated the needed water being much, much more than I mathed out.
Great point. But wouldn't it pose a power concern as the distance increases? The intensity of light decreases with respect to the distance (r) as 1/r². Let's go with the 'easy numbers' approach. Solar panel the size of 1 m² gives output of 500 W (give or take, in the end it's the ratio of solar panel area that I'm concerned about), Earth is 1 AU from the Sun, Mars is about 1.5 AU from the Sun. Formula in plain words: Power * Area of solar panel / distance ² That would mean that the power output on Mars would be about 500 * 1 / (3/2)² ~= 220 W I know that it's not unfeasible to simply stack a lot more solar panels onto the ship. But the above relationship shows that to produce same 500 W we had at around Earth's distance, on Mars we would need 2.25 m² of solar panels. Although having said that, I am aware of making a silent assumption that all of the power would be provided via solar panels. I have no idea how viable would be some form of thermal generator that uses radioisotopes (as in most satellites)… or maybe I should stop thinking in such limited terms and look for ways to put a full nuclear reactor on the ship. :D
Solar at Mars is not that big of a deal. You need bigger panels, but with the massive increases in efficiency you get each machine generation (roughly 18 months). This great write up on the ISS wings says that the ISS uses an acre of panels to generate 90Kw of power. These panels are using cutting edge engineering from the early 2000's and are not as efficient as the new stuff that they are using on, for example, Juno. the Juno panels would generate 14Kw if at earth, say 1/2 that at Mars. The thing holding you back more than anything really is the battery. Batteries are big bulky, chemical soups that are not growing in efficiency nearly as fast as the Solar stuff. As as your spacecraft will go through eclipses in orbit around a body, you need batteries to store charge. This is one reason they don't want to use solar panels on the lunar surface; they get 14 straight days of night, so you need to double power and have a massive energy sink to store the excess.
Thanks for explanation! :D I got too focused with intensity and forgot about power storage. If you would not mind, I have another question that hit me while I was reading some more on ISS. Among them was a series of articles like Staying Cool on the ISS that talks about thermal control. Apparently ISS uses ammonia to 'vent' the excess heat by forcing it to radiate IR outside of the station. It's completely understandable to have such system, not only because of their power consumption, population and amount of active machinery. Even more importantly, ammonia is pretty much unparalleled as far as heat transfer goes (at least in the concerned range of temperature). But wouldn't ammonia cause a hazard in the long run? I could not find anything more serious than this false alarm about ammonia leak, but to my understanding the more people and space are involved the higher are the chances of something going wrong. It's harder to contain, able to linger undetected before concentration gets close to hazardous levels, and isn't all that easy to filter out from the air (although not impossible, as detailed in patent of continuous electrochemical scrubber… but that requires a similarly dangerous H3PO4 chemicals so we have a Catch 22 ;)). So, after all this long-winded rambling: are the hazards linked to ammonia simply something that astronauts must accept and deal with (and I'm getting waaaay too concerned about minutia) or does it pose some real problem to long-term manned missions?
They use anhydrous ammonia. Translation- no water in it. We used the same stuff on the processing ships I worked on. Ammonia is well understood, has been used for over 100 years, is cheap, is anti-bacterial, anti-fungal and (important for space flight) light weight. Yes, it is toxic to humans. But the good thing about ammonia is that it does not need crazy insanely tolerant seals like some other gasses do. Ammonia also works well in titanium and steel piping. Ammonia leaks can be cleaned up with normal air scrubbing and water vapor (the ammonia will react with the water and remove itself from the air). Carbon Dioxide coolants are more likely to stick around, are harder to get rid of when they leak and react with steel and rubber/silicon seals. Infographic In the radiators, the ammonia goes through a phase change from gas to liquid. This results in an energy transfer. As the radiators are exposed to very cold temperatures, the heat in the ammonia moves to the colder piping, which brings the temp below the critical temp needed to reliquify the gas again initiating the phase change. This waste heat is then radiated out into space. The ammonia itself is not vented. You can also run the ammonia lines on the outside of the astronaut's pressure vessel so that when you do get leaks they do not poison the crew cabin. The thing that makes ammonia a good coolant is that it absorbs a tremendous amount of energy, boils at -30C at sea level, does not vapor lock like water based solution are prone to do, and if you vent it in space, and it gets on a space suit, you sit in the sun and the ammonia will sublimate and the UV light will knock off the Hydrogen atoms and create N2 instead. The short answer is that there may be other solutions to gaseous evaporation cooling loops in space, but they all have bigger issues than ammonia.
Before long, Musk is going to need competing enterprises to challenge, or steal the market he currently has a big leg up on... When a ligtimate option to his POV of space exploration exists, watch the timetable shrink. When a market develops, the growth curves also pick up. This is going to happen through value investors... Space tourism. When do you all think those Rockets will be made for sale?
So, one thing I noticed during the stream is that they talk about multiple "fueling" trips per crewed vehicle, but you would think that an unmanned fuel transporter would be able to fully fuel a manned vehicle of the same basic design in one trip. That, coupled with the fact that I find it hard to believe that Musk has forgotten to account for the water requirements of an Earth-Mars trip make me think that maybe there are some pretty significant engineering details that have been considered, but where glossed over for the sake of keeping the talk accessible.
Why bother with refueling? Why not just dock a propulsion module? You save the weight of the return features, and you can leave it in martian orbit and use it to refuel there for a return. But yea, interesting plan. I need to go play Kerbal some more.