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There has been enough talk on whether it is feasible for the HAB to land close enough to the earth return vehicle that I feel it is worth its own discussion. The notion that you can’t land accurately is simply wrong. It is just if you try to slow down by aero breaking small errors in the entry can lead to large errors where you land. Atmospheric disturbance such as wind and turbulence can further complicate the issue. However if you use enough fuel you can fire your reverse thrusters and land softly or put another way say you land in the wrong spot aerobreaking and you have fuel left you can fire up those rockets again and get closer. So what we should discuss is how much fuel you need to achieve a given landing accuracy.
Another question is if your landing is way off is it reasonable to drive to the MAV and what kind of a vehicle do you need to make the journey. This will depend largely on the terrain around the landing site and how quickly you can drive across it. Consequently some discussion of the landing sight is also warranted in this thread.
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I don't see it as a major concern. Zubrin discusses it in the book and I agree with his opinions.
The ship doesn't necessarily have to aerobrake and land. It can aerobrake to orbit then make adjustments to fine tune the landing if necessary. I'm sure the computers will control the entry with manual (pilot) backup. The only problem I can imagine is if there is some kind of malfunction with the landing motor. I don't know if there is only one or a set of landing motors but either way if it or they malfunction it doesn't matter anyway because they're all going to be dead.
Wind is just not much of a factor. Mars maximum wind speeds equal the force of 6 mph wind on the earth. NASA pilots should have no problems putting it right in the next parking space without touching a line.
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"The notion that you can’t land accurately is simply wrong. It is just if you try to slow down by aero breaking small errors in the entry can lead to large errors where you land... However if you use enough fuel you can fire your reverse thrusters... and you have fuel left you can fire up those rockets again and get closer."
Yes and no.
1: You cannot fire the main landing engines during aerobraking, so you cannot effect small course changes early in the decent which would substantially effect your trajectory. This is the time that you want them most, where you get the most bang for your buck (fuel mass) but this is where you can't have them, forcing you to wait until you are lower and slower to jettison the shield and ignite.
2: Yes you can make signifigant course corrections at low altitudes and speeds, but it comes at a price... Fuel. Fuel fuel fuel. LOTS of fuel. With enough fuel, you could hover helecopter like anywhere you want to go over the surface, but the amount of fuel needed for even a short hop is just horrendeous.
A large portion of the actual MarsDirect/NASA-DRM payloads that reach Mars are taken up with landing fuel, and that is just enough for minor changes and for a reliable soft landing. Absolutely not enough for very large landing site changes. Take note that it requires far more fuel to hover or radically extend touch down time then it does to simply cusion landing and jink a kilometer or two.
So actually that notion of accurate landing being impractical is true, since the amount of landing fuel is an extreme constraint.
[i]"The power of accurate observation is often called cynicism by those that do not have it." - George Bernard Shaw[/i]
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This brings me to an interesting observation. The crew vehicle is the vehicle we want to land as accurately as possible. The more mass we have on the crew vehicle the more fuel it will take to land accurately. If we reduce the mass of the vehicle the crew is landing in then more fuel will be available for low velocity course corrections. Hence it may not be a good idea to put a pressurized rover in the vehicle the crew lands in. If an interplanetary transfer vehicle is used the necessary payload for the crew landing vehicle could be kept very small. The landing vehicle could be used later as a MAV in a future mission if it is a necessary requirement to have a fully fueled MAV waiting on mars or the current mission otherwise. The HAB can be prelanded and if mobile could take the place of the pressurized rover. Otherwise the pressurized rover could be landed in a separate drop and autonomously driven to the HAB. If the pressurized rover doesn’t reach the HAB then mission planners can decide to continue with the mission or wait for another pressurized rover dropped.
Other issuers are if you use a lot of coarse corrections if the extra weight of the heat shield needed? Why is it difficult to make course corrections earlier on during arobreaking? Is it because any speed changes would increase the heating effects too much? Is their any kind of cooling system that might help solve this problem? Evaporated dry ice cooling?
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For the moment, the reuseable transfer vehicle option doesn't apear to be favored since it would require so much fuel to make both legs of the trip without refueling. A huge insitu fueled MAV that serves as TEI stage is of questionable practicality too since low MAV weight to orbit is vital to keep the mission reasonably small.
So basically you want to trade the rover, which will be needed anyway, for extra fuel on the lander? That doesn't make sense, now you have to find another way to send the rover and you don't get the extra safety of having a second LSS system plus long range mobility if even the extra fuel doesn't land you near the MAV.
The whole point is that since the manned lander doesn't have the ability to return to Earth or an orbiting ERV on its own, that there must be a safety net to reach it in case of landing error, just throwing more rocket fuel at it isn't the best idea if there is a good chance of 100's of kilometers of error.
Remote control of the rover is a scarry idea to me, there are too many things that can go wrong with it, and communicating with it by remote may not be practical over long distances at all. It will be slow going unless you have a human driver anyway. Using the rover as the HAB is clearly impractical.
As far as the actual entry sequence...
The Martian atmosphere is still thick enough to destroy the vehicle if it has no heat shield during entry. This shield must also be oriented along the angle of decent in order to work properly.
On a more fundimental level, due to the high air pressures associated with the firey plunge, small verneer engines probobly wouldn't be able to provide enough thrust to effect even a small nudge without being too heavy.
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I'm with Dook on this one.
Let's not forget that ERV 2 will be following the Hab by a few months and can land near the Hab.
The hab also has enough food for 3 years and can wait for resupply if the second ERV misses its mark.
There are just too many back up plans to worry about landing accuracy.
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I personally don't think it will be a major issue provided the long-range rover rides in the HAB module... as far as a second ERV arriving, I'm not too sure about that. If the landing error isn't very good, then even if you throw a dozen ERVs at the stranded HAB your chances of a rescue aren't nessesarrily very good.
The other big thing is abort options, that if something goes wrong on the HAB and it becomes unliveable, then the HAB crew would be stuck if they didn't have access to an ERV/MAV. The rover however might provide medium-term accomodations with its seperate LSS systems.
Other atmospheric entry issues:
The main landing engines don't have the fuel or thrust to reasonably land on Mars without the aid of a heat shield to slow the landers' velocity.
The position of the heat shield, either on the side of the vehicle or on the bottom covering the engines, will preclude main engine firing during this early phase of the decent.
A 180 degree "flip" is not desireable, even the <90 degree one demanded by the biconic aeroshell design is a little scarry.
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As far as the actual entry sequence...
The Martian atmosphere is still thick enough to destroy the vehicle if it has no heat shield during entry. This shield must also be oriented along the angle of decent in order to work properly.
On a more fundimental level, due to the high air pressures associated with the firey plunge, small verneer engines probobly wouldn't be able to provide enough thrust to effect even a small nudge without being too heavy.
How about altering the pressure forces on the vehicle some how to either turn the heat shield up some to slow the vehicle down or tilt it forward to speed the vehicle up or sideways to cause it to turn. If a fin is used the turning hinge of the fin would be protected from the shock wave and the fin could be pushed into the shockwave though hydraulics. If that lead to too much unpredictable turbulence or to violent a turning effect maybe air could be brought though the heat shield through pipes and steered by either a rudder at the end of one pipe or choosing which pipes are opened or closed. If the shape of the vehicle was near unstable then even small changes in the stresses on the shield could cause it to turn. I can see there are a lot of engineering challenges in making an accurate landing system while keeping the fuel weight down. I hope you don’t mind a little bit of speculation on how it might be done.
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The position of the heat shield, either on the side of the vehicle or on the bottom covering the engines, will preclude main engine firing during this early phase of the decent.
A 180 degree "flip" is not desireable, even the <90 degree one demanded by the biconic aeroshell design is a little scarry.
So the heat shield must cover a large part of the vehicle? Would this depend on what kind of thermal protection system we have on the vehicle besides the heat shield? Platinum coated nozzles? Half joking?
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as far as a second ERV arriving, I'm not too sure about that. If the landing error isn't very good, then even if you throw a dozen ERVs at the stranded HAB your chances of a rescue aren't nessesarrily very good.
Kurtosis can work against you hear. The only way the second hab will significantly help is if you accuracy is good to begin with and a large landing error as an abnormality and not something typical.
QUESTION: My computer program has a function that provides me with what it calls "basic statistics." Among those are Skew and Kurtosis. Your book on testing says that abnormally skewed and peaked distributions may be signs of trouble and that problems may then arise in applying testing statistics. What are the acceptable ranges for these two statistics and how will they affect the testing statistics if they are outside those limits? - Paul Jacquith
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I thought about something like that, it might be worth looking into but it won't be easy. Something like a set of deployable strakes around the edges of the heat shield to adjust trajectory. Its a possibility but the engineering will be a tricky... I have to wonder if such a mechanism able to withstand the heat and forces would be light weight enough.
And then there is the problem of actually tracking your own decent or changes to it, like how would you know you were on or off course or not. With all the ionization, locking on to landing beacon(s) may not be possible, and gyros might not handle the G-forces/vibrations accuratly.
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Lets think about the geometry for a minute...
Either you have the MarsDirect style, where the heat shield is on the bottom of the HAB covering the engines. You obviously can't fire them like this, and cutting holes in the heat shield isn't practical because the reentry gasses would probobly destroy the engines, or at least the seals around the nozzle and destroy the vehicle.
Plan B is the biconic aeroshell like on NASA DRM, where the whole top 2/3rds of the vehicle is a done shape, with one side being coverd with heat shield material all the way up to the tip. The vehicle enters on its side at an angle like the Shuttle enters at an angle, and here the engines won't do you alot of good since they aren't pointing the right direction and you have no retro rockets.
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As far as tracking goes how about a simple cantilever strain gauge accelerometer. This would help you keep track of your position between satellite corrections. As far as the geometry of the heat shield and if it possible to build one that you can steer I don’t really feel I have much knowledge to help on this issue. I hadn’t studied any entry systems in school or for my own interest. I have also never taken a course on aerodynamics. I just have a belief something is possible. I am not sure how difficult such a system would be to engineer but testing it would be perhaps a very costly Endeavour.
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and cutting holes in the heat shield isn't practical because the reentry gasses would probobly destroy the engines, or at least the seals around the nozzle and destroy the vehicle.
I was thinking that maybe you could cool the gas by forcing it to expand. Maybe by a series of divergent pipes coated in platnum and cooled by liquid helium or something.
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Sort of like this concept from lockheed:
http://www.projectconstellation.us/article....alb=sec
I think if after spiraling down you could pop a parachute in the engine area much like the shuttle landing, to slow it down tremendously and then let it go follow by a slight vertical climb. To which once you reach the max of the top of it pop a nose cone chute and drift down using the developing steering technology. At which point once engines are need fire them up and finish the landing. Have the landing legs extend just prior to landing when the craft is possibly less than 100 ft from the ground.
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Steerable parachutes with a little bit of rocket kick and glide. Interesting idea.
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It isn't so much that you steer the heat shield as the heat shield steers you, as the velocity vector will self-correct the direction the heat shield is pointed to some degree... the G-forces would be pretty extreme I think. Changing the shape of the heat shield, with extending strakes for instance, is the way to go.
No reentry gas cooling scheme will work I don't think, there is just too much of it and too much energy to do away with. Liquid helium won't do the trick even if you could store it. The entry gasses will undermine the edges of the engine nozzles and kaboom.
NASA's DRM vehicle will be a little like this, except no winglets and much fatter. Steerable parachutes will be of limited usefulness for a heavy vehicle since the air is so thin, I don't think they would be worth the effort.
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What are the temperatures during Martian atmospheric entry? Low Mars orbit involves a velocity of about 8,000 miles per hour. That's half the speed in Earth orbit and less speed than when the space shuttle broke up. Yes, one will hit the atmosphere faster than that when coming from Earth; more like 14,000 miles per hour. But one could slow into an elliptical orbit, correct the orbit, slow into a nearly circular orbit, wait and correct it, then make a third pass for landing. With satellites and based on your previous two aeropasses, you'd know the density of the upper atmosphere fairly well. Under those circumstances, one should be able to land pretty accurately, no? Even Viking came down only a few dozen kilometers from the center of its landing ellipse, didn't it?
-- RobS
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Even if you know the density and altitude, that doesn't make kilometer-accurate entry easy. I wouldn't risk such a maneuver myself.
At the moment, landing is literally hit or miss... we've got lucky, and sometimes unlikly too. Our precision isn't very good.
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And then there is the problem of actually tracking your own decent or changes to it, like how would you know you were on or off course or not. With all the ionization, locking on to landing beacon(s) may not be possible, and gyros might not handle the G-forces/vibrations accuratly.
What if prior to the manned mission 3 or 4 satellites were put in orbit to be used as a basic gps system, will the ionization affect reception of those? With only a few satellites the system would not be as accurate as Earth GPS systems but if it provided early guidance in the descent until the landing beacon was locked on they may save you being vastly off course early in the descent.
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I tend to think that an acurate landing is certianly very possible. Spirit landed within 200m of it's target, and that was a direct aerocapture. A manned mission, which would presumably aerobreak and be able to correct it's trajetory further before re-entery should be able to perform similarly if not better.
That said, an inacurate landing is still a major risk that should be planned for. Which is why either having the rover land with the hab, or have some method of getting it there is pretty critical.
As for landing correction methods, I am not so optimistic. I think you are pretty much going to have to pick a trajectory from orbit and stick with it until you hit the earth. Manuvering with the heat shield is tricky and dangerous. Overcorrect in this manner and you could potentialy tumble the craft, exposing it's unprotected side, which would spell disaster. Parachutes steerable and otherwise are out all together. Mar's atmosphere is too thin for them to be effective (which is why no mars probe has ever used them).
The only method that can work is using thrusters to correct after re-entery. This is how you are going to have to cushion your landing in any case. But as GCN points out, it is expensive in terms of fuel. Some correction is possible but not much. You will be more worried about doging boulders than you will be about landing down near the ERV.
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Whether we can or can land accurately may be a for gone problem with rising fear of possible bacterial contamination.
Not only of crew but also with respect to a sample return.
Will astronauts return from Mars?
With new evidence that bacteria could live on Mars, a leading scientist is calling on NASA to improve procedures to prevent astronauts from bringing contamination back to Earth. If necessary, that could mean the astronauts would have to spend the rest of their lives on Mars
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Parachutes steerable and otherwise are out all together. Mar's atmosphere is too thin for them to be effective (which is why no mars probe has ever used them).
Actually parachutes have been used on every recent mars probe and they've worked quite well.
After separation of the heat shield the mars probes were travelling at a speed of 7,600 m/s and an altitude of 125 km when the parachutes deployed. The parachutes slowed the vehicle down to 52-64 m/s and at a height of 50-70 m the rockets fired to further slow the probes.
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Yes plain old circular parachutes have, I'm thinking steerable ones used to actually glide and direct the landing... the low maximum speed those work at and the thin air make them impractical.
Packing extra landing fuel is another strike I think against the MarsDirect arcitecture. If extra fuel is called for, there won't be any way to carry it without going nuclear.
I put a little more faith into heat shield geometry changes, like strake flaps around the edges. The high corrective force from the rest of the heat shield and the small changes nessesarry make it worth looking into.
This idiocy about biological contamination is anti-science, precautionary-principle, feelings-not-fact stupidity... rest of their lives indeed, nonsense, that is kind of opinion that people who don't want to see us leave the Earth spout. The crew will have a solid two and a half years to find out if there are any microorganisms before reaching Earth, and since the only part of the vehicle coming back is the crew return capsule (probobly CEV derived), that could be herimetically sealed and the capsule taken to a biohazard lab... at least they would be back on Earth.
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With new evidence that bacteria could live on Mars, a leading scientist is calling on NASA to improve procedures to prevent astronauts from bringing contamination back to Earth. If necessary, that could mean the astronauts would have to spend the rest of their lives on Mars
*Give me a break. The Apollo astronauts were safely decontaminated when they came back to Earth; they didn't have to spend the rest of their lives on the Moon!
Any excuse not to do Mars. The cards are being stacked against it, even to the point of being obvious and lame.
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