Landing on Mars

Impaler · 2012-06-26 01:14:57

Were getting off topic, lets fork off a surface habitat thread.

RobS: Pure Oxygen!!! Do you have any idea how dangerous that is?

Rxke · 2012-06-26 01:28:46

not that much at lower pressure

RobS · 2012-06-26 06:00:04

I was using 1 atmosphere and a tenth atmosphere as examples. In my novel I assumed a standard Martian pressurized space was 1/3 of an atmosphere, 1/5 atmosphere oxygen and the rest nitrogen and argon. That'd be 3.3 tonnes per square meter.

Rune · 2012-06-26 09:09:29

RobS wrote:

Where pressurizing space is concerned, keep in mind that a pressurized space is like an upside down suspension bridge. If you are pressurizing to full atmospheric pressure, the upward force is 10 tonnes per square meter. If you are using 1/10th atmosphere pure oxygen, it's 1 tonne per square meter. All that upward force has to be opposed through the edges of the dome, so you need some huge piles of regolith or, more likely, pile-driven pylons frozen into place to anchor the dome.

It's cool weight works the other direction and you can balance both, then. Imagine an arbitrarily big hole sealed with a 25mT/m2 (10/0.4=25) homogeneous cover, pressurized at 1atm and... net load on the structure is zero and you can use bubblegum to build it, no matter the size.

Rune. Totally off-topic, too, but I'll get back on it on the next post... I've got a lot of stuff to reply to.

TwinBeam · 2012-06-26 09:16:11

Impaler wrote:

TwinBeam: Are you actually proposing they build the pressure vessels of a habitat from plastic-film and support hoops? Cause that's what you would be dealing with given 1 mt of cargo at a time. The Mercury capsule was just over a mt and they said it was 'worn' not 'ridden'. Your EVA work hours are far too low to achieve a construction project of any magnitude, building a barn on the Earth takes longer then that.

Your later post is correct, this is getting off topic. But please allow me to add some data to my previous post to explain, since it is key to the question of HOW to land things, and you are still objecting to the feasibility of the "multiple smaller deliveries and assemble" approach.

I took 25mT of mass from Zubrin's hab sketch, and assumed a design based on twenty five modular, folding, 1mT shipments of maybe 250 components, well designed for ease of assembly (e.g. all connectors built in - very few separate nuts and bolts).

200 sq-m steel building construction kits with a LOT more components and manually added nuts and bolts, take about 1.2 man-hr per sq meter to build on Earth, where components weigh at least 2.5x more.

So figure about 2 man-hr per sq-m on Mars, not counting tele-operated robotic assistance that does any heavy lifting.
A hab 8m across -~50sq-m - should take around 100 man-hr, or about 9 days for a crew of two.

So, having estimated this from several angles now (kit buildings on Earth, EVA construction of ISS), 2 weeks seems like a reasonably good ball-park estimate for the structure and basic air and power supply. A week or maybe two more to finish the interior and robotically bury it for shielding.

GW Johnson · 2012-06-26 09:39:22

Low-pressure pure O2 atmospheres were what we used in Mercury, Gemini, and Apollo, plus Skylab. It worked fine; you just have to be careful to use nonflammable materials. Ditch the petroleum plastics, for example.

NASA's standard was right at 1/3 atm (253 mmHg, 338 mbar) and still is, in space suits. However, you don't need that much. Around 20-25% of an atm (152-190 mmHg, 203-253 mbar) is just fine, as Paul Webb demonstrated with his elastic spacesuit mechanical-counterpressure garment back in 1969.

They stayed on Skylab 185 days in a 1/3-atm O2 atmosphere, and it worked just fine.

GW

RobS · 2012-06-26 09:47:48

The main reason I favor 1/3 atmosphere over 1/5 atmosphere pure oxygen, is because in a settlement you have other concerns, such as cooking (air pressure lowers the boiling temperature of water and changes cooking times and techniques) and the need to give patients pure oxygen in hospitals. On Mars it may be better to use an oxygen partial pressure more like that in Tibet or the Andean plateau, which is closer to 15% of an atmosphere. The human body adjusts by creating more red blood cells. If such a person went to the hospital where the total air pressure were 1/3 of an atmosphere, an oxygen mask would provide them more than double their usual oxygen supply. If the rest were argon, which doesn't create the bends, one could use lower pressure counterpressure suits as well and have easy in and out access. One also has to consider children and agriculture; some plant species transpire more water at lower air pressures and don't adjust well to them. Presumably the settlement's atmosphere would also have several times more carbon dioxide than the Earth's atmosphere, while still being low enough for humans.

Rune · 2012-06-26 10:10:42

GW Johnson wrote:

I used the data from the Glenn Research Center links you sent me to put together a sort of model of the Mars atmosphere for entry purposes. Extending the Glenn model to 100+ km altitudes is not very good, because the lapse rate is most likely wrong way up there. But the pressure is probably ballpark correct, and the density profile I got is conservatively too dense. Below around 28 km altitudes, it's probably not too bad a model.

I've had a look, and from that inconsistency you found with temperature I assume my help wasn't that great. Try looking at the atmospheric model here:

http://www.ssdl.gatech.edu/papers/confe … 6-0076.pdf

The charts are already made, and especially the one with mach numbers looks very right to me (page 7). For more stuff on reentry (I'm documenting myself, so why shouldn't you all share the results of my google-fu):

Sizing of an Entry, Descent, and Landing System for Human Mars Exploration

Atmospheric Environments for Entry Descent and Landing (not limited to Mars)

The silly NASA charts, useful for the references, mainly.

GW Johnson wrote:

I am still defeated by the notions of posting illustrations on this forum. I know those links you gave me are in English, but I do not share a dictionary with the writers of those instructions. So, I posted the model with the curves I got over at my "exrocketman" site: http://exrocketman.blogspot.com

And I am determined to make you learn, whether you want to or not! Here is a very Inception-like example:

The url in the middle of the image is a popular image hosting site I in no way endorse, but still use 'cause it's the first I remembered off and had an account. In this case (and you can check out how I made this post by cliking the quote button, in the following screen you will see a plain text version of what I wrote between quote tags, [ quote] and [/ quote], so you can check out with a little time and the preview tool how I did each trick I used here, from named links to pictures). The main thing is, change the url in the image for any url (also called "link") that takes you to a picture, in this case: "http://img24.imageshack.us/img24/5959/thisisatest.jpg", and when written between the [img ] and [/img ] tags, the forum will display it as an image (I've added a couple spaces to foul up the tags and make them visible in this case).

...get it now? I know I explain myself like shit, but it really isn't anything complicated... and the "preview" button can catch any mistakes you made, like leaving open a tag.

Rune. This is the informational post. It required, by the way, several dozen tags opened on firefox, more than a few minutes of google, opening of paint, and waiting for the forgotten password to my image account to arrive to my mail, so I'll leave the opinions for some other time, k? I am still in exams after all...

Rune · 2012-06-26 10:18:44

RobS wrote:

If the rest were argon, which doesn't create the bends, one could use lower pressure counterpressure suits as well and have easy in and out access.

Just one quick comment more... that is an awesome idea! Not many people look into that kind of problems, and having to wait every time you do an EVA like in ISS is horribly time-consuming, and risky.

RobS · 2012-06-26 10:24:00

Well, I am not positive argon doesn't cause the bends! Nitrogen might not either if it is just a tenth of an atmosphere of pressure.

RobS · 2012-06-26 12:50:12

Actually, it sounds like argon is worse than nitrogen where the bends is concerned!

http://www.advanceddivermagazine.com/ar … elium.html

So if you are using 1/3 atmosphere, 1/5 (or 1/6) atmosphere of oxygen and the rest nitrogen/argon, you set your pressure suit on 1/3 atmosphere pure oxygen and gradually reduce the pressure. Then you won't get the bends and have no decompression time.

GW Johnson · 2012-06-27 09:50:27

I don't know about dilution gases, because there's not a lot of low total-pressure experience with them.

But, in a pure O2 atmosphere, there is a partial-pressure offset you have to take into account when deciding what level of pure O2 pressure is adequate, and for whom. The "biggie" is water vapor inside the wet lung passages: a constant 47 mmHg no matter the total pressure. The small one is CO2 in the exhalation, but it's negligible to first order.

I did these calculations a while back and posted them over at "exrocketman". Midoshi helped me figure out how to do this over a year ago, before the great crash. The stuff is already plotted. Pure O2 with the water vapor offsets included. The bar graph in the figure makes it perfectly clear how to do this.

For most of us flatlanders, 20-25% of an atmosphere of pure dry O2 supply inside the helmet is adequate. At 20%, it's no worse than flying up to 10,000 feet here without oxygen. 25% matches sea level. Somebody already accustomed to life in the open at around 15,000 feet in the mountains would do just fine on 15% of an atmosphere pure O2. Many of us could become accustomed to it, yes, but it takes time, and you have to handle the inevitable cases of "mountain sickness". They can never tolerate a p-total that low.

NASA's 1/3-atm O2 in the suits is needlessly high. But, decompressing from an oxygen-nitrogen atmosphere near 1 bar to 1/3 atm pure O2 is easier than to a lower pure-O2 P-setting. That's why they continue to require 1/3 atm or the compression equivalent. It would be just as easy to use a lower P-total a bit richer in O2, and thereby ease the decompression required going to 15-25% atm in a pure O2 suit. But they haven't done that, due to the inertia of tradition.

To find these calculations and graphs, go to http://exrocketman.blogspot.com, and use the navigation-by-date-and-title tool underneath my photo/profile. You are looking for the article dated 1-21-2011, titled "Fundamental Design Criteria for Alternative Space Suit Approaches". I was looking at mechanical counterpressure suits when I did this.

GW

Last edited by GW Johnson (2012-06-27 09:53:08)

GW Johnson · 2012-06-27 14:05:36

Hi Rune:

Two things. (1) you found better reports than I did in my simple google search. Thanks. I printed those out. I think I can revise my Mars atmosphere model to be good enough to serve, at least for rough feasibility calculations. Based on a quick read-through of your links, it appears the hypersonic deceleration at Mars is inadequate (you end up at M3 too low), and the available aero-decelerator supersonics are way short of adequate, for anything of a size and ballistic coefficient that we might really be interested in. I'm thinking retro thrust all the way down really is the answer. Low thrust hypersonics, and high thrust supersonic to touchdown. Aero-decelerators may or may not be feasible at all.

(2) this url you are talking about, could that be some sort of tag for the images I have been posting at exrocketman? If I could figure out what those are, could I use that link to make an illustration there appear here?

GW

Rune · 2012-06-28 09:44:38

GW Johnson wrote:

(2) this url you are talking about, could that be some sort of tag for the images I have been posting at exrocketman? If I could figure out what those are, could I use that link to make an illustration there appear here?

Exactly, it's the internet address for that file/page. You know, what appears in your address bar when you are looking at an image. For the images in your blog, it goes something like this:

http://1.bp.blogspot.com/-Cwfu4JCrSDA/T … rofile.bmp

... but it looks better when you put it between tags like this (check it out by quoting this post if you want to):

You can get the address of any picture on the internet, not just yours, by right-clicking on it, clicking "see image" (or whatever looks similar in your browser) and copy-pasting the address of the picture that comes up. Oh, and .bmp is a very wasteful file archive. Paint or any other image program can change them into .jpg (or save them as such from the beggining), which takes much less data for each image and looks just as good.

GW Johnson wrote:

(1) you found better reports than I did in my simple google search. Thanks. I printed those out. I think I can revise my Mars atmosphere model to be good enough to serve, at least for rough feasibility calculations. Based on a quick read-through of your links, it appears the hypersonic deceleration at Mars is inadequate (you end up at M3 too low), and the available aero-decelerator supersonics are way short of adequate, for anything of a size and ballistic coefficient that we might really be interested in. I'm thinking retro thrust all the way down really is the answer. Low thrust hypersonics, and high thrust supersonic to touchdown. Aero-decelerators may or may not be feasible at all.

Yup, you either can get your beta in the vicinity of 150-200, or you can forget about chutes, that's also my take. Inflatable heatshields might take us there, but then you can get very low betas, and then you don't need chutes. If you have to take a very big ballistic coefficient... yes, then you just have to go with rockets all the way. But of course, I want to see numbers before I say some method is the best!

So int he end the lesson I take from all of this is that supersonic retropropulsion and inflatable heatshield should both be two very high-priority technology developments. Which they aren't right now.

Rune. Methinks that, at some level, someone doesn't want to run out of excuses not to go.

TwinBeam · 2012-06-29 22:40:26

An even more extreme "assemble on Mars" approach to reduce the problems of landing stuff on Mars: bootstrapping.

Crash-land payloads of raw materials near the first site - probably designing the raw materials payloads to shatter on impact for easier handling. This requires aerobraking and some guidance, to crash near the first site, but only small rockets to control the flight path, and no parachutes and no need to land intact using powerful rockets. More useful mass deposited.

Using the same methods used for Mars rovers, deposit small robots, automated production tools, some electronics for the base life support equipment, etc. Maybe use five landers - lose any 2 and the mission can continue, though hampered.

The human crew would move to Phobos and teleoperate robots on Mars to gather the raw materials and produce bigger tools, which in turn would be used to produce most of the base (by mass). (If the equipment landing failed, the crew could briefly explore Phobos, and then return to Earth.)

After about 2 years of Mars base-building, a 2nd mission would arrive with a Mars lander or landers for the first settlement team. It would also bring more crew and equipment to expand the Phobos base down to Stickney Crater (for better radiation protection - orbital crew would switch between the two locations for average lower radiation exposure and occasional artificial gravity).

The bootstrap packages are certainly the biggest question mark - what to include for fastest and most secure bootstrap. The crash landers don't try to solve the hard problem of landing big masses on Mars efficiently - just slowing down and maybe guidance to a desired crash site. The crew lander would take a brute force approach - making it somewhat easier, though "expensive". In essence, more mission mass would be concentrated on getting humans down safely, at the expense of needing to spend longer in orbit, building up equipment on the surface.

One possible bonus of this approach - with plenty of fuel capacity, the lander could be capable of return to orbit, after re-fueling.

TwinBeam · 2012-06-29 22:42:24

An interesting article on auto-rotation as an alternative for Mars landing.
It seems to focus on fairly small payloads, however.

Void · 2012-06-30 08:59:48

I am inclined to think what you mentioned should be considered.

I have tried to work though a "Similar" set of notions.

The following is a thought exercise for the most part but illustrates some advantages of alternate schemes:

Using a "SkyCrane" type landing system, after ejection from an aeroshell, and after having been slowed down sufficiently, a "Cargo" of "Chain" could be released, to depend below the "SkyCrane". The upper set of links would be of the strongest matrials, and below that progressively weaker materials.

Strong metals, weak metals, plastics, edible materials.

Strapped on to the skycrane could be an inflatable shelter, or not, something that needs a greater degree of protection.

As the assembly was deployed, the chain would be too heavy for the "SkyCrane" to have any hope of landing it gently, rather the decent would be very rapid.

At the last moments of landing, the first part of the chain, the edible structures would hit hard, and very likely shatter to some degree, but the load would be lessened. The "SkyCrane would fire special rockets to give an immediate additional reduction in speed. The whole assembly would be traveling sideways to some degree, so that the chain would not all fall on top of the previously deposited materials. At some point just before impact of the "SkyCrane" an explosive bolt would disconect the upper end of the chain from the "SkyCrane". The "SkyCraine" would then either softland, or hardcrash, or fly back into orbit. I favor softland. It would have released the majority of it's cargo, and so the engines would be capable of halting it's inertia, and also of allowing it to hover and land.

Concerns would be "Backlash" where the chain might rebound up and hit the "SkyCrane/Lander". A vigorous sideways motion might help to protect from that.
Alternately the chain would be released substantially above the point where it touched the ground, and with a sideways motion it shoud differentiate as it impacts.
I would think the speed of impact should be slow enough that some of the chain would be intact, and useful as chain.

In some cases, starting the deployment of the deposite of chain at the top of the sloping landform and ending at the bottom of a depression such as a crater might also help, but would require almost perfect precision.

As for humans landing I wonder if in the end they might dare to be in spacesuits only with a rocket pack? If so, they would need to have a reliable rocket pack, and again extreme precision as to land where the deployed resources are, or they would almost certainly die.

Crash landing some stuff first is also an additive option.

Presuming these people landed would have tools and a shelter and an energy source, they might build what else they needed with the depoyed materials, and local materials.

I choose chain because it has some of the attributes of a solid, and some attributes of a fluid, especially when it would be vibrating.
This should allow the dissapation of impact energy over an extended period of time compaired to an impact of a big solid lump of whatever.

Last edited by Void (2012-06-30 09:42:00)

RobS · 2012-06-30 09:05:05

This is all much more complicated than a simple lander! If you are going to get stuff within 15 meters/50 feet of the surface and hover there for a few seconds, you might as well land everything safely on legs.

Void · 2012-06-30 09:12:15

Actually I only said hover to indicate that after the chain load was expelled, the engines would have enough power to hover, povided they had fuel. Of course landing the engine assembly ASAP, before the fuel ran out would be the preference.

For example if the engines had enough power to hover a one ton assembly, and the total weight with chain was 5 tons, then without releasing the chain the speed of descent would be enormous. The timing of the disconnection of the upper end of the chain would be critical. It would have to happen soon enough that the "Lander" could overcome it's inertia, and also be close enough to the ground as to not linger in hover mode. Certainly not a "Personed" process, but an ultra high speed robotic process. And the chain materials would have to be of great utility or it would not be worth it.

It is a process somewhere between crash landing and a somewhat soft landing.

Again I am not proposing it as a defninte solution, but a thought experiment I have been working on.

I am not even sure why it would be needed to save the engine section from impact except to soft land a inflatable habitate, or some other sensitive machine or if that engine section was to be turned into an assent rocket for return to orbit.

There is much to challenge in this, I simply present it as thinking, not as an effort to overturn previous work, but as in the crash landing notions presented to suppliment other various previously considered options.

I actually want to avoid hovering, unlike that skycrane which is to deploy the next rover.

Last edited by Void (2012-06-30 09:44:05)

Void · 2012-06-30 09:35:18

Asside from what I recently posted, and in line with what TwinBeam posted, I wonder if it would be possible to hard land a collection of bags or canisters of very cold frozen mixture of water and Hydrogen Peroixde, without it exploding? Say at 20K? If so then that could serve as an immediate Oxygen supply and also water. It a previously landed device can provide Oxygen and water, then not needed, but I think that getting water will be hard. Anyway, again, an option perhaps.

Last edited by Void (2012-06-30 09:45:30)

RobS · 2012-06-30 10:09:53

Considering that you have enough delta-v to get everything within 100 mph/150 kmph of landing, the remaining delta-v for a soft landing is nothing. I think the sky crane idea is to avoid trying to drive the rover off a tipped landing platform. If astronauts are receiving the stuff, that's not an issue. The sky crane also relies on a very light weight cable. No reason making a chain out of frozen tang and beef stroganoff; use a light cable and crash land the rest at 100 mph or soft land it with the engine if you can.

Last edited by RobS (2012-06-30 10:11:10)

Void · 2012-06-30 11:09:47

For your intentions, yes that is also an option.

However, I have no intention to susspend anything from the chain but chain.

The chain if it is metal or even perhaps plastic is a tool.

The chain if it is food, is a collection of food joined, so that it can be extracted from the dirt and dusted off. Perhaps it would a soluable so that it could be immersed in water to make a soup of some kind, and yet build with a bit of strength. Perhaps beef jerky and a glue of soup dried soup broth?

The chain if it is plastic can serve as a tool, but could be fed into a 3D printer to make items required and desired.

The Metal, if it were brass would be useful, a brass chain can be of use, otherwise it can be melted and processed into tools or construction materials.

Steel chain would be of some value, otherwise, perhaps it could be forged into tools, using solar concentrated focus, and blacksmith methods.

Cable is useful, but when it gets screwed up it is screwed up. Chain can be cut with a cutter, and can be joined with a new forged link, or a specially built one that will mechanically join together.

The point would be to slow it to a speed where impact would not render it useless or unrecoverable. That way less fuel used to deliver it. The rocket engines used to slow it are immediatly put to a new task as soon as the chain either is stopped from pulling the assembly down when it strikes the ground, or is simply released from the assembly to fall on it's own. The engines which previously were used to slow down the chain then are reused to soft land or hard land the engine assembly, to protect the engine assembly or a further cargo, or to just protect the cargo, such as an inflatable shelter.

If the engine assembly is protected, then while the mission is occuring refueling from Martian atmosphere could be occurring at the same time, so that it can deliver the people back up to orbit. So it gets used 3 times.

GW Johnson · 2012-06-30 19:06:01

Rune:

Your better search links paid off. I now have fairly good models for the atmospheres of Earth, Mars, and Titan, extending to altitudes over 100 km in all cases (about 900 km at Titan). These atmosphere models are now posted with plots and tables over at "exrocketman". That would be http://exrocketman.blogspot.com, in an article dated 6-30-12. There's a lot of text, 3 tables, and 18 figures over there.

These plus some data regarding entry conditions in one of those reports will enable me to run crude design sizings on a chemical one-way Mars lander, a chemical two-way Mars lander, and a nuclear two-way reusable Mars lander. I plan on doing these as "universal" designs, suitable for Mars, Titan, Mercury, our moon, and the airless moons of Jupiter and Saturn. Sort of a one-design-fits-all approach. This will take a while for me to do. Stay tuned .......

GW

RobS · 2012-06-30 22:56:25

I just found this on the Mars Society website at http://www.marssociety.org/home/press/n … arsmission.

Note in the EDL section near the end that Zubrin says the Martian atmosphere is dense enough to decelerate a Dragon to 340 meters per second. That's slow enough for deployment of rockets for the rest, though he says a parachute could slow the craft to 70 m/sec.

The Use of SpaceX Hardware to Accomplish Near-Term Human Mars Mission
posted May 16, 2011 6:50 AM by Michael Stoltz [ updated May 20, 2011 11:12 AM ]
Robert Zubrin, Pioneer Astronautics, 05.15.11

The recent announcement by the entrepreneurial Space Exploration Technologies Corp. (SpaceX) that it intends to field within two years a heavy lift rocket capable of delivering more than twice the payload of any booster now flying poses a thrilling question: Can we reach Mars in this decade?

I believe the answer is yes. In this paper, I will lay out a plan to make use of the soon-to-be-available SpaceX systems to accomplish near-term human Mars exploration with minimal technology development. First, I will layout a baseline mission architecture and plan. In the next section, I will discuss various technology alternatives available within the selected mission architecture. Then, in the following section, I will discuss alternative mission architectures. I will then conclude with some overall observations bearing on the question of sustained exploration and settlement of Mars.

It may be noted that the author is not an employee of the SpaceX company, and does not have detailed knowledge of the SpaceX systems. It will take the hard work and ingenuity of the SpaceX engineers to develop configurations and systems that can make these ideas a reality. Nevertheless, it is apparent that if an approach such as that recommended here is adopted, the requirements and capabilities numbers can be made to converge. We can reach Mars in our time.

1. Baseline Mission Plan

Here’s how it could be done. The SpaceX Falcon Heavy will have a launch capacity of 53 metric tons to low Earth orbit. This means that if a conventional hydrogen-oxygen chemical rocket upper stage were added, it could have the capability of sending about 17.5 tons on a trajectory to Mars, placing 14 tons in Mars orbit, or landing 11 tons on the Martian surface. The same company has also developed and is in the process of demonstrating a crew capsule, known as the Dragon, which has a mass of about 8 tons. While its current intended mission is to ferry up to 7 astronauts to the International Space Station, the Dragon’s heat shield system is overdesigned, and is capable of withstanding reentry not just from Earth orbit, but from interplanetary trajectories. It’s rather small for an interplanetary spaceship, but it is designed for multiyear life, and if we cut its crew from 7 to 2, it should be spacious enough for a pair of astronauts who have the right stuff.
Using these basic tools, a Mars mission could be done utilizing three Falcon Heavy launches. One would deliver to Mars orbit an unmanned Dragon capsule with a kerosene/oxygen chemical rocket stage of sufficient power to drive it back to Earth. This is the Earth return vehicle (ERV).

A second launch will deliver to the Martian surface an 11 ton payload consisting of a Mars Ascent Vehicle (MAV) employing a single methane/oxygen rocket propulsion stage, a small automated chemical reactor system, 3 tons of surface exploration gear, and a 10 kilowatt power supply, which could be either nuclear or solar. The MAV would land with its propellant tanks filled with 2.6 tons of methane, but without the 9 tons of liquid oxygen required to burn it. This oxygen could be made over a 500 day period by using the chemical reactor to break down the carbon dioxide that composes 95 percent of the Martian atmosphere. Since the reactor and the power system together only weigh about 2 tons, using such technology to generate the required oxygen in-situ rather than transporting it saves a great deal of mass, and offers the further benefit of providing copious power and unlimited oxygen to the crew once they arrive. Combined, the 11.6 tons of methane/oxygen propellant is sufficient to deliver a 2 ton crew cabin (equal in dry mass to the lunar ascent vehicle used during the Apollo missions) from the Martian surface to high Mars orbit where it can rendezvous with the ERV.

Once these elements are in place, the third launch would occur, which would send a Dragon capsule with a crew of two astronauts on a direct trajectory to Mars. The capsule would carry 2500 kilograms of consumables, sufficient, if water and oxygen recycling systems are employed, to support the two-person crew for up to three years. Given the available payload capacity, a light ground vehicle and several hundred kilograms of science instruments could be taken along as well.

The crew would take six months to reach Mars, after which they would land their Dragon capsule near the MAV. They would then spend the next year and a half exploring Mars. Using their ground vehicle for mobility and the Dragon as their home and laboratory, they could search the Martian surface for fossil evidence of past life that may have existed in the past when the Red Planet featured standing bodies of liquid water. Going further, they could set up drilling rigs to bring up samples of subsurface water within which native microbial life may yet persist to this day. If they find either, they will prove that life is not unique to the Earth, but is a general phenomenon in the universe, thereby answering a question that thinking men and women have wondered upon for millennia.

At the end of their 18-month surface stay, the crew would transfer to the MAV, take off, and rendezvous with the ERV. This craft would then take them on a six-month flight back to Earth, whereupon it would enter the atmosphere and splash down to an ocean landing.

2. Technical Alternatives within the Mission Architecture

a. MAV and associated systems

In the plan described above, methane/oxygen is proposed as the propulsion system for the MAV, with all the methane brought from Earth, and all the oxygen made on Mars from the atmosphere. This method was selected over any involving hydrogen (either as feedstock for propellant manufacture or as propellant itself) as it eliminates the need to transport cryogenic hydrogen from Earth or store it on the Martian surface, or the need to mine Martian soil for water. If terrestrial hydrogen can be transported to make the methane, about 1.9 tons of landed mass could be saved. Transporting methane was chosen over a system using kerosene/oxygen for Mars ascent, with kerosene coming from Earth and oxygen from Mars because methane offers higher performance (Isp 375 s vs. Isp 350 s) than kerosene, and its selection makes the system more evolvable, as once Martian water does become available, methane can be readily manufactured on Mars, saving 2.6 tons of landed mass per mission compared to transporting methane, or about 3 tons per mission compared to transporting kerosene. That said, the choice of using kersosene/oxygen for Mars ascent instead of methane oxygen is feasible within the limits of the mass delivery capabilities of the systems under discussion. It thus represents a viable alternative option, reducing development costs, albeit with reduced payload capability and evolvability.

b. ERV and associated systems.

A kerosene/oxygen system is suggested for Trans-Earth injection. A methane/oxygen system would offer increased capability if it were available. The performance improvement is modest, however, as the required delta-V for TEI from a highly elliptical orbit around Mars is only 1.5 km/s. Hydrogen/oxygen is rejected for TEI in order to avoid the need for long duration storage of hydrogen. The 14 ton Mars orbital insertion mass estimate is based on the assumption of the use of an auxiliary aerobrake with a mass of 2 tons to accomplish the bulk of braking Delta-V. If the system can be configured so that that Dragon’s own aerobrake can play a role in this maneuver, this delivered mass could be increased. If it is decided that the ~1 km/s Delta-V required for minimal Mars orbit capture needs to be done via rocket propulsion, this mass could be reduced to as little as 12 tons (assuming kerosene/oxygen propulsion). This would still be enough to enable the mission. The orbit employed by the ERV is a loosely bound 250 km by 1 sol orbit. This minimizes the Delta-V for orbital capture and departure, while maintaining the ERV in a synchronous relationship to the landing site. Habitable volume on the ERV can be greatly expanded by using an auxiliary inflatable cabin, as discussed in the Appendix.

c. The hab craft.

The Dragon is chosen for the primary hab and ERV vehicle because it is available. It is not ideal. Habitation space of the Dragon alone after landing appears to be about 80 square feet, somewhat smaller than the 100 square feet of a small standard Tokyo apartment. Additional habitation space and substantial mission logistics backup could be provided by landing an additional Dragon at the landing site in advance, loaded with extra supplies and equipment. Solar flare protection can be provided on the way out by proper placement of provisions, or by the use of a personal water-filled solar flare protection “sleeping bag.” For concepts for using inflatables to greatly expand living space during flight and/or after landing, see note in Appendix.

3. Alternatives to the Selected Mission Architecture

a. Direct Return.

In an ideal world, direct return from the Martian surface using in-situ produced propellants is the way to go. This, of course, is the basis of the Mars Direct plan, which other things being equal, would be my preference. However, under the assumption that this is a near-term mission using soon-to-be-available systems with minimal technology development, that is not feasible. For example, direct return of a Dragon capsule from the Martian surface in one stage using hydrogen/oxygen propellant produced from Martian water would require about 50 metric tons of propellant. This would require 50 kilowatts of 24-hour power to produce, which, assuming a nuclear reactor is not available, means a solar array of about 5000 square meters. Such an array would likely weigh at least 10 tons, thereby blowing the mission mass budget, and be difficult to deploy by automated systems as well. In addition, assuming a water concentration of 4% by weight in the soil, obtaining 50 tons of Martian water would require mining 1200 tons of soil, which is a non-starter. Using Martian water in combination with atmospheric CO2 to produce methane/oxygen instead of hydrogen/oxygen would cut the power requirement by about 40% and the mining requirement by 60%, but the plan still remains unfeasible within the limits of the available systems. Thus the use of a lightweight LEM-type vehicle to perform Mars ascent and rendezvous with a Dragon placed in a highly elliptical Mars orbit is necessary if the mission mass requirements and delivery capabilities are to converge.

b. Double rendezvous

An alternative to the plan described here might be to fly the crew to Mars in the same Dragon used for the ERV (i.e., a “mothership”), and fly another Dragon to the Martian surface to provide a surface hab. The crew would then rendezvous with the MAV, and take it down to land near the surface hab, which they would live in for 1.5 years, after which they would ride the MAV back up to the ERV. This architecture is feasible in principle, but inferior to the one selected because it requires two orbital rendezvous per mission instead of one, does not allow the ascent propellant to be made in advance of the launch of the crew, and lands the crew separate from substantial living quarters or extended life support capability, without any countervailing advantages.

4. General Observations

The proper goal of a human Mars mission program should be sustained exploration followed by settlement. This can only be done if costs are kept low. This plan creates sufficiently low cost mission architecture to enable sustained exploration. Falcon Heavy launches are priced at about $100 million each, and Dragons are presumably even cheaper. Adopting such an approach, we could send expeditions to Mars at half the mission cost currently required to launch a Space Shuttle flight. In addition, both Dragons employed in the mission are re-used: one remaining on site to contribute to the growing Mars base, and the other returned to Earth. It will be observed that no orbital infrastructure, advanced orbital operations, advanced propulsion, or even surface nuclear power systems (although the 10 kilowatt Topaz demonstrated by the Soviet program would fit the bill) are required to enable the mission. This, plus the fact that the mission can be done using a booster soon to be available minimizes development cost and time, and moves the potential timeframe of the mission from the indefinite future to the near-present.

For settlement, cheap one-way transportation to Mars is required. In addition, cargos larger in scale both in mass and in dimension need to be delivered. This will require development of a true heavy lift vehicle, with at least an 8 meter and preferably a 10 meter fairing, and launch capabilities of over 100 tonnes to orbit. Furthermore, if costs are to be lowered, reusability is desired. However reusability needs to be placed in perspective. The most important part of a space transportation system to make reusable is the lowest stage, since this is the most massive (therefore offering the greatest reusability savings), and adding mass to it (to make it reusable) does not cause any increase in the mass of the stages above it. On the other hand, making upper stages or interplanetary transfer systems reusable only saves a small amount of hardware, but causes the mass of the stages below them to increase. Thus reusability needs to be implemented in steps from the bottom-up, rather than from the top-down (as was unfortunately done in the Shuttle.)

Using the mission architecture described here, and the soon to be available Falcon Heavy and Dragon, the first human missions could be done and an initial outpost could be established on Mars during the present decade. With the advent of a heavy lift vehicle capable of delivering ~9 m diameter hab modules in the 30 ton class one-way to Mars, the subsidized settlement of Mars could begin, with such return flights as remain necessary continuing to be conducted by the FH/Dragon-derived systems. If the heavy-lift vehicle can evolve to reusability, starting with its lowest stages, costs of one-way transport to Mars could be lowered further, eventually reaching the point where individuals of fairly ordinary means would be able to pay their own way, freely venturing forth to start new lives on a new world.

Appendix: Notes Concerning Various Mission Issues

1. Zero Gravity Health Effects.

There is no need for zero gravity exposure. Artificial gravity can be provided to the crew by tethering the Dragon off the TMI stage, in the same way as is recommended in the baseline Mars Direct plan.

2. Radiation.

Cosmic ray radiation exposure for the crew is precisely THE SAME as that which would be received by those on any other credible Mars mission, all of which would use the six month Conjunction class trajectory to Mars, both because that is the point of diminishing returns (the "knee of the curve") where Delta-V trades off against trip time, and because it is uniquely the trajectory that provides a 2-year free return orbit after launch from Earth. Assuming the baseline mission, the total cosmic ray dose would be no greater than that already received by a half-dozen cosmonauts and astronauts who participated in long duration missions on Mir or ISS, with no radiation induced health effects having been reported. (Cosmic ray dose rates on ISS are 50% those of interplanetary space. The Earth's magnetic field does not shield effectively against cosmic rays. In fact, with a crew of six, the current planned ISS program will inflict the equivalent of 30 man-years of interplanetary travel GCR doses on its crews over the next decade. This is an order of magnitude more than that which will be received by the crew of the mission proposed here.) There are enough consumables on board to provide shielding against solar flares.

3. Aerocapture.

The preferred method of Mars capture is aerocapture, rather than direct entry. This means that the Dragon aeroshield, which has some lifting capability, may well be adequate. This is a complex problem, but a back of the envelope calculation indicates that the Dragon’s shield size is in the ballpark. Thus, consider a loaded Dragon system with an entry mass of 17000 kg, an effective shield diameter of 4 meters, a drag coefficient of 1, coming in with an entry velocity of 6 km/s at an altitude of 25 km, where the Mars atmospheric density is 1.6 gm/m3. Setting drag equal to mass times deceleration, it can be seen that that the system would decelerate at a speed of 42 m/s2, or a little over 4 gs. It could thus perform a 1 km/s deceleration in about 25 seconds, during which time it would travel about 140 km. This deceleration is sufficient to capture the spacecraft from an interplanetary trajectory into a loosely bound highly elliptical orbit around Mars. If the perigee is not raised, the craft will reenter again, and again, progressively lowering the apogee of its orbit, until either a desired apogee for orbital operations is achieved or the craft is committed to entry for purposes of landing. That said, if a larger aerobrake were desired, this could be created by adding either a flex-fabric or inflatable skirt to the Dragon core shield.

4. EDL.

Using just its aeroshield for deceleration, the Dragon would have a terminal velocity of around 340 m/s on Mars at low altitude (air density 16 gm/m3). So we could either give it a rocket Delta-V capability of 600 m/s (a 20% mass hit assuming storable or RP/O2 propulsion, Isp~330 s) to land all propulsive, or we could use a drogue to slow it down (a 20 m diameter chute would slow it to ~70 m/s) and then employ a much smaller rocket Delta-V for landing.

5. Living Volume.

The habitable volume of the Dragon capsule is admittedly lower than optimal. However it should be noted that with 5 cubic meters per crew member, it is 2.5 times higher than the 2 cubic meters per crew member possessed by Apollo crews. Alternative comparisons include 9 cubic meters per crew member on the Space Shuttle, or 8 cubic meters per crew member on a German U-Boat (Type VII, the fleet workhorse) during WWII. This would be uncomfortable, but ultimately, workable by a truly dedicated crew.

However these limits can be transcended. The Dragon has a 14 cubic meter cargo area hold below the aeroshield. Into this we could pack an inflatable hab module, in deflated form, but which if inflated, could be as much as 6 m in diameter and perhaps 8 m long, thereby providing an additional three decks, with added useful volume of 226 cubic meters and a total floor space of 85 square meters, 85% as much as that in the Mars Society's MDRS or FMARS stations, which have proved adequate in size for crews of six. After Trans-Mars injection, the Dragon would pull away from the cargo section and turn around, then return to mate its docking hatch with one in the inflatable. It would then pull the inflatable out of the cargo hold, much as the Apollo command module pulled out the LEM. The inflatable could then be inflated. The other end of the inflatable would be attached to the tether, which is connected to the TMI stage, for use in creating artificial gravity.

Upon reaching Mars, the inflatable could either be expended, along with the tether system and TMI stage, prior to aerocapture. Alternatively, and optimally, the tether and TMI stage alone would be expended, but the inflatable deflated and retained for redeployment as a ground hab after landing.

Extra space could be also be provided on the ground by using a 4th launch to pre-land another Dragon loaded with supplies, including one or more inflatable modules which could be set up by the crew after they land.

6. Overall Risk.

The mission architecture is much safer than any based on complex mega systems requiring orbital assembly, since the quality control of orbital assembly does not compare with that which can be accomplished on the ground. It would be better to have a crew of four, but if we are to do it with Falcon Heavys, a crew of two is all we can do. While such a crew size lacks a degree of redundancy otherwise desirable, it also offers the counter benefit of putting the fewest number of people at risk on the first mission. It's quite true that not flying anywhere at all would be safer, but if you want to get to Mars, you have to go to Mars.

Last edited by RobS (2012-06-30 23:00:09)

GW Johnson · 2012-07-02 11:38:35

Bob Zubrin is a really smart man, and that plan (and its alternatives list) is a good one. Some of us, myself included, differ with some of the details. It is this debate which refines the mission and vehicle designs such that the astronauts (who are really test pilots at this stage of the game) are not killed.

I have concerns about months spent in a tight volume re: sanity, and that question also relates to the centrifugal artificial gravity Zubrin calls for. His proposal is two cable-connected modules.

I don't believe the bed-rest studies insofar as what level of gee is therapeutic. I do believe that they successfully show ill health effects if you don't get up and move around, even here at one gee on Earth. So how do you get up and move around inside a capsule that small? Your spin gravity won't be effective medically if you cannot get up and move around. That's a very important thing to remember about artificial gravity.

Sanity effects: A Dragon with two in it might resemble the confinement in a typical small Tokyo apartment. But, typical Tokyo apartment dwellers spend most of their day outside that apartment. These astronauts cannot do that. Unless, as Zubrin suggests, there is an additional inflatable module docked to the capsule. Where he and I differ is that I think it needs to be larger than the minimal thing you can hide in the unpressurized cargo space of a Dragon. One or two of Bigelow's modules might serve, though.

I don't much care for cable-connected modules as the way to spin for artificial gravity. This is because you have to stop the spin, untie things, and re-dock to a hard vehicle configuration for each and every mid-course or trajectory trim maneuver (and you WILL have them). That's too much trouble to carry out, and way too many opportunities for failure. Why not assemble your vehicle as one long string of docked modules, and just spin it head-over-heels as a rigid object? No EVA required to stop the spin and maneuver, just thruster firings. Much safer, not to mention easier.

Using ISRU for the return fuel means you need a system proven to work in-situ on Mars. It will need to be checked out on one of the unmanned probes before you send the men. But if you rely on small two-or-three docked-item assemblies from Falcon-Heavy launches, then ISRU return fuel is required. I see no proposals to get that checkout job done.

If it's more than two docked items launched by the same rocket, you'll need to do the docking in LEO before you depart, for better control and safety reasons. The volume and spin gravity issues may end up requiring orbital assembly instead of direct flight, precisely because 3-5 items may be required, even for a 2 or 3 man mission.

Zubrin is just about right regarding radiation. I pretty much agree. I'd prefer 20 cm of water to protect against X-class solar flare events, which is a bit more than he suggests.

Zubrin is dead-nuts-on right about making the attempt: you can't go to Mars unless you actually go. Risk is inevitably involved. But, remember, there's nothing more expensive than a dead crew. Those two things have to be balanced, quite carefully. Bad mission design and vehicle engineering has killed people for centuries. Space is no different, and a lot harsher and more hostile to life.

Let's just say I view most of the direct-flight scenarios with a jaundiced eye, for exactly the reasons listed above. Plus, the more minimalist you make your mission, the more likely it will fail. That's the history.

GW

Last edited by GW Johnson (2012-07-02 11:43:38)

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