Landing on Mars

RobS · 2012-06-21 20:55:54

I like the "log cabin" idea! But all the experts seem to think it's better to send one lander than four, probably because of the multiplier of expense, extra fuel, and more that can go wrong (nonfatally, that is). That's a lot of carbon dioxide filters to replace and other basic maintenance to do. When ISS had a crew of 3, two of them were busy with maintenance.

Impaler · 2012-06-21 23:24:54

Rune wrote:

Sorry if it sound like I'm teasing you but... have you read the link I provided? There's you rational way of getting a big surface for aero-deceleration, and it cuts the useful payload of an Ares V to 60mT apart from the massive, heat-shielded shroud, which masses 50mT in itself. As to shape, that only affects L/D, not the ballistic coefficient (well, it's not the dominant factor, to be more precise, Cd also plays a part), so it makes no difference one way or the other, and a delta wing is more wasteful of structural weight than a conic section close to a circle. Just... trust me, as long as you don't intend to fly, just decelerate in a controlled trajectory, then you are better served with more-or-less oval shapes. There's a reason they are the ones used and suggested.
As to sidemount configurations, while they may make a lot of sense for clustering first stages around second stages (like Angara, Delta IV, Falcon Heavy... they must be in fashion), they are not so nice for payloads. Too close to the business end, and it works very bad from a structural standpoint (all those torques on the primary structure take weight to handle).
Parashields /inflatable heatshields may be an ever better solution than shrouds, with regards to weight fractions. Or so their researchers claim, at any rate. I would put money in certifying both for interplanetary reentry speeds so at least one pans out and we can skip shroud limitations completely, while potentially adding reusability. I think some inflatable test has actually flown, but at much lower suborbital velocities.

Rune. Sadly, no real money goes to the useful stuff being developed... this problem first appeared decades ago, and the solutions suggested then, I'm sure.

Do you mean the "Guided Entry Performance of Low Ballistic Coefficient Vehicles at Mars"? Cause I don't see anything their about using a launch-shroud as a Heat-shield, in any case at nearly half the total LEO payload it seems very inefficient for Mass utilization. Is it heat-tiled all around or only on one side?

Your criticism of side-mounting when it comes to torque are valid but I've never seen any numbers on how much this increased the structural masses involved, obviously a newly designed heat-shield wing could be designed with adequate structure for the launch load, but the ability of the Ares-V/SLS to accommodate the side-mounting would be a more serious issue as the Ares-V/SLS might not be able to handle the torque not having been designed for that role (still you would think it's External-tank heritage would make it at least conceivable), but if that were the case we would simply default to attachment on the nose which might even be preferable. The Sierra-Nevada DreamChaser vehicle is basically a small delta wing vehicle on the nose of a EELV so a large winged vehicle on the end of a SHLV rocket seems very reasonable.

As for 'Too close to the business end' this sounds baseless, neither Shuttle nor Buran suffered from being near the rocket exhaust as their was in fact considerably above and away from it. The Challenger disaster ware solid rockets burned through the external fuel tank would have destroyed ANY payload ANY WARE on the vehicle and just demonstrates that Solids are a death-trap (which is why Ares V/SLS isn't fit to launch crew IMHO). The though Buran was immune to this scenario because it used liquid boosters.

TwinBeam: Your naive assumption that loss of mission dose not occur until the last astronaut dies is patently false, In reality a mission would be horribly compromised with the very first lose of life as the loss of available man-hours would cut into mission-objective activities (science) and reduce safety margins to the point ware nothing other then survival could be done and the mission is basically a complete failure at achieving its scientific goals.

Also the safety of a landing vehicle just dose not go up linearly as more mass is used as 90% of the things that can cause a disaster are timing, guidance, parachutes, rocket-start-up, none of which become more reliable simply because they are bigger. Each additional vehicle is a very nearly linear increase of what can fail so you end up with the 20% chance of mission failure not 0.000625%, this is just basic mission risk calculus.

The degree of callousness with regard to risking human life for some on this forum makes Zubrin look like a bleeding-heart sometimes.

Last edited by Impaler (2012-06-21 23:29:52)

louis · 2012-06-22 07:00:47

RobS wrote:

I like the "log cabin" idea! But all the experts seem to think it's better to send one lander than four, probably because of the multiplier of expense, extra fuel, and more that can go wrong (nonfatally, that is). That's a lot of carbon dioxide filters to replace and other basic maintenance to do. When ISS had a crew of 3, two of them were busy with maintenance.

But most experts call for (a) larger tonnages than are necessary and (b) ignore the ability to pre-land supplies robotically (as we land Rovers now).

The ISS is a complete hotch-potch of a thing...designed piecemeal. I never use it as much of a guide to anything. Two people, from a three man crew, got to the Moon no problem. By the end I think the mission time was something like three weeks. That was 40 years ago. I'd start there.

Rune · 2012-06-22 07:35:56

TwinBeam wrote:

Assuming a L/D of 0.2, a capsule starts falling when drag force goes below about 2g's (4m/s^2 / 0.2 = 20m/s^2) - no point using rocket before that, as you get free lift to stay in the air and get free braking.
Assuming a 30T, 8m diameter vehicle, and getting low enough that air density is around 0.01kg/m^3, at 2g's you'd still be clipping along at about 1430m/s.
If you're going to rocket decelerate at constant 2g's beyond that point, you need another 72 seconds with average rocket deceleration of 1g (+ avg 1g aerobraking) to kill horizontal velocity - about 720m/s deltaV equivalent.
Assuming an average of 2m/ss falling acceleration (4m/ss minus average 2m/ss aerodynamic lift), you'd be heading down at 144m/s if you did no vertical braking. So rocket braking at 2g (net 1.6g after gravity, 16m/ss), you'd need to slow the fall for another 9 seconds - 180m/s more. Total 900m/s effective delta-V.
If on the other hand you use the rocket purely for lift from 2g drag (20m/ss) down to 0.4g (4m/ss), you'll have shed all but 450m/s horizontal velocity, or 980m/s shed before it's cheaper to use the rocket to brake. Average aerodynamic lift of 0.2*(12m/ss avg drag accel) = 2.4m/ss average lift, so average of 1.6m/ss rocket acceleration needed to stay at a fixed altitude. With average of 12m/ss avg deceleration, that's 980/12 = 82sec at 1.6m/ss for 130m/s rocket deltaV. Then rocket brakes at 1.8g avg (2g with 0.2g avg drag) for another 23sec - 400m/s deltaV. And it's falling at 90m/s after that, so another 5.6sec at 2g (net 1.6g) to kill that, for 110m/s more deltaV. Total net of 640m/s rocket effective deltaV.
Or about 30% less rocket acceleration required.

First, to vindicate GW and me a little bit, I'll say your idea is not really at odds with our comments: you turn on the engines when you can't get enough "umph" form the air alone. Having said that, I love it when someone shows me numbers to support their idea, and the result is not what I expected. Going by gut feeling, I would have said you were just paying bigger gravity loses, but apparently I would have been wrong.

As to the scenario you have painted in particular, that is close to 600 beta, so the descent trajectory is going to look different than the ones I got my hands at. I assume you got the terminal velocity from drag and density? If so, what altitude-density is that? 'Cause you have to have enough space and G's to slow down from there, and if you don't have the room to do it in vertical, that is more gravity losses ('cause a vertical descent takes the least time, and gravity losses are a function of time, but I have a feeling you know all this already). Also, consider that if the landing craft is meant to be refueled and launched (it's a classic MAV with ISRU on the surface), asking 10G's or even more out of it when it is mostly empty of fuel is not really that much. Changing the G's changes the analysis you did back there a bit, but I don't think much.

Rune. The more I think about it, the more useless the chutes look.

GW Johnson · 2012-06-22 09:53:41

Error correction: in post 121 above, I misremembered what I read about Curiosity's landing. The not-quite terminal velocity hanging on the chute approaching 2 km altitude from above is 100 m/s not 300 m/s. I have since calculated a soundspeed using the Glenn RC Mars atmosphere model, and it's right at 243 m/s from 2 km to the surface. That's 0.41 Mach for a terminal velocity at their chute mass loading, which is pretty near all that we can do. Compare that to terminal velocities in the 20-30 mph range for cargo and round personnel chutes here on Earth (0.026 to 0.039 Mach).

Rune: I'm glad you pointed me to that Glenn RC site. I have used it to generate profiles of temperature, pressure, density, and soundspeed to high altitudes on Mars. Below roughly 28 km, it looks pretty good. Above that, in the free-molecule flow regime (extreme low densities, too much mean free path length), the temperatures look anomalously low compared to Earthly profiles, which get very hot in the ionosphere. The soundspeeds look all wrong. I don't think I'd rely on that model for calculating entry drag and lift, the Mach numbers would look all wrong. Above 115 km, it predicts temperatures below absolute zero, so I cut that off as 4 deg K min.

Twinbeam: I think what we're talking about is not as different as it sounded at first. After having pored over that atmosphere model and using it to calculate Newtonian-flow stagnation pressures at circular-orbit velocity, I'm showing around 2 mbar pressure on the heatshield with vacuum behind, at beginning of entry, in the vicinity of 122-140 km altitude. I don't trust these numbers as accurate, but they are crudely representative.

Early in entry, while density is low, drag forces (and lift) are low. This lift and drag deficit could be made up with retro thrust through (or around) the heatshield, because flying tipped for lift points the rocket a bit downward, and that's what you were talking about. The only thing to "argue" about is how far to tip the heatshield.

Getting enough lift to flatten the entry trajectory up high has real benefits. It's best to do this as far from the surface as possible, for flight safety purposes.

Later, in mid entry trajectory, there's enough density and speed to fly fairly flat with little thrust at all.

Finally, late in the entry trajectory as Mach decreases under about 3 or 4, you have the density, but not the speed squared effect to have much drag or lift. Again, flying tipped for lift, using rocket retro thrust to make up the drag and lift deficits, could have real benefits. It keeps the trajectory flat and away from the surface. You'd like this to be around 10-20 km, maybe.

For one thing, you could pop the chutes at a lower, safer Mach, and do it higher up. That way, more of the descent could take place transonically to subsonically on the chute. You could use very little retro thrust, or even none. Save fuel.

Terminal velocities at reasonable chute mass loadings seem to be around half a Mach at 0-2 km on Mars. Heavily loaded chutes would still be supersonic. So chutes alone will never land anything of significant size on Mars. That's just plain rocket braking, from about 2 km or so altitude, at relatively high thrust.

Essentially, that's what Curiosity will do, except they are not making up deceleration deficits during entry or chute descent. Their final rocket braking scheme is a little complicated, but makes sense for a one-way probe of large mass.

GW

RGClark · 2012-06-23 00:51:21

Remember the key problem is getting the large surface area for an aerobrake at low added mass. This should be doable by inflatable heat shields as are being investigated by NASA's Game Changing Technology office:

HIAD: Changing the Way We Explore Other Worlds.

A giant cone of inner tubes assembled sort of like a child's stacking ring toy may some day help cargo, or even people, land on another planet, return to Earth or any destination with an atmosphere.
NASA calls the inflatable spacecraft technology Hypersonic Inflatable Aerodynamic Decelerator or HIAD, for short.
The HIAD could give NASA more options for future planetary missions, because it could allow spacecraft to carry larger, heavier scientific instruments and other tools for exploration.

http://www.nasa.gov/offices/oct/game_ch … index.html

Bob Clark

RGClark · 2012-06-23 02:13:25

RGClark wrote:

GW Johnson wrote:
"Lower heating if decelerating at higher up" is really lower peak skin temperatures at lower ballistic coefficient. The total integrated BTU's (KW-hr) absorbed over the trajectory is actually higher in that case, but that's an easier problem to solve than high skin temperatures. Control of survivable gees is the real key to how this trajectory is selected.
For a winged vehicle here on Earth, a "ballistic coefficient" is better expressed as "wing loading", which is vehicle weight divided by wing planform area. To obtain the benefit of earlier deceleration higher up, this needs to look more like a small light aircraft (10-20 lb/sq.ft) than the shuttle or a jet fighter (100-200 lb/sq.ft). Peak skin temperatures are under 2000 F, which ceramics, or very, very, very heavily-cooled Inconel-X, can survive. Better odds on the ceramics.
For a space capsule here on Earth, it is vehicle weight divided by heat shield broadside area. This has typically been about 4 times the shuttle/jet fighter range (400-800 lb/sq.ft). These vehicles punch very deep into the air before they slow significantly. Peak skin temperatures (stagnation point, leading edges) is around 3500-4000 F. That's why Shuttle had ablative carbon-carbon leading edge and nose cap pieces, and why Mercury, Gemini, Apollo, and Dragon had/have ablative phenolic composite heat shields.
PICA-X on Dragon (and the other new capsules) is actually nothing but a modernized version of the 1960-ish vintage silica-phenolic, which is really nothing more than what the old capsules used in the 1960's and 1970's.
Think about it: if you can actually achieve such a low wing loading/ballistic coefficient (around 10-20 lb/sq.ft), you could survive re-entry here on Earth (from orbit) with a steel truss "airframe" and a ceramic fire-curtain cloth skin, as long as there was some sort of insulating standoff between the frame and the skin, and some sort of ventilation inside to absorb the BTU's. In other words, a ceramic fabric-skinned variant of a Piper Cub might actually be a feasible re-entry vehicle from LEO.
Anything that might work here would work on Mars. The heating there is less demanding. I see some experiments that need to be done here!!!!
GW
Thanks for that. Your suggestion of getting a lightweight structure with high wing area, or equivalent, reminded me of the Lockheed X-33:
X-33.
http://www.astronautix.com/lvs/x33.htm
It was only to weigh 63,000 lbs dry, compared to the space shuttle orbiter at ca. 200,000 lbs. Note that the problem of the composite propellant tanks not holding up when pressurized would not be a problem here since we would remove them for this purpose. In fact this would make the weight even lighter. This page gives the total propellant tank weight as 15,200 lbs:
Marshall Space Flight Center
Lockheed Martin Skunk Works
Sept. 28, 1999
X-33 Program in the Midst of Final Testing and Validation of Key Components.
http://www.xs4all.nl/~carlkop/x33.html
So conceivably the weight might be as low as 48,000 lbs, though actually the propellant tanks provided structural rigidity. So we would have to add some strengthening members if they were removed, but the extra weight would quite likely be much less than the 15,200 lbs of the tanks. The reference area for the X-33 is in the range of 1600 sq.ft. So if not too much added weight for strength was required for the tankless version, we might get a wing loading of 48,000 lbs/1600 sq.ft. = 30 lbs/sq.ft.
The volume you would get by removing the tanks on the X-33 is about 300 m^3, which is about the volume of the payload bay on the shuttle orbiter. So it could contain about the same volume as the Bigelow BA 330 inflatable habitat proposed for Mars missions or orbital or surface habitats.
...
Bob Clark

Actually it would be even better than this. The X-33 had engines to liftoff from Earth fully fueled. For this purpose, these wouldn't be needed, or it would use much smaller engines just for landing the unfueled mass on Mars. The thrust for the X-33 given on the Astronautix page was 513,000 lbs. vacuum. Reportedly the aerospike engines on the X-33 had a 40-to-1 T/W ratio. So their weight on the X-33 would be about 13,000 lbs. So the weight would be reduced to 35,000 lbs by removing them, and the wing loading to 35,000 lbs/1600 sq.ft = 22 lbs/sq.ft.
But another key factor would be the lift/drag ratio. For the X-33, the hypersonic L/D was about that of the space shuttle, in the range of 1 to 1.2. How does the L/D ratio affect the question of landing large masses on Mars?

Bob Clark

Last edited by RGClark (2012-06-23 02:15:47)

Matthew Heins · 2012-06-23 15:53:01

Hi.

I'm not a technical type, and I'm new, so I can't really comment directly on all of the engineer talk going on here.

But whenever I read or hear discussions of the technical problems -including perhaps alleged showstoppers- for a Human Mission to Mars, I ask myself this question:

"What was the technical capability of the U.S.A in 1961, and what did they never-the-less accomplish in 1969 (and earlier)?"

I don't think the gizmos are the problem in going to Mars.

The problems are rather social and organizational.

Once we have those in line, it really is just an issue of money and time, money and time -and genius techies like some of y'all here (but those are all over the place nowadays).

And lastly if anyone is curious, I am a recent reader of Zubrin's Case for Mars (etc.), a recent joiner of the Mars Society, and I am all for Mars Direct straight-up, or Semi-, or any further modification of these. I am okay with Dragon Direct, but I would really like to see a return to a true heavy booster and the big missions it would allow. I would say that these are -or will be when they find out about any of this- the typical opinions of the average person who will really make a MarsShot fly -the folks that pay for it.

"Back to where we turned wrong, then upwards!" is always a bigger seller than "Beyond the cutting edge technology" and "We can get by with less".

The only thing I would really change about Mars Direct or the Design Reference Mission is that (at latest by Mission 2 or 3) I would "double down" on the whole thing and send a "Construction Crew" back to one of the previous landing sites, along with the "Exploration Crew" of the standard Zubrin/NASA missions.

I would say 1.5 - 3 years of surface time with decent capability is enough time to find Base-Building necessities near enough to the exploration landing sites to get on to building a more permanent and flexible set-up than the "habs" -especially since the initial landing sites should have been very well-selected using orbiter and robot lander data.

Anyway, hope to have many interesting talks in the future here (but also hope that the Society gets a forum going itself).

SpaceNut · 2012-06-23 16:33:38

RobS wrote:

I like the "log cabin" idea! But all the experts seem to think it's better to send one lander than four, probably because of the multiplier of expense, extra fuel, and more that can go wrong (nonfatally, that is). That's a lot of carbon dioxide filters to replace and other basic maintenance to do. When ISS had a crew of 3, two of them were busy with maintenance.

Since we know that we are not able to deliver the large tonnage to the surface that is needed so why not make it smaller packaged so that we can do what we will need.

louis · 2012-06-23 16:50:28

SpaceNut wrote:

RobS wrote:
I like the "log cabin" idea! But all the experts seem to think it's better to send one lander than four, probably because of the multiplier of expense, extra fuel, and more that can go wrong (nonfatally, that is). That's a lot of carbon dioxide filters to replace and other basic maintenance to do. When ISS had a crew of 3, two of them were busy with maintenance.
Since we know that we are not able to deliver the large tonnage to the surface that is needed so why not make it smaller packaged so that we can do what we will need.

Sometimes the simplest is the truest.

I don't know why people persist in planning to land large tonnages in one go , when you can get the tonnage you need to the surface in multiple missions.

RGClark · 2012-06-24 02:22:42

Matthew Heins wrote:

Hi.
And lastly if anyone is curious, I am a recent reader of Zubrin's Case for Mars (etc.), a recent joiner of the Mars Society, and I am all for Mars Direct straight-up, or Semi-, or any further modification of these. I am okay with Dragon Direct, but I would really like to see a return to a true heavy booster and the big missions it would allow. I would say that these are -or will be when they find out about any of this- the typical opinions of the average person who will really make a MarsShot fly -the folks that pay for it.
"Back to where we turned wrong, then upwards!" is always a bigger seller than "Beyond the cutting edge technology" and "We can get by with less".
The only thing I would really change about Mars Direct or the Design Reference Mission is that (at latest by Mission 2 or 3) I would "double down" on the whole thing and send a "Construction Crew" back to one of the previous landing sites, along with the "Exploration Crew" of the standard Zubrin/NASA missions.
I would say 1.5 - 3 years of surface time with decent capability is enough time to find Base-Building necessities near enough to the exploration landing sites to get on to building a more permanent and flexible set-up than the "habs" -especially since the initial landing sites should have been very well-selected using orbiter and robot lander data.
Anyway, hope to have many interesting talks in the future here (but also hope that the Society gets a forum going itself).

Welcome to the forum.

Bob Clark

RGClark · 2012-06-24 02:40:38

SpaceNut wrote:

Since we know that we are not able to deliver the large tonnage to the surface that is needed so why not make it smaller packaged so that we can do what we will need.

Actually, we don't "know" that. To be more precise, we don't know how to do it now, but that is not the same as saying we know we can't do it.
The research on this is ongoing and I think multiple solutions will be available near term.
However, I'm not opposed to multiple small deliveries to the surface. One question that needs to be solved is landing them nearby each other, minimizing the landing ellipse in the parlance of the current Mars robot landers. Actually, a proposed solution to the question of landing large payloads has an influence on this question as well.
That's using a high lift/drag ratio lander. One of the selling points of the space shuttle was its high cross-range, enabling more precise landings. A high L/D ratio Mars lander would reduce the need for a large area aerobrake while at the same time enabling more precise landings.

Bob Clark

Impaler · 2012-06-24 04:27:40

The current landing mass limit is only a single ton, no amount of 'assembly' of bits that small is going to make a viable human habitat. I'd estimate 10 mt usable payload as an absolute lower limit for assembly to even work and it would be nightmarish difficult. Even the largest payload fairing of a SHLV (such as SLS) isn't going to increase that very much cause the heat-shield area is the limiting factor. If this new EDL tech that we must create can break this existing limit then we would want to go right up to the limit of this new tech to minimize assembly for all the reasons Clark mentioned and more. Maybe that new EDL tech maxes out at 100 mt, maybe at 10 mt we can't predict until it's developed and has been tested a few times. If its high enough then the launch vehicle becomes the limiting factor and their would actually be a need for something like SLS.

Matthew Heins: Welcome to the forums and for making me the old-new-guy. I'd advise you to take Zubrin with a grain of salt, a lot of his engineering doesn't provide anywhere near enough safety margin and he's unjustifiably contemptuous of any new technology because he believes everything can be done 'right now' with 'more money' and 'more stomach for risk'. This whole thread exists to address the fact he just hand-waved away the limits in EDL tech.

SpaceNut · 2012-06-24 08:23:15

True RGClark "we don't know how to do it now, but that is not the same as saying we know we can't do it" with regards to the large payloads being landed on the surface of Mars but if we keep waiting for that break edge technology we are not ever going.... its not like you do not have time to build from what you land on mars for a safe return home....
The shuttles d/l was with a bit of a problem for mars as there as no runways, the down mass of the 100 ton orbiter was about 15t give of take.( exact numbers are in one of the old threads)... so that is a problem....

welcome to Newmars Matthew Heins ....

Rune · 2012-06-24 09:35:08

Re: RGClark:
Exactly the inflatable test I was thinking about. And yeah, that seems to be the way to go for manned payload: big non-rigid heatshield to get beta under control, and terminal rocketry. Precision would also be greatly increased, since both methods provide active steering. The big source of error on reentries is the time falling on chutes, when you are at the mercy of wind.

As to vehicles that are designed for lifting off again from the surface... they pretty much have all they need already, right? Including low densities, since they are mostly tank for the ride back up. They could brake harder and longer than required for landing, just because they have to lift off afterwards. Well, unless they have to land the ascent fuel, then things change, but I don't think that is the smarter way to go about it.

In summary... I would correct you a bit: we have a pretty good idea as to how to go about it. Only no one has put real money to prove or disprove the current ideas.

Re: Matthew Heins:

Welcome! You have the right attitude

Re: Louis & Space nut

As to "go with what we have now"... like Impaler says, what we have now is approximately capable of landing one a half tonnes (MSL). Building the ISS for 25mT and 5mT chunks took a crew of three the better part of 10 years, with long stretches of time unmanned at first, and very frequent resupplies. Not to mention a shitload of EVA's and constant telerobotic operation of most of the station systems form the ground. Just to put things in perspective, there are just not enough man-hours in a whole martian campaign with several missions to set up all the stuff the first mission will require to be operational as soon as they land or shortly afterwards. That is why mission planners want to land one-piece independent habitats that can work as a standalone through the entire mission and do so from the minute it touches down, and that is why that mass kind of drives the landing system requirements. Oh, and most landers/ascent vehicles capable to handling the crew are approximately the same weight, at least within the same order of magnitude, so the EDL requirements are quite similar too. The ballpark is about 50mT at entry, it may be half if you are an optimist, or twice that if you are a pessimist, but that is the approximate order of magnitude of all I've seen. It is also way beyond the capabilities we have "right now". But it wouldn't be too hard to improve those capabilities, if a serious effort was initiated.

Rune. MSL essentially copies technology derived form Apollo after all (Viking), with a fancy LIDAR on top and no landing struts to call it new.

GW Johnson · 2012-06-24 10:27:09

Rune:

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 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

All the guys:

This kind of model is a prerequisite to figuring out a landing scheme for landing large masses on Mars, either one-way or two-way. The bulk of the hypersonic deceleration should be taking place close to (or perhaps at, or even under) the 28 km I identified, and the chute supersonics and subsonics should be well below this altitude. At least it's a start.

I can rough out vehicles using pencil and paper from data like this, but it's just crude, order-of-magnitude stuff. Anything trustable in detail would require a trajectory code, which I don't have. Here's a first cut atmosphere model for those of you who might have such codes.

I'm going to dig out my pencil and solve this damned lander problem once and for all. One-way chemical, two-way chemical, and two-way nuclear thermal. This is going to take a while. Expect nothing quickly.

GW

TwinBeam · 2012-06-24 22:00:45

Rune wrote:

Re: RGClark:
Building the ISS for 25mT and 5mT chunks took a crew of three the better part of 10 years, with long stretches of time unmanned at first, and very frequent resupplies. Not to mention a shitload of EVA's and constant telerobotic operation of most of the station systems form the ground. Just to put things in perspective, there are just not enough man-hours in a whole martian campaign with several missions to set up all the stuff the first mission will require to be operational as soon as they land or shortly afterwards. That is why mission planners want to land one-piece independent habitats that can work as a standalone through the entire mission and do so from the minute it touches down, and that is why that mass kind of drives the landing system requirements.

About 160 components were added to ISS in an equal number of EVAs - 1000 person-hours of EVA activity. So about 6 person-hours per component, in weightlessness, where things take 2x to 3x longer than they did in training on Earth. Those modules would each be about the same mass as one Mars hab. But let's assume that number of components is the key variable, keep the 6 hours per component figure, and assume about 25 one-ton modules landed - 15T of structure, 10T of equipment to install. Assembly time about 150 person-hours with the use of a small crane. Assuming teams of 2 in suits and tele-robotic support by 2 more crew operating from orbit or inside a lander, about 75 work hours - perhaps three weeks. Not too bad if you're going to be there for a year. The crew would probably need an inflatable "construction shack" to rest in.

Also the hab would likely be air-tight, and filled with air after the first 2 weeks, and just need "shirt-sleeve" indoors finishing work for the last week, while tele-robotic earth movers finish burying the hab for shielding.

If the "log cabin" or some similar in situ resource usage scheme were used, the site might be cleared and all the "logs" filled tele-robotically before landing, potentially shaving the out-door work phase to 1 week or at least avoiding making it take longer on the surface, while probably shaving the landed mass to under 20T including the additional construction equipment required (e.g. a "log filler/packer").

Impaler · 2012-06-25 05:36:46

GW: How much dose the Martian Entry velocity effect EDL, I'm wondering if it is worth-while to enter Low Martian orbit by something like SEP (not that hard to do cause their are no Van-Alan belts and the gravity well is shallow). I suspect that the slower entry velocity results in less total thermal energy imparted to the heat-shield and lower max g's but doesn't do anything for the altitude at which you drop to terminal velocity. It might be worth it but I'm doubtful, we often hear about how Orion and Dragon's heat-shields are capable of 'Lunar-return' implying that this is substantially harder then returning from LEO. If this difference is indeed significant on Earth could it also be on Mars?

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. The rate of EVA work over time sounds about right though, approximately 8 hour shifts ever other day. In the later Apollo landings they were doing 8 hour EVA each day for 3 days but that's probably unsustainable high for such physically demanding work, also their would be internal maintenance activities that the short duration expendable nature of the LEM didn't require. The Apollo 17 EVA's totaled 22 hours duration for 44 man-hours on the moon, but even this small amount of work required the Astronauts to have a 'cabin' that was 2 mt (LEM assent stage minus assent propellant). So even under conditions as spartan as the LEM your habitat is still the driver for landing mass, not to mention the fact it would be utterly suicidal to be dependent on such labor intensive, error prone construction for the critical crew surface survival phase of the mission.

Last edited by Impaler (2012-06-25 05:42:20)

GW Johnson · 2012-06-25 11:48:33

Impaler: I dunno precisely how much a different entry orbit affects the heating drag. But, unless you're coming in at around escape velocity or more, it wouldn't be a lot different than any other entry. I used the surface circular orbit velocity as "representative" in my calculations: escape/square root of 2. From almost any closed orbit at low altitude the entry interface speed would be close to this figure.

From what I read, the heat shield on the Spacex Dragon is rated for a "free return" from Mars at some 50,000 ft/sec, beyond Earth escape velocity (about 36,000 fps) by quite a margin. Just a crude guess says such a shield might survive entry from LEO (about 25,000 fps) maybe 4 times before you ablate through it. KE proportional to speed squared, heating proportional to KE.

GW

GW Johnson · 2012-06-25 11:54:24

Here's a notion for a quick habitat using something similar to a Bigelow inflatable. Have your equipment on a core structure to which the inflatable "shell" is already packaged around. This core has hard ends where the legs are. It sits on its side on those legs and inflates as an off-centered cylinder such that there is more volume above than below. The airlocks are also on the ends. You unpack your equipment inside, just mount nothing on the inflatable wall. Through something resembling an air mattress over it and inflate to not quite 7 mbar, as insulation and a meteroid shield. Throw an opaque tarp over that, and stake it all down. Voila: instant inflatable habitat, with all the gear already inside. Land it as a pallet and just set it up like a giant tent. There's probably a vertical-axis version, but stairs would be less convenient inside.

GW

louis · 2012-06-25 13:40:20

GW Johnson wrote:

Here's a notion for a quick habitat using something similar to a Bigelow inflatable. Have your equipment on a core structure to which the inflatable "shell" is already packaged around. This core has hard ends where the legs are. It sits on its side on those legs and inflates as an off-centered cylinder such that there is more volume above than below. The airlocks are also on the ends. You unpack your equipment inside, just mount nothing on the inflatable wall. Through something resembling an air mattress over it and inflate to not quite 7 mbar, as insulation and a meteroid shield. Throw an opaque tarp over that, and stake it all down. Voila: instant inflatable habitat, with all the gear already inside. Land it as a pallet and just set it up like a giant tent. There's probably a vertical-axis version, but stairs would be less convenient inside.
GW

Sounds good to me. It's remarkable that NASA has had so long to think about this sort of thing and not come up with anything specific.

My fave for creating habitat once there: dig a trench with the (1500 kg) mini digger you bring with you. Plaster the floor and sides with ISRU cement. Maybe embed air pipes for heating. Then place the steel sheets you have manufactured on Mars over the trench. Cover with a couple of metres of regolith.

Could be very good for creating farm hab space.

Not sure how we deal with the air lock problem but I like the idea of using water to create an ice barrier externally. When you want to exit you melt the ice and the water drains away. After entry you freeze the water again (partly by exposure to the Mars atmosphere).

Would require a lot of energy perhaps, but that is something we should have plenty of.

I think it's worth looking into - we need a simpler way of air locking than importing huge steel doors or attempting to manufacture them on Mars.

GW Johnson · 2012-06-25 14:03:09

ISRU cement is an unknown for a while yet. There's some ideas floating around, but cement as we know it here won't set there: too cold. "Icecrete" is one candidate, but you have to protect it from sublimation by a coating or by burial, and you have to insulate heavily to keep it from melting.

There's an underwater habitat-of-sorts idea floating around, too. In places on Mars where there is a buried glacier, you can just melt out a huge pond, and cover it with re-frozen pack ice and a regolith cover over that. If the pack ice is thick enough, the pressure in the water underneath may be high enough to support life in a wet suit and SCUBA, not a pressure suit. Takes about 6-7 meters of ice, I think.

In the pond, use lights to support photosynthesis in an aquaculture environment, and the waste heat from the lights keeps the pond from refreezing. There's no pressure dome; this could cover acres and acres, as big as desired. You import organic matter and organisms, and grow water plants and animals. Some could be fresh, others saline.

You could do this same under-ice thing in the trench you suggested, Louis. You just need a sealant of some kind to keep the liquid water from sinking into the subsurface geology.

GW

RobS · 2012-06-25 17:07:10

Mars surface habitats need not mass too much. Zubrin was figuring on a 250 kilogram inflatable attached to a Dragon (the Dragon providing the life support) to provide 90 cubic meters of space per astronaut. Michael Duke's plan for a south polar base assumed a total of 8 tonnes for a lunar hab (including life support; I don't think he assumed an inflatable because there were no inflatables tested when he wrote his paper). The Mars Direct Earth Return Vehicle massed about 5 or 6 tonnes, I think, for the hab part, including life support.

In my novel I assumed a flying saucer shaped habitat, a flat tuna can basically, inflatable, with wiring already installed, to which one added life support equipment and plastic pipes for water/sewage and plastic air vents for air circulation. I assumed they glued hard plastic sheets down to stiffen everything, moved in furniture from the landing vehicles, added flame-proof "wall paper" to customize or personalize the look of each bedroom and room, and all that took a month or so to set up. Once the thing is inflated one can work inside in a shirtsleeve environment to do all the work. The hab was a single story with a basement with a curved floor underneath, some of which could be used for storage, and a curved roof that provided an "attic" over part of the main area. Stiffening the entire structure were scores of separate inflatable ribs to hold the shape. I figured the whole thing would mass about 12 tonnes, including life support equipment. Later they added sandbags and sprayed water over the top, then covered it with white parachute material to prevent evaporation of the water. They also added metal support beams inside to support the added weight. I also assumed it came with two airlocks already installed and the inflatable was initially packed inside them. Once the structure was inflated the airlocks moved to opposite sides of the hab, 12 meters apart. To the airlocks they later "docked" pressure suit donning facilities, access to greenhouses, and industrial modules for production of plastics and PGM/nickel-iron refining (all of which were additional mass). A second hab was inflated nearby and provided redundancy and place for anyone angry to escape to. The habs were connected by pressure tunnels.

Last edited by RobS (2012-06-25 17:10:55)

louis · 2012-06-25 17:34:25

GW Johnson wrote:

ISRU cement is an unknown for a while yet. There's some ideas floating around, but cement as we know it here won't set there: too cold. "Icecrete" is one candidate, but you have to protect it from sublimation by a coating or by burial, and you have to insulate heavily to keep it from melting.
There's an underwater habitat-of-sorts idea floating around, too. In places on Mars where there is a buried glacier, you can just melt out a huge pond, and cover it with re-frozen pack ice and a regolith cover over that. If the pack ice is thick enough, the pressure in the water underneath may be high enough to support life in a wet suit and SCUBA, not a pressure suit. Takes about 6-7 meters of ice, I think.
In the pond, use lights to support photosynthesis in an aquaculture environment, and the waste heat from the lights keeps the pond from refreezing. There's no pressure dome; this could cover acres and acres, as big as desired. You import organic matter and organisms, and grow water plants and animals. Some could be fresh, others saline.
You could do this same under-ice thing in the trench you suggested, Louis. You just need a sealant of some kind to keep the liquid water from sinking into the subsurface geology.
GW

Yes, that's an interesting idea - creating large areas of aquaculture...

While we are on the subject of large spaces, you could throw in my small canyon idea - find a a narrow canyon over which you can put in a roof. Maybe bulldoze and compact material at either end. You could, very cheaply have a large living space that could be pressurised and in which you could create a kind of biosphere (though with venting and inputs).

On the cement, I was thinking of this as a smooth lining for the interior of the trench. I have always imagined you would have some sort of low pressure work tent over and keep the space at a desired temperature, so I would hope cold would not be an issue in itself.

RobS · 2012-06-25 17:47:16

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.

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