Landing on Mars

RobS · 2012-03-22 17:28:39

Have you all seen this?

http://www.universetoday.com/7024/the-m … ed-planet/

It basically says that you can't land a large manned vehicle on Mars with the standard heat shield-parachute-thruster combination because the heat shield can't be large enough to slow the vehicle down to Mach 2 (when you can deploy parachutes) before you hit the ground. The atmosphere is too thin. But it's too thick to fire thrusters straight ahead of you at Mach 2+ because the exhaust plume is too dynamic and the resulting shaking could shake your vehicle apart. It advocates a "hypercone," a big inflatable structure, at Mach 5, to slow down the ship. I suppose a super-large heat shield, assembled in Earth orbit, would do it as well; that possibility is hinted at.

GW Johnson · 2012-03-22 17:54:27

I am guessing that, in part, that is why the Super Draco thrusters on Dragon are canted roughly 45 degrees off axis. You "heat shield it" partway through re-entry, then burn to make sure you're down to around Mach 2 (that's a ribbon chute limit, it's actually closer to Mach 2.5), deploy a chute, then chute plus thrusters to land. Or just thrusters to land.

Dragon is a one-way vehicle to Mars's surface like that, so we're likely talking unmanned probe or cargo carrier. The canted thruster plumes avoid most of the hypersonic instabilities you spoke of.

The other reason they are canted is for launch escape service on the manned version of Dragon. Match made in heaven. I like well thought-out systems.

GW

louis · 2012-03-22 19:23:54

RobS wrote:

Have you all seen this?
http://www.universetoday.com/7024/the-m … ed-planet/
It basically says that you can't land a large manned vehicle on Mars with the standard heat shield-parachute-thruster combination because the heat shield can't be large enough to slow the vehicle down to Mach 2 (when you can deploy parachutes) before you hit the ground. The atmosphere is too thin. But it's too thick to fire thrusters straight ahead of you at Mach 2+ because the exhaust plume is too dynamic and the resulting shaking could shake your vehicle apart. It advocates a "hypercone," a big inflatable structure, at Mach 5, to slow down the ship. I suppose a super-large heat shield, assembled in Earth orbit, would do it as well; that possibility is hinted at.

NASA is fond of quoting these problems, but they sound more like excuses to me. I am sure Musk is attacking the problem and coming up with solutions.

A couple of points, I believe that orbital and aerocapture can slow down the vehicle substantially without need for heat shield or huge retro rockets. Personally I favour a specialist lander, not too large- in fact as small as possible. On landing, perhaps we could have a robot pressurised vehicle ready and waiting for them. They could transfer to that, and begin assembling pre-delivered supplies in the base area. The expandable habitat could then be activated and they could transfer to that from the pressurised vehicle.

RobS · 2012-03-22 20:44:42

I was wondering about Dragon's canted thrusters. But do we know for sure that will solve the problem? Intuitively, it seems like they should.

Louis, you will need some sort of heat shield because the first time you hit the Martian atmosphere, you are coming in from interplanetary space and moving at over 12,500 mph/20,000 kmph. Orbital velocity is 8,000 or 8,500 mph (about 13,000 kmph), so you're going to enter the atmosphere at least at that speed, and there's nothing you can do about it unless you fire thrusters, which is a waste of fuel.

Regarding cargo landers, the concern (Zubrin era) was whether you could be sure to land near them. I think we are beyond that concern now; even with an upper atmosphere of unpredictable density, NASA has been landing fairly close to its target landing spot. Even so, keep in mind the inconvenience of assembling supplies in one spot when your four landers are 3, 5, 8, and 10 kilometers away from you. I think I read that a Falcon Heavy could place 11.5 tonnes on the Martian surface. Figure that's about 10 tonnes of supplies.

GW Johnson · 2012-03-22 21:49:55

RobS:

Sure, the canted thrusters will work. That idea worked very long ago for the tractor rockets of the escape towers on Mercury and Apollo, and also for parachute delivery of very heavy items like tanks, with last-second rocket braking. It was the Soviets who actually fielded this parachute/rocket braking.

If one is using a descent trajectory that involves at least some rocket braking (so you can steer arbitrarily) during the hypersonics, then precision landings become possible, regardless of the atmospherics. The missing piece is a radar transponder to home-in on. Once a first landing at any given site has been made, that is how one precisely guides subsequent properly-equipped vehicles down to exactly the same site.

GW

RobS · 2012-03-23 07:13:14

GW Johnson, I wasn't asking whether canted thrusters would work. That's a simple matter of physics. I was wondering whether we are sure it would solve the problem that is caused when one fires engines into a hypersonic air stream. If you fire your engines straight in the direction of movement, the hypersonic airstream and exhaust gasses build up in front of the vehicle and buffet it violently; so I gather from the article listed above. Here's the exact quotation:

But using current thruster technology in Mars’ real, existing atmosphere poses aerodynamic problems. “Rocket plumes are notoriously unstable, dynamic, chaotic systems,” said Manning. “Basically flying into the plume at supersonics speeds, the rocket plume is acting like a nose cone; a nose cone that’s moving around in front of you against very high dynamic pressure. Even though the atmospheric density is very low, because the velocity is so high, the forces are really huge.”

Manning likened theses forces to a Category Five hurricane. This would cause extreme stress, with shaking and twisting that would likely destroy the vehicle. Therefore using propulsive technology alone is not an option.

End of quote.

Canted thrusters at least get the rocket plume mostly out of the way, but will that be enough? No one has tested such a system, though I suppose an expert would be able to make some calculations, and eventually someone will perform a wind tunnel test. A test could also be performed high in the earth's atmosphere.

Last edited by RobS (2012-03-23 07:14:01)

louis · 2012-03-23 14:54:49

GW - Yes, I wasn't arguing against a heat shield of some kind being required. But let's slow the thing down gradually and then certainly use retro thrusters and parachutes for the descent.

louis · 2012-03-23 14:59:53

RobS wrote:

GW Johnson, I wasn't asking whether canted thrusters would work. That's a simple matter of physics. I was wondering whether we are sure it would solve the problem that is caused when one fires engines into a hypersonic air stream. If you fire your engines straight in the direction of movement, the hypersonic airstream and exhaust gasses build up in front of the vehicle and buffet it violently; so I gather from the article listed above. Here's the exact quotation:
But using current thruster technology in Mars’ real, existing atmosphere poses aerodynamic problems. “Rocket plumes are notoriously unstable, dynamic, chaotic systems,” said Manning. “Basically flying into the plume at supersonics speeds, the rocket plume is acting like a nose cone; a nose cone that’s moving around in front of you against very high dynamic pressure. Even though the atmospheric density is very low, because the velocity is so high, the forces are really huge.”
Manning likened theses forces to a Category Five hurricane. This would cause extreme stress, with shaking and twisting that would likely destroy the vehicle. Therefore using propulsive technology alone is not an option.
End of quote.
Canted thrusters at least get the rocket plume mostly out of the way, but will that be enough? No one has tested such a system, though I suppose an expert would be able to make some calculations, and eventually someone will perform a wind tunnel test. A test could also be performed high in the earth's atmosphere.

I've wondered before now about a cradle for the lander, so the thrusters would be spaced well away from each other. I do feel there must be a solution to the problem.

Also, can someone explain why you can't slow down in the vacuum of space first?

louis · 2012-03-23 15:11:38

I think part of the EDL problem is that NASA have been thinking in terms of a pretty big lander that will serve all the crew's needs for an extended stay. I think if we can strip down the lander to a minimum tonnage, then a lot of the issues become manageable.

GW Johnson · 2012-03-23 17:46:37

Oops, sorry if I insulted somebody. Didn't mean to.

I'm already very sure the crudely-45 degree canted Super Dracos will work during entry anywehere, because the thrusters on all the capsules and shuttles we ever flew worked. Effectively, they're canted thrusters, too, just different angles. It's only right into the airstream that's unstable.

Somebody asked why you couldn't reduce velocity before entry. You can, it just costs a lot of propellants. With chemical rockets, the Isp and practical mass ratios limit you to using as much aero deceleration as you can get. With solid core nuclear, you just begin to get into the regime of Isp at which you can reduce velocity before entry, and still maintain a decent mass ratio.

Advanced gas core nuclear concepts do even better, but we never built any of those, just solid core. And we quit even that in 1973, right before we were to fly them. The guys who did it are mostly dead now. But I met 3 survivors last August. They're in their 80's now. The "engineering art" dies with them.

Solid core nuclear is enough to rocket-burn to landing from low Mars orbit, and still rocket-burn back to orbit, in a single stage vehicle of "reasonable" mass ratio. In other words, we could build a single-stage, re-usable "landing boat" out of that technology!

Send one lander and a lot of propellant for it, and make many landings with it, refueling in orbit after every trip. I was looking at 10% payload, 20% inert structure and equipment, and 70% propellant (liquid hydrogen), for the round trip, at 30-degrees out-of-plane maneuver both ways! 60 ton lander, 6 ton payload. Wow!

Do you like that prospect? I sure do.

GW

RobS · 2012-03-23 18:46:37

Actually, I wonder whether slowling down in orbit really helps that much. Let's say you are in a circular low Mars orbit at the altitude of 100 miles (160 km, 160,000 meters). You fire your engines and stop dead in space.

Now you fall like a rock towards Mars, gaining 3.8 meters per second per second.

s = 1/2a t2 (s = distance traveled, a = acceleration from Martian gravity, t2 = time squared) where 1/2a is 1.9 meters per second per second and s = 160,000 meters. So t-squared is 160,000/1.9 = 84,210; square root and you get a t (time) of 290 seconds. 290 x 3.8 = 1,102 meters per second = 3,967 kilometers per hour = 2,459 mph.

So you'd still approach the surface at almost mach 4; too fast to deploy a parachute, and you would gain almost all that speed before the atmosphere was thick enough for a parachute to be useful, anyway. Basically, it does no good to fire an engine in orbit and slow yourself down up there.

But I think canted thrusters make sense and most likely solve the problem. Of course, at a 45 degree angle, a delta-v of 1000 meters per second will only produce a vertical delta-v of 707 meters per second because it also produces a horizontal delta-v of 707 meters per second, if I recall my trigonometry right.

Oh, regarding Louis's point that a small lander might solve the problem: the article I provided the link for was an interview with the guy who designed the descent systems for Spirit, Opportunity, and the Mars Science Lab. He said the parachute/thruster combination couldn't even be scaled up for Mars Science Lab. Basically, the parachute/thruster combo doesn't work when the payload exceeds about a tonne.

Last edited by RobS (2012-03-23 18:49:56)

louis · 2012-03-23 20:13:43

RobS wrote:

Actually, I wonder whether slowling down in orbit really helps that much. Let's say you are in a circular low Mars orbit at the altitude of 100 miles (160 km, 160,000 meters). You fire your engines and stop dead in space.
Now you fall like a rock towards Mars, gaining 3.8 meters per second per second.
s = 1/2a t2 (s = distance traveled, a = acceleration from Martian gravity, t2 = time squared) where 1/2a is 1.9 meters per second per second and s = 160,000 meters. So t-squared is 160,000/1.9 = 84,210; square root and you get a t (time) of 290 seconds. 290 x 3.8 = 1,102 meters per second = 3,967 kilometers per hour = 2,459 mph.
So you'd still approach the surface at almost mach 4; too fast to deploy a parachute, and you would gain almost all that speed before the atmosphere was thick enough for a parachute to be useful, anyway. Basically, it does no good to fire an engine in orbit and slow yourself down up there.
But I think canted thrusters make sense and most likely solve the problem. Of course, at a 45 degree angle, a delta-v of 1000 meters per second will only produce a vertical delta-v of 707 meters per second because it also produces a horizontal delta-v of 707 meters per second, if I recall my trigonometry right.
Oh, regarding Louis's point that a small lander might solve the problem: the article I provided the link for was an interview with the guy who designed the descent systems for Spirit, Opportunity, and the Mars Science Lab. He said the parachute/thruster combination couldn't even be scaled up for Mars Science Lab. Basically, the parachute/thruster combo doesn't work when the payload exceeds about a tonne.

Last para first, but they are talking about a hypersonic deceleration in the atmosphere, are they not?

My proposal would be to stop dead in orbit and not allow the craft to accelerate to Mach 4 - you are using retro thrust all the way. But this is why it is important to have a small lander so you devote enough propellant/fuel to the deceleration stage.

Let's remember that this was essentially the Apollo solution and it worked well. The only issue there was that they didn't have supplies on the lunar surface, to allow an extended stay. They had to pack up and go within 72 hours or whatever.

If I remember rightly, x5 should get you to the surface, so maybe with a 3 tonne lander, you need 15 tonnes of propellant and fuel, so call it a 20 tonne craft in all.

RobS · 2012-03-23 21:56:53

And every tonne you put in Mars orbit requires you to launch about four tonnes, so your three-tonne lander masses 20 tonnes in Mars orbit and 80 tonnes on the earth's surface! Why would you do that, if a heat shield, parachute, and canted thrusters solve the problem and mass maybe half a tonne for a three-tonne lander? Furthermore, if a Mars mission requires 50 or 60 tonnes of stuff for four people for 18 months (Mars Direct requires 53 tonnes for four) and you're landing everything three tonnes at a time, you need about twenty landers launched by 20 80-tonne rockets. No, you don't want to do that.

According the Falcon Heavy's wikipedia page, a Falcon heavy can put 53 tonnes into low earth orbit and 14 tonnes to trans-Mars injection. I have also seen somewhere it can place 11.5 tonnes on the Martian surface.

Last edited by RobS (2012-03-23 21:58:20)

louis · 2012-03-24 08:27:09

RobS wrote:

And every tonne you put in Mars orbit requires you to launch about four tonnes, so your three-tonne lander masses 20 tonnes in Mars orbit and 80 tonnes on the earth's surface! Why would you do that, if a heat shield, parachute, and canted thrusters solve the problem and mass maybe half a tonne for a three-tonne lander? Furthermore, if a Mars mission requires 50 or 60 tonnes of stuff for four people for 18 months (Mars Direct requires 53 tonnes for four) and you're landing everything three tonnes at a time, you need about twenty landers launched by 20 80-tonne rockets. No, you don't want to do that.
According the Falcon Heavy's wikipedia page, a Falcon heavy can put 53 tonnes into low earth orbit and 14 tonnes to trans-Mars injection. I have also seen somewhere it can place 11.5 tonnes on the Martian surface.

Well firstly I would say I am not at all dogmatic on the best way forward, so I look at this entirely pragmatically.

However, there are some considerations here:

1. The fewer descent elements in your technology, the less there is to go wrong. If retro, parachute and heat shield - three elements - are all crucial then failure in just one is enought to kill the mission and cause loss of life, as we saw with the shuttle. So, you are building in complexity. And also, the three technologies have to work together.

2. The more complexity we build in the more the development costs rise. That's why Musk's technology costs a fraction of the Space Shuttle - because he has always aimed for simplicity based on tried and trusted technologies. It is the development costs that at the really big element in Mars missions. This is why putting just a small rover on Mars costs hundreds of millions of dollars - because there are thousands of people working on the project over maybe 10-15 years. So, keeping it simple can save huge amounts of money.

3. Shipping tanks of fuel to Mars orbit is, in principle, not such a difficult task. We (NASA and the ESA) have now sent many craft there accurately. So, it doesn't require any major technological innovation.

4. I think 50-60 tonnes of stuff is an over-estimate. I would say 40 tonnes to the surface for six people is all that is required - but possibly far less. Of course there is the issue of fuel/propellant for a return, which can be addressed in various ways.

Regarding the robot landings, with my approach I think it would be more like 15 launches, maybe launched over 8 years, so averaging about 2 a year, or maybe 3-4 every launch window. Is that really such a huge task for a project one would be throwing billions at? Some say $40billion. Even if it was a billion per launch (not a figure I would accept) you've only used up $15billion. These robot landings would actually be a lot simpler than the Mars Rover missions. You would develop your standard EDL technology for the robot landers and then replicate that for each robot mission.
You can land pretty accurately using transponders on the ground and orbiting satellites to fix position. You could land a simply mini rover with on board camera to check out all supplies and ascertain their location to pin point accuracy. You could even get the mini rover to draw a landing zone circle for the human crew lander to zero in on.

GW Johnson · 2012-03-24 10:21:56

For a rocket-burn landing, that's what we did on the airless moon. It's exactly the same as a gravity-turn ascent, just done in reverse.

Now, on Mars, there's enough atmosphere to be a heating problem, both ways. What speed you will be going when you hit atmosphere depends mostly on the altitude of your parking orbit. So, park fairly high to reduce speed at entry.

Hit the sensible atmosphere well into your gravity-turn-in-reverse trajectory, at speeds well below the orbit speed. You'll do canted burn with assistance from heat shield aero drag, then canted burn with assistance from a parachute (or series of chutes) once you're below about M2.5-ish. Then canted burn to touchdown. I think that's what they're really going to do with one-way Spacex Dragons on Mars.

The point is to use aero drag to reduce the thrust required during what is otherwise a continuous-burn descent. Reducing thrust reduces propellant demand. It's not a huge effect, but every bit helps. There's no such help during ascent, but there is also no need to cant the engines. Even during descent, once the hypersonics are over, there is no need to cant the engines. Straight-axis thrust eliminates the cosine loss.

Now, the instability of straight-axis thrust with the jet straight into the oncoming hypersonic stream is a real problem. Whether computer-controlled attitude thrusters can cope with it, well, some experiments need to be done. My intuition suggests it is possible, but your attitude thruster system needs some real force to it. If it could be done, the need for canted propulsion engines during descent disappears.

For the delta-vees needed to do this job on Mars, either as a one-way descent vehicle, or as a two-way lander, "reasonable" mass ratios and the rocket equation generally seem to rule out chemical Isp's, except as staged vehicles. What is "reasonable" for mass ratio? Well, that depends on whether the vehicle is to be used once, or many times.

What we are used to building is one-shot throwaway rocket stages, for which inert fractions in the 8-10% range seem feasible at this time. Maybe a tad higher for those with heat shields and landing legs, say 10-15%.

A reusable vehicle, on the other hand, needs to be tougher than an old boot in order to take the abuse of repeated spaceflight entry conditions (and they are very abusive, even on Mars). I would point to airplanes, which are very reusable flight vehicles. Transports and bombers for decades have been in the range of 40-50% inerts. So was the X-15 rocket plane (40% inert).

The lander we are discussing in this thread is not an airplane, and likely has no wings at all. It's a different structure. But I would hazard a guess that a reusable "landing boat" vehicle would have an inert fraction closer to the 20-25% range than anything we've seen in prior rocket vehicle designs. It's simply not strong if the material isn't there. And that material has a weight to it.

So, that's what brought me to the nuclear rocket technology for a single-stage two-way Mars landing boat: about 20% inerts puts you into a simple tradeoff of payload fraction vs propellant fraction. At Isp 400-class, I couldn't solve the problem single stage. At Isp 1000-class, I could, and with an out-of-plane capability to boot. That's solid-core nuke, something we know we could build, because we did it before.

Now, in my design study, I did not cant the engines, I only had one. But I could have used 3 or 4 small ones instead. Those could be canted slightly on descent at a cosine loss, and gimballed straight on ascent. I'd fire through the heat shield from near center. It simply lays out better as a big conical vehicle that way, with a cockpit near the tip of the cone, away from the radioactive stuff.

Anyhow, that's my two cent's worth.

GW

RobS · 2012-03-24 12:56:01

The problems with developing solid core nuclear engines are political. No private company will do it, so it'll be a government program. Environmentalists will fight it every inch of the way. Testing will require a huge, expensive indoor facility because no one will dare blast potentially radioactive exhaust into the atmosphere, the way they did with NERVA. Launches will have protests, which is bad publicity. Congressmen will be lobbied by all sorts of pressure groups. Testing in earth orbit will be tricky. Politics currently is about 90% irrational, judging by the campaigns being waged right now, which is a bad ratio to deal with when one is talking about nuclear engines. You'd never know that climate science is nearly unanimous about man-made heating of the Earth, judging from the words of politicians.

And I think nuclear engines will be unnecessary, anyway. Zubrin was figuring on a powered delta-v of 700 meters per second for landings, which has a pretty favorable mass ratio. Converting hydrogen and atmospheric carbon dioxide into methane and oxygen should be reasonably reliable and a cheap technology to develop. If solar panel technology continues to improve, you may not even need a nuclear surface reactor. Zubrin figured a 3.5 tonne reactor could generate 80 kilowatts. I wonder whether we can develop a deployable 50-kilowatt solar power array (inflate ribs with pumped Martian air to unroll them across the surface) massing about 5 tonnes. If so, we could go without nuclear, which would save a lot of headaches.

Once we have a small base on Mars, we'll have a water well and won't need terrestrial hydrogen at all. Escape velocity from Mars is half that of earth's and 2/3 the velocity needed to reach earth orbit from the terrestrial surface. What cargo could a reusable Falcon carry from the Martian surface to orbit? From the Martian surface to trans-earth injection? Has anyone figured that out?

Last edited by RobS (2012-03-24 12:57:45)

GW Johnson · 2012-03-25 09:46:02

For nuclear engines, I have often said in other threads that the best place to test such things is on the very near-by moon. Testing any kind of rockets requires a stable thrust stand where you wring it out before you ever consider flying.

Solar PV power is possible in large sizes. The shuttle bay was 65 feet long (about 20 m). The insides of those doors were a solar array, rated at a max of 25 KW. That's now an older design, but it still gives you a notion of the size of the thing vs its power. (The second layer of door panels was the waste heat radiator.)

I quite agree that the politicians are completely perfidious and behave nonsensically with respect to space, to nuclear, or to any other science or technology issues one cares to name. They won't change until we hold them responsible for what they have done, and have not done.

GW

SpaceNut · 2012-03-25 12:07:09

We have lost a thread that talked about the orion SM with thrusters that were part of the escape systems rather than the LES....

Russel · 2012-03-30 09:33:30

There's a bunch of things that I'm intrigued about regarding Mars missions but this is the bit related to landings.

Let's suppose the following two scenarios are viable.

A: This is the "conventional" scenario. Some of your hardware is already in place on the surface of Mars, but when you land, you land seated inside some larger structure which also happens to be all or a large part of your habitat. The conventional wisdom here is some kind of biconic heat shield or perhaps a ballute or some form of large (earth orbit assembled) structure which also serves as a very large heat shield. Or some combination of the above.

B: In this scenario you land everything on Mars first except for the humans. The humans arrive in one or two dragon style capsules. You have the capability of precision landing, but just to be sure you also carry some form of lightweight transport.

To my mind, trying to land humans along with something that weighs upwards of 60 tonnes, along with all the EDL issues that comes with that is taking more risk with the crew than needed.

If you decouple the crew and the habitat then you end up with scenario B. Of course this has its own risks and complexities.

Just briefly, if you were flying to Mars, what would you bet your life on?

louis · 2012-03-30 16:31:36

Russel wrote:

There's a bunch of things that I'm intrigued about regarding Mars missions but this is the bit related to landings.
Let's suppose the following two scenarios are viable.
A: This is the "conventional" scenario. Some of your hardware is already in place on the surface of Mars, but when you land, you land seated inside some larger structure which also happens to be all or a large part of your habitat. The conventional wisdom here is some kind of biconic heat shield or perhaps a ballute or some form of large (earth orbit assembled) structure which also serves as a very large heat shield. Or some combination of the above.
B: In this scenario you land everything on Mars first except for the humans. The humans arrive in one or two dragon style capsules. You have the capability of precision landing, but just to be sure you also carry some form of lightweight transport.
To my mind, trying to land humans along with something that weighs upwards of 60 tonnes, along with all the EDL issues that comes with that is taking more risk with the crew than needed.
If you decouple the crew and the habitat then you end up with scenario B. Of course this has its own risks and complexities.
Just briefly, if you were flying to Mars, what would you bet your life on?

For Mission 1, I'd go with B. I'd do it the Apollo way because every Apollo landing and ascent was successful.

I'd be reassured to know that we will have lots of ways of checking that the supplies and habitat landed are in good working order.

In the longer term I think we would have much bigger landers. But that would be some years down the line, I think.

Russel · 2012-03-31 07:00:45

Lets take the above principle further. That is you take no compromises with the human cargo but take your engineering risks with everything else.

What that would mean for instance is designing a mission that gets the humans there faster, but use the slow lane for for everything else. Now that's not strictly about landing but it does affect how you land. And I also know that this has been said before its surprising how many missions choose slower routes because of the need to ship the humans inside the hab.

What I have in mind here is that there's been a lot of talk about using aerobraking into a Mars orbit.

For me that's a good way to save on fuel mass for things like your hab, your ISRU, or an orbiter.

But I've never been terribly happy with missions that propose to fly the humans along with all that, and then take the risk of aerobraking.

Also, for the non-human side of the landing this is where a large in earth orbit heat shield comes in handy since it can serve a dual role for both aerobraking and then landing.

One last idea I'd like to run past the experts. What if, instead of things like ballutes or large shields, or some combination thereof, what you're actually building is basically a deep sea anchor. Its something you design to create a lot of drag but it projects hundreds of metres or more into the slipstream of the craft. It could be based on cables, or it could even have a backbone. Onto that you could attach semi right structures that are designed to create drag, or even inflatable drag structures. The point being is the whole design is inherently stable.

Any thoughts?

louis · 2012-03-31 17:14:25

Russel wrote:

Lets take the above principle further. That is you take no compromises with the human cargo but take your engineering risks with everything else.
What that would mean for instance is designing a mission that gets the humans there faster, but use the slow lane for for everything else. Now that's not strictly about landing but it does affect how you land. And I also know that this has been said before its surprising how many missions choose slower routes because of the need to ship the humans inside the hab.
What I have in mind here is that there's been a lot of talk about using aerobraking into a Mars orbit.
For me that's a good way to save on fuel mass for things like your hab, your ISRU, or an orbiter.
But I've never been terribly happy with missions that propose to fly the humans along with all that, and then take the risk of aerobraking.
Also, for the non-human side of the landing this is where a large in earth orbit heat shield comes in handy since it can serve a dual role for both aerobraking and then landing.
One last idea I'd like to run past the experts. What if, instead of things like ballutes or large shields, or some combination thereof, what you're actually building is basically a deep sea anchor. Its something you design to create a lot of drag but it projects hundreds of metres or more into the slipstream of the craft. It could be based on cables, or it could even have a backbone. Onto that you could attach semi right structures that are designed to create drag, or even inflatable drag structures. The point being is the whole design is inherently stable.
Any thoughts?

Hi Russel,

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?

RobS · 2012-03-31 19:50:40

If we're talking about using aerobraking to go into Mars orbit after a Hohmann trajectory or a six month trajectory, there's no problem. Mars has plenty of air for that and it has been used many times for incremental aerobraking (lowering the apoapsis of an orbit).

But if you are going faster than that--a 4 or 5 month transit between planets, for example--you hit the atmosphere much faster, which also means you transit through it in less time. Regular heat shields are too small for that. Big extra-wide ones will help and ballutes (which are still a theoretical idea) will help. Either way, one is talking about rather high gee forces. We may develop technology that can reliably use the Martian atmosphere, though.

Landing is a third problem because, according to the article I read recently, the atmosphere can't slow a large vehicle below about Mach 2 or 2.5 before the Martian surface gets in the way. Heat shields would have to be much larger. Parachutes can't work in speeds over Mach 2 or 2.5, either; they're shredded. If you open a parachute at Mach 2, it has to be 100+ meters in diameter, which may not be practical. And rocket engines have difficulty working when flying into the "thicker" air near the surface at such high speeds. Perhaps canted engines that fire at 45 degrees will work.

It's hard to appreciate this problem without personal experience. In July 1976 I was in the science control room of Viking 1 as it fired its engines and headed for the surface. It was rather frightening; the thing fell like a rock. All we could look at was a simple, small black and white tv screen that showed Viking's altitude in feet and velocity in feet per second. Maybe there was other data, but I don't remember what. Whenever you did the division, it always seemed like it was going to impact in less than 30 seconds. Of course, at first Viking was mostly moving forward, not downward, so the numbers were a bit deceiving. But if I recall, the whole operation took less than 5 minutes. When you fire the deorbit burn in low Mars orbit, you go down, and fast!

RobS · 2012-03-31 21:01:41

I thought you all would enjoy reading this extract from my diary.

The Viking 1 Landing on Mars
Extract from Robert H. Stockman’s occasional diary, entry of July 21, 1976

Man’s first unmanned landing on Mars took place at 5:11:50 a.m. PDT on July 20, 1976. This date is the 7th anniversary (to the day) of Apollo 11, man’s first manned landing on the moon. Thus July 20 will forever be engraved in history books. It was also Tuesday (French, Mardi—Mars day) which makes the time chosen for the landing doubly auspicious.

The time was also one requiring certain sacrifice for the members of the Viking Mission team. I was on LIFT-SAG (Lander Imaging Flight Team—Science Analysis Group) which is another way of saying the team responsible for scientific analysis of the first picture sent back from the surface of Mars.

Neither I nor my roommates [a bunch of Brown University graduate students renting a big Pasadena apartment on NASA’s dime] wanted to do an all-nighter, so we arrived at JPL at 3:00 a.m. I slept from 7:30 to 1 a.m., so I was in pretty good condition; some of my other roommates, who are on LIFT, had gotten much less sleep. JPL was incredibly crowded, with VIPs (Ray Bradbury was there and family members of employees. The LIFT area was a bit tense; everyone was standing around, talking excitedly or worriedly, and no one was working. Whenever a Viking News Bulletin came over the t.v. (JPL produced its own Viking News program to feed over the hundreds of t.v. monitors all over the Lab) everyone rushed to the Conference Room to watch. Besides news updates, many mission officials and scientists were interviewed. All were excited; some sounded high. That was the way it was—everyone was incredibly tense and incredibly excited.

Yet the landing was perfect. Continuous coverage started at about 4:30 [a.m.], but the actual entry, deployment of the parachute, and firing of the final retros all occurred in about 6 minutes. No one knew exactly when the spacecraft would touch down; the thickness and composition of the Martian atmosphere was too unknown to be certain how much it would decelerate the landing. Throughout the final minutes, there was also the nagging thought that signals from Mars took 19 minutes at the speed of light. A command in response to a problem, without any time to make a decision, would arrive 38 minutes after the problem occurred. Consequently the lander was completely on its own, its onboard computer making all decisions about when to do what in all circumstances.

The final 6 minutes were terrifying. Deceleration occurred incredibly fast. An altitude of 490,000 feet would be announced; next 460,000 feet. The lander hit the atmosphere at 500,000 feet at 15,000 feet per second. Gee force climbed very fast; 4, 5, 6, 7, 8, 8.6 gees! I’d heard the top gee load was only 7.5 gees, so this scared me a lot. The Viking seemed to be falling like a rock, but all the curves flashed on the t.v. monitors indicated that everything was going perfectly.

At 19,000 feet it had slowed to about 2,500 feet per second and the parachute deployed. Every milestone in the mission had split second timing—the Viking always only seconds from a crash, if something failed or hesitated. The parachute had to be exploded out of the capsule with a mortar—the Martian air was too thin to pull it out. What if the mortar didn’t fire? Or it blew a hole in the parachute?

But it didn’t. At 2500 feet, 200 feet per second, the ‘chute was ejected and the terminal engines fired. The landing took place at 5:12:07, only 17 seconds late—not bad, the estimated time of error before landing was +- 8 minutes!

Everything worked perfectly. When “Touchdown!” was announced, everyone applauded and cheered. It was an incredible relief to know the landing was good—that we still had our jobs!

[I’ll type the other half—when we saw the first rocks—another day.]

louis · 2012-04-01 06:49:54

RobS wrote:

If we're talking about using aerobraking to go into Mars orbit after a Hohmann trajectory or a six month trajectory, there's no problem. Mars has plenty of air for that and it has been used many times for incremental aerobraking (lowering the apoapsis of an orbit).
But if you are going faster than that--a 4 or 5 month transit between planets, for example--you hit the atmosphere much faster, which also means you transit through it in less time. Regular heat shields are too small for that. Big extra-wide ones will help and ballutes (which are still a theoretical idea) will help. Either way, one is talking about rather high gee forces. We may develop technology that can reliably use the Martian atmosphere, though.
Landing is a third problem because, according to the article I read recently, the atmosphere can't slow a large vehicle below about Mach 2 or 2.5 before the Martian surface gets in the way. Heat shields would have to be much larger. Parachutes can't work in speeds over Mach 2 or 2.5, either; they're shredded. If you open a parachute at Mach 2, it has to be 100+ meters in diameter, which may not be practical. And rocket engines have difficulty working when flying into the "thicker" air near the surface at such high speeds. Perhaps canted engines that fire at 45 degrees will work.
It's hard to appreciate this problem without personal experience. In July 1976 I was in the science control room of Viking 1 as it fired its engines and headed for the surface. It was rather frightening; the thing fell like a rock. All we could look at was a simple, small black and white tv screen that showed Viking's altitude in feet and velocity in feet per second. Maybe there was other data, but I don't remember what. Whenever you did the division, it always seemed like it was going to impact in less than 30 seconds. Of course, at first Viking was mostly moving forward, not downward, so the numbers were a bit deceiving. But if I recall, the whole operation took less than 5 minutes. When you fire the deorbit burn in low Mars orbit, you go down, and fast!

But you can fire retro rockets before you get to the atmosphere can't you? That's why I say we shouldn't be parsimonious with the propellant for the human flight. We might throw in a bit of orbital capture as well I think, there's nothing wrong with a couple of weeks of orbiting to help slow things down if that is indeed of assitance to the EDL process.

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