Landing on Mars

RobS · 2012-04-01 08:51:32

Right now, Louis, by definition we must be parsimonious. Last night I was listening to Zubrin's 68 minute talk (on Youtube) about the Mars Semi-Direct Plan he proposed. The talk was at last year's Mars Society annual conference. Someone asked him a very interesting question near the very end of the talk: you have shown we can send 2 or maybe even 3 astronauts with three Falcon Heavy launches, so why not double them up, use six, and fly a more comfortable mission? Basically, the person who asked the question was proposing what I had just posted to the forums a few hours earlier, in terms of size of mission!

Zubrin's answer was very interesting: that if you start adding more of this, someone else will say, no, we also need more of that, then someone else will say we need some of this too, then someone else will say it's getting too big; let's use a new high-technology engine, and the next thing you know the whole plan is so big and expensive it gets canceled. So, Zubrin said, I am intentionally proposing the smallest practical mission possible, because every else does the opposite.

So yes, you can add fuel to slow down the spacecraft before it reaches the Martian atmosphere. But where do you stop? Coming toward Mars from a 6-month trajectory, you approach at something like 6 kilometers per second. With hydrogen and oxygen fuel you'll need about three tonnes of fuel for every tonne of payload; with methane and oxygen, about four tonnes for every tonne of payload (including the fuel tanks and engines). Why would you do that, when a heat shield can burn off most of that speed, a parachute masses about 5 to 7% of the total payload, and the final retro rockets require about one tonne of fuel for every three or four you are landing? You're roughly doubling or tripling the size of the mission and that also increases its complexity; more launches that could fail, more orbital rendezvous that could be difficult, more tanks that could leak, more engines that could blow up.

Furthermore, if the mindset "let's add this" gets going in the planning committee, pretty soon they'll be adding heated toilet seats (the Japanese astronauts will want them; apparently they're very common in Japan), everyone will want better food, more gym equipment, a bigger surface vehicle, etc. The mission will explode in size because every basketball hoop will require three times its mass in fuel.

So I think Zubrin is right in this case: you need to aim for the practical minimum. KISS ("keep it simple, stupid"; a popular colloquial expression) is a really important principle to keep in mind when planning a mission to Mars. It's very easy for us to forget, sitting here in cyberspace spinning out mission plans in a political and engineering vacuum.

louis · 2012-04-01 17:07:15

RobS wrote:

Right now, Louis, by definition we must be parsimonious. Last night I was listening to Zubrin's 68 minute talk (on Youtube) about the Mars Semi-Direct Plan he proposed. The talk was at last year's Mars Society annual conference. Someone asked him a very interesting question near the very end of the talk: you have shown we can send 2 or maybe even 3 astronauts with three Falcon Heavy launches, so why not double them up, use six, and fly a more comfortable mission? Basically, the person who asked the question was proposing what I had just posted to the forums a few hours earlier, in terms of size of mission!
Zubrin's answer was very interesting: that if you start adding more of this, someone else will say, no, we also need more of that, then someone else will say we need some of this too, then someone else will say it's getting too big; let's use a new high-technology engine, and the next thing you know the whole plan is so big and expensive it gets canceled. So, Zubrin said, I am intentionally proposing the smallest practical mission possible, because every else does the opposite.
So yes, you can add fuel to slow down the spacecraft before it reaches the Martian atmosphere. But where do you stop? Coming toward Mars from a 6-month trajectory, you approach at something like 6 kilometers per second. With hydrogen and oxygen fuel you'll need about three tonnes of fuel for every tonne of payload; with methane and oxygen, about four tonnes for every tonne of payload (including the fuel tanks and engines). Why would you do that, when a heat shield can burn off most of that speed, a parachute masses about 5 to 7% of the total payload, and the final retro rockets require about one tonne of fuel for every three or four you are landing? You're roughly doubling or tripling the size of the mission and that also increases its complexity; more launches that could fail, more orbital rendezvous that could be difficult, more tanks that could leak, more engines that could blow up.
Furthermore, if the mindset "let's add this" gets going in the planning committee, pretty soon they'll be adding heated toilet seats (the Japanese astronauts will want them; apparently they're very common in Japan), everyone will want better food, more gym equipment, a bigger surface vehicle, etc. The mission will explode in size because every basketball hoop will require three times its mass in fuel.
So I think Zubrin is right in this case: you need to aim for the practical minimum. KISS ("keep it simple, stupid"; a popular colloquial expression) is a really important principle to keep in mind when planning a mission to Mars. It's very easy for us to forget, sitting here in cyberspace spinning out mission plans in a political and engineering vacuum.

Well, I am certainly a minimalist when it comes to missions. I have only somewhat reluctantly come round to the idea of a pressurised rover on the basis that the Musk's launch costs are going so low.
I would respond with the following points:

1. Mass costs follow launch costs in the main. You have to remember that Musk has already reduced launch costs from about $20,000 per Kg to $5000 and says he is going to reduce them further by a factor of ten or more. I am not saying I completely accept his figures, but this is the difference between the Zubrin era and the Musk era. I could double the mass of the mission, but I wouldn't be doubling the cost between 1985 and 2015 or indeed between now and say 2020.

2. My proposal is not the standard proposal, since I am arguing for a much smaller transfer vehicle with most supplies being delivered separately in robot landings. So, the propellant balance sheet is not as you claim. And you have to factor in the development costs for your parachute and your heat shields. Only once you've done all that can you compare and contrast.

3. Zubrin's reasoning is weak. Musk has shown that the private sector can slash costs when led by a determined individual. There is no reason to suppose Musk couldn't apply the same approach with a Mars mission.

4. It is seeing this as a "science and exploration" mission which will really ratchet up the costs. If we see it as an ISRU and revenue generation mission we will get entirely different results.

RobS · 2012-04-01 17:52:17

I am intrigued by your approach, Louis, and I think you are right: the era of private space exploration may be at hand, and it will be conducted under different rules, more practical and more economic. Government space exploration has been oriented around science and, franky, political concerns like creating jobs.

But even so, there's no reason to spend $500 million to send an extra 150 tonnes of propellant to Mars to avoid using a heat shield and parachute. They are well developed technologies, we know what materials to use on Mars (we've used how many heat shields and parachutes there?), and they aren't expensive to develop and build. Maybe he retrorocket technology is expensive, since it appears that's an issue. But we don't know that for sure, and neither of us are technical experts.

I am also rather concerned about very small landers for human crews. The first crew needs to bring with it the stuff it needs if it misses the landing site; otherwise, they're dead. The exception is if we've already landed a dozen or so vehicles of similar size on Mars and they've all come down exactly where they are supposed to.

I am also uncertain how easy it'll be to build up a large unmanned base with robots and such before we send humans. So much can go wrong that a person with a screwdriver can fix, we may need people there early on. If we need people there early on--which will also make the mission more attractive to the public--then we need to be sure to land enough stuff with them to guarantee their survival. You don't want them to land 500 km from stuff, have no way to get to it, and starve to death slowly on CNN. Your company wouldn't survive, nor would a government Mars program. That's not a question of engineering, but of emotions. And the exploration of Mars is an emotional topic at its heart; otherwise we'd all be saying, let's go back to the moon.

louis · 2012-04-01 19:21:24

RobS wrote:

I am intrigued by your approach, Louis, and I think you are right: the era of private space exploration may be at hand, and it will be conducted under different rules, more practical and more economic. Government space exploration has been oriented around science and, franky, political concerns like creating jobs.
But even so, there's no reason to spend $500 million to send an extra 150 tonnes of propellant to Mars to avoid using a heat shield and parachute. They are well developed technologies, we know what materials to use on Mars (we've used how many heat shields and parachutes there?), and they aren't expensive to develop and build. Maybe he retrorocket technology is expensive, since it appears that's an issue. But we don't know that for sure, and neither of us are technical experts.
I am also rather concerned about very small landers for human crews. The first crew needs to bring with it the stuff it needs if it misses the landing site; otherwise, they're dead. The exception is if we've already landed a dozen or so vehicles of similar size on Mars and they've all come down exactly where they are supposed to.
I am also uncertain how easy it'll be to build up a large unmanned base with robots and such before we send humans. So much can go wrong that a person with a screwdriver can fix, we may need people there early on. If we need people there early on--which will also make the mission more attractive to the public--then we need to be sure to land enough stuff with them to guarantee their survival. You don't want them to land 500 km from stuff, have no way to get to it, and starve to death slowly on CNN. Your company wouldn't survive, nor would a government Mars program. That's not a question of engineering, but of emotions. And the exploration of Mars is an emotional topic at its heart; otherwise we'd all be saying, let's go back to the moon.

Well, I am not dogmatic on EDL. Whatever works... but when I hear NASA playing up the difficulty of EDL I do wonder whether the simplest thing is not to slow down the craft to near a dead halt and proceed that way to the surface (after the lander detaches). It may cost an extra $500m but how much will the development of a more complex EDL will cost?

I think if we are not confident of getting a lander to land where we want it within 50 metres, then we shouldn't be going to Mars. We'll have orbiters, transponders on the ground and all sorts of equipment on board. We can have a robot rover actually draw a landing circle on the surface. There can be no other result than that we land accurately as a helicopter would on a rig at sea.

We never had an unsuccessful Apollo landing or lunar surface mission and that was nearly 50 years ago. Technology has moved on. I would think the small lander would allow the crew to survive for at least several days. But they should be within a couple of kms of a pressurised rover and an expandable habitat plus vital supplies of oxygen, water and food.

I wouldn't overemphasise the "robot base" side of things. Really most supplies will be landed and kept in position. But we will probably have one of two elements operational e.g. a water mining rover robot and a rover inspection vehicle.

RobS · 2012-04-01 20:01:57

The difference between Mars and the moon is the former's atmosphere. Apollo 17 landed within a hundred meters or so of where it was supposed to land and quite near the Surveyor spacecraft, which it visited per schedule. Winds can blow a ship around on its way down. But Musk is confident he can land a Falcon stage or Dragon capsule on a pad, and the Earth has winds, too. I suppose an extra delta-v of 100 or 200 meters per second is enough to compensate. But 200 meters per second is probably close to a tonne of fuel, and people want to avoid that the first few times.

The Viking spacecraft were supposed to come down within a "landing ellipse" on the Martian surface that was something like 70 kilometers long and 25 wide. I don't remember the exact size; I'm sure one can look it up on the web. The border of the ellipse represented a 99% probability of landing within. It actually came down fairly close to the center; I'd say it was a third of the way from the center to one of the ends, so maybe 10 or 15 kilometers away from the dead center. The ellipses for Spirit and Opportunity were smaller, and the ellipse for Curiosity is even smaller. So we are getting better. If the landing ellipse for a manned mission is only five kilometers long and two wide, we really have no problem. If we have a spare 100 or 200 meters per second of delta-v, we can steer onto a landing pad within that ellipse, too.

As for spending an extra 500 million on fuel, you're talking about doing that several times, so it adds up. We can test a martian landing system in the upper atmosphere of the Earth; if you can bring a capsule to a dead stop at 100,000 feet (where the air pressure is Martian) from Mach 2, then you can do it on Mars. And then you test the result once or twice with cargo landers on Mars before trying it with people. The problem is probably worth solving.

RobS · 2012-04-01 20:16:11

P.S. I just asked Google how big the landing ellipses were:

Viking: 200 km long and 100 kw wide (I was off!)
Mars Pathfinder: the same
Mars Exploration Rovers: 100 km by 20 km
Curiosity: 25 km by 20 km.

In the last case, if the base is smack dab in the middle, you have a 99% chance of arriving within 12.5 km east-west and 10 km north-south. That's not bad.

SpaceNut · 2012-04-01 20:20:44

The trouble with hitting a small landing zone is being able to repeat the pin point target on the follow up mission for reuse of the initial camp site. That said being able to move what gets landed when its in the 20 plus tonnes is much more difficult.....

louis · 2012-04-01 20:46:05

RobS wrote:

The difference between Mars and the moon is the former's atmosphere. Apollo 17 landed within a hundred meters or so of where it was supposed to land and quite near the Surveyor spacecraft, which it visited per schedule. Winds can blow a ship around on its way down. But Musk is confident he can land a Falcon stage or Dragon capsule on a pad, and the Earth has winds, too. I suppose an extra delta-v of 100 or 200 meters per second is enough to compensate. But 200 meters per second is probably close to a tonne of fuel, and people want to avoid that the first few times.
The Viking spacecraft were supposed to come down within a "landing ellipse" on the Martian surface that was something like 70 kilometers long and 25 wide. I don't remember the exact size; I'm sure one can look it up on the web. The border of the ellipse represented a 99% probability of landing within. It actually came down fairly close to the center; I'd say it was a third of the way from the center to one of the ends, so maybe 10 or 15 kilometers away from the dead center. The ellipses for Spirit and Opportunity were smaller, and the ellipse for Curiosity is even smaller. So we are getting better. If the landing ellipse for a manned mission is only five kilometers long and two wide, we really have no problem. If we have a spare 100 or 200 meters per second of delta-v, we can steer onto a landing pad within that ellipse, too.
As for spending an extra 500 million on fuel, you're talking about doing that several times, so it adds up. We can test a martian landing system in the upper atmosphere of the Earth; if you can bring a capsule to a dead stop at 100,000 feet (where the air pressure is Martian) from Mach 2, then you can do it on Mars. And then you test the result once or twice with cargo landers on Mars before trying it with people. The problem is probably worth solving.

As far as I know, to date none of the landings have been made with the benefit of radio triangulation via transponders and orbiters. That's probably because the robot EDLs have been pretty much uncontrollable once committed. But with a human craft powered by retro rockets, that doesn't apply.

I've been meaning to post this marvellous video of the DC Clipper landing back on its pad - incredible! Have you seen it before...? Shows what can be done. Having seen that, I think one can say you can do pretty much anything with retro rockets if you've got enough propellant. But if we can throw in orbital capture, all to the good. Here's the clip -

http://www.youtube.com/watch?v=JzXcTFfV … re=related

RobS · 2012-04-01 20:52:26

No, none of the landings had access to any ground signals. You are right, that would allow steering and much more precise landing, without much more delta-v.

I have never seen the Delta Clipper clip before, but had heard about the precise landings and the unfortunate end of the program. The Falcon X and the Dragon are supposed to do the same thing.

louis · 2012-04-02 02:23:55

RobS wrote:

No, none of the landings had access to any ground signals. You are right, that would allow steering and much more precise landing, without much more delta-v.
I have never seen the Delta Clipper clip before, but had heard about the precise landings and the unfortunate end of the program. The Falcon X and the Dragon are supposed to do the same thing.

Seeing how manouverable that craft is, makes me think that perhaps the Dragon would be very suitable for Mars exploration as well. Perhaps you could have two dragons, one with the main crew and the other,perhaps radio controlled, which would contain a propellant making facility - so that together they could roam the whole planet.

Russel · 2012-04-02 10:33:40

louis wrote:

Yes some of those thoughts have crossed my mind. I've never seen why we shouldn't expend more propellant on a direct shot to Mars for the humans in a much smaller craft than would be necessary if you were jumbling supplies with the humans.
I wasn't aware aerobraking was especially risky - do you mean in the sense there isn't much margin of error for positioning or you might skim off into outer space? With Mars orbiters in place might that not be such a concern?

Aerobraking has several risks. One is the obvious - you miscalculate and you either hit the planet or fly off into space. I think they've got that one reasonably well in hand.

The more subtle point is that a smaller miscalculation can make a huge difference to the G forces. And that doesn't matter so much if its non manned.

There is a limit to how light you can make even a direct trajectory because to some extent your minimum mass is set by how much radiation shielding you have to have.

It is true though, that if you put your non manned stuff into a very slow trajectory - which is what you'd get with a low powered electric drive - then you can probably do that without any aerobraking anyhow.

Russel · 2012-04-02 10:46:38

I should also add, that to the extent you can keep unnecessary mass from travelling with your crew, that simplifies the problem of taking extra propellant along to slow down after a direct trajectory.

I'm sure there are some here willing to do the sums, but the thing is, its not adding much complexity to the mission simply tanking up with more fuel to start with.

What it does do is rule out being able to take anything fancy with you - meaning even the return craft would probably have to be pre-positioned - again another candidate for a slower, cheaper trajectory.

Its hard to talk about landings without not also considering the entire mission isn't it

And while we're at it, there is of course another variation. You attach a dragon or similar to a space hab. Head off to Mars. As close as possible to Mars your crew move into the capsule, it detaches, and then the capsule heads towards a direct landing. The space hab then does a course correction and then uses aerobraking. It gets parked in orbit to form part of the return mission.

Btw, I'm still curious to know if anyone has cottoned on to the idea of a "deep sea anchor" approach to landing (or indeed aerobraking) ?

SpaceNut · 2012-04-02 19:37:24

Aerobraking/ Aerocapture only get the craft to slow into an orbit which is the starting point of the landing cycle known as EDL...it requires the heatshield, parachutes and rockets to get the speed down to make it possible to land....

RGClark · 2012-04-03 11:54:36

There is a link to report on "Red Dragon" that discusses this question of landing on Mars in this thread on NasaSpaceFlight:

Re: Red Dragon
« Reply #297 on: 12/12/2011 10:19 PM »
http://forum.nasaspaceflight.com/index. … #msg838572

For landings that use aerobraking, rather than retro-rockets, the problem is one of the "ballistic coefficient", as discussed in this article:

Mars Exploration Entry, Descent and Landing Challenges.
http://www.4frontierscorp.com/dev/asset … rs_EDL.pdf

It's proportional to the ratio of landed mass to area of the air brake. The problem is because of Mars thin atmosphere the air brake's surface area has to be large for a large landed mass which would mean an even heavier mass. (Note: in some sources the ballistic coefficient is defined in terms of the ratio of the weight, in Newtons, to the surface area.)
So the air brake would have to look like it does here:

How to Go to Mars--Right Now!
Human exploration of Mars doesn't need to wait for advanced rockets, giant spaceships, or lunar base stations.
By Robert Zubrin / June 2009
http://spectrum.ieee.org/aerospace/spac … ight-now/1

and here:

Aerobrake.
http://smpritchard.deviantart.com/art/A … -276129026

But it would have to be of a lightweight material to be this large without incurring too large a weight penalty while at the same time having great heat resistance for reentry. One solution that has been proposed is a ballute. The SpaceX PICA-X material used for the Dragon heat shield might also work since it appeared to undergo minimal degradation on Earth reentry so it's possible it could be made thinner to get a larger heat shield at low weight.

Bob Clark

Rxke · 2012-04-03 15:48:06

Off topic, but it's very heart-warming to see you back RobS !

Russel · 2012-04-04 08:11:21

RGClark wrote:

There is a link to report on "Red Dragon" that discusses this question of landing on Mars in this thread on NasaSpaceFlight:
Re: Red Dragon
« Reply #297 on: 12/12/2011 10:19 PM »
http://forum.nasaspaceflight.com/index. … #msg838572
For landings that use aerobraking, rather than retro-rockets, the problem is one of the "ballistic coefficient", as discussed in this article:
Mars Exploration Entry, Descent and Landing Challenges.
http://www.4frontierscorp.com/dev/asset … rs_EDL.pdf
It's proportional to the ratio of landed mass to area of the air brake. The problem is because of Mars thin atmosphere the air brake's surface area has to be large for a large landed mass which would mean an even heavier mass. (Note: in some sources the ballistic coefficient is defined in terms of the ratio of the weight, in Newtons, to the surface area.)
So the air brake would have to look like it does here:
How to Go to Mars--Right Now!
Human exploration of Mars doesn't need to wait for advanced rockets, giant spaceships, or lunar base stations.
By Robert Zubrin / June 2009
http://spectrum.ieee.org/aerospace/spac … ight-now/1
and here:
Aerobrake.
http://smpritchard.deviantart.com/art/A … -276129026
But it would have to be of a lightweight material to be this large without incurring too large a weight penalty while at the same time having great heat resistance for reentry. One solution that has been proposed is a ballute. The SpaceX PICA-X material used for the Dragon heat shield might also work since it appeared to undergo minimal degradation on Earth reentry so it's possible it could be made thinner to get a larger heat shield at low weight.

Bob Clark

More to the point, the more surface area you present the higher you start to brake and the lower the peak heating. As a consequence if you go far enough in terms of larger and larger heat shields or rather drag creating structures, the less restrictions on materials.

I just want to raise one point again, since its an important one. There's two vital services. One is heat shielding per se. The other is simply drag. They don't have to happen in the same place or structure.

The other thing I want feedback on is the idea of a deep sea anchor. Its a device designed to create drag that trails behind the craft.

The neat thing is you can make more use of structures in tension.

One very simple form of this is simply a long.. long.. cable. Along that cable you attach a succession of self inflating structures.. basically a hybrid of parachute and ballute.

You have to design it long enough so that each successive drag element isn't too shadowed by the previous one. But the essence is that its self stabilising - it will always generate a drag force in the right direction - unlike shields that are positioned below a craft.

The point is to get yourself to that desirable point where you've got a large ratio of area to mass which has knock on effects the most obvious of which is more diffuse heating plus the fact that most of the air you're heating is behind you.

I wonder if anyone's thought of this or would care to make a comment?

Void · 2012-04-04 18:53:15

Well, I guess I will take the bait and risk a beating.

I suggest the use of a carbon nano-tube ribbon, in conjunction with a roton on the nose of the entry vehicle. If carbon-nanotubes are so strong, then how about a strong Ribbon/Flag (But a long one)

I know that rotons have outlived their welcome, but in this case, I am not so much suggesting helicoptering down to a landing with a roton, but using the combination of roton and ribbon, to slow the craft down for the next event, which could be a parchute and thrusters, or just thrusters, and maybe with the assistance of the roton.

I see the roton as starting the slowdown by largely thrusting perpendicular (With canting allowed) to expand the trailing vacuum bubble. This might help to slow the
craft down.

Then from the roton arms, from a compartment in the roton arms a ribbon deployed and allowed to trail the craft. The twisting motion of the roton on the base of the
ribbon being expected to cause it to form a helical 3D presentation to the stream of atmosphere around it.

I do fear flapping like a flag, which although that flapping might convert spacecraft momentum to sound waves and heat, it might also tear the machine apart.

Anyway that is why I involked the roton twisting, to try to counterbalance the twisting force with the linear force of the atmospheric stream to cause a flexible ribbon
to have a 3D profile while making a high speed entry to the atmosphere. I presume that this happens after the maximum heating event on the heat sheild of the craft.

Anyway at the upper end of the ribbon could be a container, which woud block the air stream, and help to keep the ribbon strait and not flapping. In that compartment could be a baloon or parachute as you might like, or a thruster pack.

If it were a thruster pack, then the ribbon would remain attached to the craft all the way down, and that thruster pack way above on the ribbon would fire to keep theribbon erect and could also assist in the final landing.

Alternatelly after the ribbon and roton had slowed the craft sufficiently, they would be ejected and a parchute deployed, and finally thrusters as normal from the
bottom of the craft.

Please remember that I am way out of my normal area of thinking here, and we were asked for ideas.

Obviously a cloth ribbon of carbon-nanotubes makes me think it would be about as strong as a ribbon could be, but I am just guessing.

Last edited by Void (2012-04-04 18:56:47)

GW Johnson · 2012-04-05 10:00:21

My engineering intuition suggests that trailed drag devices won't be very effective, because of "shadowing" in the wake of the main vehicle. Even if the tow cable were miles long, this effect would still obtain, we've seen it in the persistent ionized trails the space shuttle left behind during its re-entries.

To make a towed drag device effective, it would have to be wider than than the wake left behind by the vehicle, which is somewhat wider than the vehicle itself. That means you are looking at some sort of conical ring-shaped ballute inflatable. Anything in the middle shadowed by the wake is ineffective as a drag device. But it would be inherently dynamically stable to use such a towed drag device.

I haven't go a clue how to build such a device, but it would need every bit the same heat protection as the vehicle itself, and so would its tow cable, being immersed in incandescent plasma. That's a very harsh environment for all known materials. Interesting idea, though. Certainly deserving of tests.

It would be rather smart to include test articles on ferry flights to ISS, and release them for test coming home.

GW

Russel · 2012-04-05 11:34:23

I'm pretty sure that if you make your drag cable (or truss or whathaveyou) long enough you will find enough air to resist. No, I'm not sure long that is.. hundreds of metres to kilometers.

What I actually had in mind was a string of drag elements - not just one. The reason is it makes it easier to ship. That plus redundancy and in future you could even arrange to have a string you can prune as you go.

Various things come to mind. One is a semi-rigid structure using a hoop and a ring of fabric to create the drag. Leaving some porosity may actually help. Another idea is ballute donuts.. yes a string of them.

The point about doing this is to create a large enough drag that you don't experience the sorts of incandescent heating we're usually used to. And by implication that includes the bottom of the craft you're trying to protect.

I would love to see a simple test from earth orbit. One simple configuration for packing purposes is simply a "stack of pancakes". just a series of discs or perhaps inverted cones that simply pay out like a series of parachutes. I guess if you were really fussy you could use a pyro just to be sure to be sure. And of course a weighted instrument package.

As to the above regarding rotons.. Hmm.. Not sure.. I guess you could borrow from that a bit and take advantage of induced spin to stabilise your structure or even use the spin the add rigidity to inflatable parts. Would require a serious turnbuckle tho..

p.s. I guess one approach is a very simple but very large inverted cone with suitable openings. Think worlds biggest umbrella ~200m dia. And you even have the freedom of manned flights to do assembly-on-the-way. The trick is to make sure it doesn't land on you after you've landed

Void · 2012-04-05 18:44:17

Well, I have been given information both to the negative of what I suggested, and also concepts also suggested.

Might I suggest, that instead of an inverted cone, a net shaped like a badmitton "Birdie". I guess without a roton, you could try to make it rigid, but that makes me wonder how it is stowed and deployed.

With a roton, it could drag behind in the shadow, but be spun to expand outside of the shadow.

To avoid burning up, then not deployed until, speeds are less than 1/2 mile per second? I don't know how bad the heating is then. I am also decending into archaic units from your point of view, but it is comfort from mine.

Anyway with a roton, this could be controllable. (Maybe) You could spin it into the air flow to the degree that you dared, and let it be swept back into the air stream shadow, to make sure the descilleration rate is comfortable, and the heating is not excessive.

A spinning net should be deployable, and also yield more drag for the amount of device mass than a continuous cone of material. It may shed heat better also, provided it is not pushed into the air stream too harsly. When you get rid of it I don't know. Maybe when it is possible to depoy a parachute. I am inclined to wonder if retaining the roton engines would make sense however all the way down, since they could assist on the landing, and they are already paid for. However, then the parachute and net would have to ride in a compartment mounted on the rear of the roton. How much love a parachute would have for a spining roton, I don't know not much I am guessing. Maybe the roton could run with the net. The roton would shut off, and the net being ejected would pull the parachute out. When the parachute was ejected the roton could start up again for the final landing.

Again I know you guys are levels more practiced and I will not continue beyond the point where I can reasonably speculate on these things.

GW Johnson · 2012-04-05 18:49:59

"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

Void · 2012-04-05 22:52:01

I think I am seeing your particular drift. I would really like to see your plan when it emerges because I will bet it will be practicle or almost there. And no it would not require rotons unless they are truely worth including.

Russel · 2012-04-06 09:23:03

The thing about heating is, if the heating is done well behind your craft it doesn't matter.. you're just toasting gas molecules. Its only when you're heating something near your craft that you need to worry about insulation.

RGClark · 2012-04-06 10:39:11

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.

BTW, since the X-33 could weigh 1/3rd to 1/4th the weight of the space shuttle orbiter, I suggested that a plan that was announced last year for a commercial, next-generation shuttle system:

Next Gen Shuttle-Capable vehicle interest as secret effort to save orbiters ends.
December 19th, 2011 by Chris Bergin
http://www.nasaspaceflight.com/2011/12/ … ters-ends/

should use instead the X-33 in place of the shuttle orbiter.

This would mean you could carry more payload for this STS 2.0, if you will, than the original shuttle system. Ironically, the fully orbital version of the suborbital X-33 was considered to be a "next-generation" shuttle:

Lockheed Vying to Design Replacement for Shuttle : Space: Skunk Works in Palmdale competes against McDonnell Douglas Aerospace and Rockwell International.
March 14, 1995|JEFF SCHNAUFER | SPECIAL TO THE TIMES
http://articles.latimes.com/1995-03-14/ … ll-douglas

However, strictly speaking in being single stage to orbit, this orbital version would have been quite different than the space shuttle orbiter. But having the X-33 in place of the shuttle orbiter in a multi-stage system would be more in keeping with the idea of a "next generation" shuttle.

Bob Clark

Last edited by RGClark (2012-04-06 10:41:40)

GW Johnson · 2012-04-08 16:01:07

I was not so very much a fan of X-33's 8% structural inert fractions. Turns out I was right: those propellant tanks fractured under their own weight, when stood on end.

I am very much a fan of very low ballistic coefficients/wing loadings for re-entry. I like the idea of an extension of the venerable old Piper Cub as a re-entry vehicle. Absorbing BTU's is easy. High skin temperatures is not.

Sort of poetic justice, in a way. I rather like stick-and-rudder flying. I learned to fly in a Piper J-5, if anybody knows what one of those is.

GW

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