4 ms·

Can someone explain to me why it isn't possible to steer the capsule with the heat shield such that it can be used to deplete virtually all of its speed? I can

by jsmcgd 14y ago

Can someone explain to me why it isn't possible to steer the capsule with the heat shield such that it can be used to deplete virtually all of its speed? I can imagine a system where the capsule pitches up near the end of its run so that it starts gaining altitude and then deploys a parachute for landing once it has reached its apogee. I appreciate that their system probably is the best design as they are a smart bunch, however I haven't heard the reason's why some of the more conventional mechanisms weren't used. Another thing is the crane mechanism, which is to protect the rover from the debris kicked up from the rocket motors. I was wondering why the rover couldn't just be encased in a simple lightweight protective box and then placed directly on the deck? Does anyone have any links to the other designs that were considered and rejected?

gus_massa 14y ago
I think that to do that maneuver you must have wings, and the atmosphere of Mars is thin, so you need bigger wings, and the structure to hold the wings in place give them enough strength. So I think it's easier to use a parachute.
Sharlin 14y ago
I don't think the capsule has the right aerodynamic properties to allow it to work as a proper lifting body in the 0.01 ATM Martian atmosphere. We don't really have any experience about lifting bodies on Mars, even less than we have about huge supersonic parachutes. You can't land on Mars with just a parachute anyway, not with a 900 kg payload. The main problem with Mars is that its atmosphere is just dense enough that it complicates things but not dense enough that it's really helpful.
- jsmcgd 14y ago
  Apparently according to this video [1] (about half way through) the capsule is a lifting body however I was thinking of something like a simple guided arrow, perhaps some fins that are extended on a telescopic shaft. You could possibly then put a tractor rocket at the end of this shaft to slow descent and keep the rocket plumes away from the main body and the ground. Regarding parachutes, is it not the case that if you need more performance, that you simply increase their size? [1] http://www.youtube.com/watch?feature=player_embedded&v=SwXe_X4UKoM http://www.youtube.com/watch?feature=player_embedded&v=S...
  - Tuna-Fish 14y ago
    Unless the atmosphere is so thin that the added weight of the parachute and it's support hurts more than it helps. 0.6 kPa is really that bad. The effectiveness of a parachute grows at a square of your airspeed. This means that every planet has a minimum speed you can successfully slow down to using parachutes with a given system. For realistic loads and conventional materials, on Mars that is about 50m/s. Which is just too fast.
  - ballooney 14y ago
    > I was thinking of something like a simple guided arrow, perhaps some fins that are extended on a telescopic shaft It's one thing to say it, it's another to design it such it has a chance of working. Telescoping mechanisms with deployable aerodynamic structures sounds very complicated to me, compared to the thrusters they're using. > You could possibly then put a tractor rocket at the end of this shaft to slow descent and keep the rocket plumes away from the main body and the ground. The sky crane already does this. > Regarding parachutes, is it not the case that if you need more performance, that you simply increase their size? 'Performance' is lots of things, presumably you mean drag. Increasing the diameter does increase the drag, but it also increases the opening forces, which requires a stronger structure, which means more mass. You don't have the luxury of much margin when every gram in your landing system has cost the us taxpayer thousands of dollars and is a gram that you can't use for science.
pigboy 14y ago
The capsule and heat shield do deplete virtually all of its speed. With Mars' thin atmosphere, the terminal velocity is still quite high and the parachute/rocket combination is needed to take it all the way down. AFAIK this is a relatively conventional system, with the exception of the very last moment of touchdown, which uses descent rockets and the sky crane. Perhaps the most similar landing was by Viking, which made the final descent on rockets. Those thrusters were expressly designed (that is, compromised) to produce as little disturbance of the landing site as possible. The only thing Curiosity is doing that is really new is the sky crane. If there are any errors in understanding the dynamics of rocket plus lengthening pendulum in Mars gravity, then all is lost. I think it is likely that some extra attention has been paid to this part of the mission.
lmm 14y ago
>I can imagine a system where the capsule pitches up near the end of its run so that it starts gaining altitude and then deploys a parachute for landing once it has reached its apogee. The martian atmosphere is really thin (I've heard it said that it would pass for vacuum in a high school science classroom). They've got that giant parachute doing as much deceleration-through-atmospheric-drag as possible, and it's still not enough. (And as a sibling post said, fancy atmospheric manoeuvring would require huge wings) >Another thing is the crane mechanism, which is to protect the rover from the debris kicked up from the rocket motors. I was wondering why the rover couldn't just be encased in a simple lightweight protective box and then placed directly on the deck? A box with rocket motors on? That'd still churn up the ground that the rover then has to drive through to go anywhere. Or do you mean dropping it without the cable? The current rovers (MER) were deployed that way, with airbags, but this one's too big to do that with.
- jsmcgd 14y ago
  Yeah I was thinking of landing it without the cables, inside a protective box and then opening the box once all the dust and debris has settled or blown away. You wouldn't need any airbags as the current system is able to achieve a descent of only 1.5mph, meaning it could be placed gently on the surface.
  - lmm 14y ago
    Right, but even after the dust has blown away your rocket downdraft has basically dug a pit that your rover is now at the bottom of.
    - HeyLaughingBoy 14y ago
      You also need to consider that the landing may be successful but the vehicle's drive system may be rendered inoperable for any number of reasons. In that case, you'd want the surrounding ground to be as undisturbed/uncontaminated as possible so you could still do some science in that location without being able to move around.
maaku 14y ago
> Can someone explain to me why it isn't possible to steer the capsule with the heat shield such that it can be used to deplete virtually all of its speed? They do exactly that. But Mars' atmosphere is so thin that terminal velocity of the capsule is still supersonic. > I can imagine a system where the capsule pitches up near the end of its run so that it starts gaining altitude and then deploys a parachute for landing once it has reached its apogee. Parachutes don't help enough in 0.007atm. Even with the biggest chutes possible (remember: they're heavy!), you'd still be hurtling towards the ground like a missile. > Another thing is the crane mechanism, which is to protect the rover from the debris kicked up from the rocket motors. I was wondering why the rover couldn't just be encased in a simple lightweight protective box and then placed directly on the deck? Weight. Believe it or not, the sky crane delivers more payload to the surface. > I appreciate that their system probably is the best design as they are a smart bunch, however I haven't heard the reason's why some of the more conventional mechanisms weren't used. > > Does anyone have any links to the other designs that were considered and rejected? http://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/20090007730_2009006430.pdf http://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/2009000...
- ballooney 14y ago
  > They do exactly that. But Mars' atmosphere is so thin that terminal velocity of the capsule is still supersonic. Also, that kind of blunt body is stable hypersonically, but unstable supersonically. As soon as you get to a mach number where a parachute can deploy stably (about mach 1.8 and below) you deploy it to stabilise the thing, and of course the drag is much greater than entry body alone. >> Another thing is the crane mechanism, which is to protect the rover from the debris kicked up from the rocket motors. I was wondering why the rover couldn't just be encased in a simple lightweight protective box and then placed directly on the deck? The point of the sky crane is not just to avoid kicking up dust, this notion seems to have become the received wisdom in the media though. The reasons include: -Having rovers drive off from on top of things is difficult, it puts the CoG of the lander right up and if you have to drive off a sloped ramp having landed on a slope, the ramp can be really very sloped. -Rocket forces nearer the ground can be quite an interesting control problem, in terms of the forces generated when you're near the ground and where all the exhaust goes. -Having rocket fuel tanks landing on spikey rocks has the potential to really ruin your day. -Knowing when to turn the rocket engines off (i.e. successfully detecting landing) is actually quite a hard problem. It's a very noisy problem, and getting it wrong was what destroyed Mars Polar Lander (i.e. it thought it landed when it was still up in the air, and so fell out of the sky). The skycrane has the rocket motors on the whole time and goes off an crashes. Lots of this is explained in the video I linked to somewhere else in this family of comments.
ballooney 14y ago
Everybody just watch this. It's Tom Rivellini, one of the lead mechanical designers of the landing mechanism, explaining how it works and how it evolved and what was considered and what was rejected. It's not a pithy over-produced 4 minute video, it's the real stuff for the technically interested. It answers all your questions. http://www.jpl.nasa.gov/events/lectures_archive.cfm?year=2009&month=8 http://www.jpl.nasa.gov/events/lectures_archive.cfm?year=200... > I haven't heard the reason's why some of the more conventional mechanisms weren't used. What's a conventional mechanism when you're landing on another planet?

by jsmcgd 14y ago

gus_massa 14y ago

I think that to do that maneuver you must have wings, and the atmosphere of Mars is thin, so you need bigger wings, and the structure to hold the wings in place give them enough strength. So I think it's easier to use a parachute.

Sharlin 14y ago

I don't think the capsule has the right aerodynamic properties to allow it to work as a proper lifting body in the 0.01 ATM Martian atmosphere. We don't really have any experience about lifting bodies on Mars, even less than we have about huge supersonic parachutes. You can't land on Mars with just a parachute anyway, not with a 900 kg payload. The main problem with Mars is that its atmosphere is just dense enough that it complicates things but not dense enough that it's really helpful.

jsmcgd 14y ago
Apparently according to this video [1] (about half way through) the capsule is a lifting body however I was thinking of something like a simple guided arrow, perhaps some fins that are extended on a telescopic shaft. You could possibly then put a tractor rocket at the end of this shaft to slow descent and keep the rocket plumes away from the main body and the ground. Regarding parachutes, is it not the case that if you need more performance, that you simply increase their size? [1] http://www.youtube.com/watch?feature=player_embedded&v=SwXe_X4UKoM http://www.youtube.com/watch?feature=player_embedded&v=S...
- Tuna-Fish 14y ago
  Unless the atmosphere is so thin that the added weight of the parachute and it's support hurts more than it helps. 0.6 kPa is really that bad. The effectiveness of a parachute grows at a square of your airspeed. This means that every planet has a minimum speed you can successfully slow down to using parachutes with a given system. For realistic loads and conventional materials, on Mars that is about 50m/s. Which is just too fast.
- ballooney 14y ago
  > I was thinking of something like a simple guided arrow, perhaps some fins that are extended on a telescopic shaft It's one thing to say it, it's another to design it such it has a chance of working. Telescoping mechanisms with deployable aerodynamic structures sounds very complicated to me, compared to the thrusters they're using. > You could possibly then put a tractor rocket at the end of this shaft to slow descent and keep the rocket plumes away from the main body and the ground. The sky crane already does this. > Regarding parachutes, is it not the case that if you need more performance, that you simply increase their size? 'Performance' is lots of things, presumably you mean drag. Increasing the diameter does increase the drag, but it also increases the opening forces, which requires a stronger structure, which means more mass. You don't have the luxury of much margin when every gram in your landing system has cost the us taxpayer thousands of dollars and is a gram that you can't use for science.

pigboy 14y ago

The capsule and heat shield do deplete virtually all of its speed. With Mars' thin atmosphere, the terminal velocity is still quite high and the parachute/rocket combination is needed to take it all the way down. AFAIK this is a relatively conventional system, with the exception of the very last moment of touchdown, which uses descent rockets and the sky crane. Perhaps the most similar landing was by Viking, which made the final descent on rockets. Those thrusters were expressly designed (that is, compromised) to produce as little disturbance of the landing site as possible. The only thing Curiosity is doing that is really new is the sky crane. If there are any errors in understanding the dynamics of rocket plus lengthening pendulum in Mars gravity, then all is lost. I think it is likely that some extra attention has been paid to this part of the mission.

lmm 14y ago

>I can imagine a system where the capsule pitches up near the end of its run so that it starts gaining altitude and then deploys a parachute for landing once it has reached its apogee. The martian atmosphere is really thin (I've heard it said that it would pass for vacuum in a high school science classroom). They've got that giant parachute doing as much deceleration-through-atmospheric-drag as possible, and it's still not enough. (And as a sibling post said, fancy atmospheric manoeuvring would require huge wings) >Another thing is the crane mechanism, which is to protect the rover from the debris kicked up from the rocket motors. I was wondering why the rover couldn't just be encased in a simple lightweight protective box and then placed directly on the deck? A box with rocket motors on? That'd still churn up the ground that the rover then has to drive through to go anywhere. Or do you mean dropping it without the cable? The current rovers (MER) were deployed that way, with airbags, but this one's too big to do that with.

jsmcgd 14y ago
Yeah I was thinking of landing it without the cables, inside a protective box and then opening the box once all the dust and debris has settled or blown away. You wouldn't need any airbags as the current system is able to achieve a descent of only 1.5mph, meaning it could be placed gently on the surface.
- lmm 14y ago
  Right, but even after the dust has blown away your rocket downdraft has basically dug a pit that your rover is now at the bottom of.
  - HeyLaughingBoy 14y ago
    You also need to consider that the landing may be successful but the vehicle's drive system may be rendered inoperable for any number of reasons. In that case, you'd want the surrounding ground to be as undisturbed/uncontaminated as possible so you could still do some science in that location without being able to move around.

maaku 14y ago

> Can someone explain to me why it isn't possible to steer the capsule with the heat shield such that it can be used to deplete virtually all of its speed? They do exactly that. But Mars' atmosphere is so thin that terminal velocity of the capsule is still supersonic. > I can imagine a system where the capsule pitches up near the end of its run so that it starts gaining altitude and then deploys a parachute for landing once it has reached its apogee. Parachutes don't help enough in 0.007atm. Even with the biggest chutes possible (remember: they're heavy!), you'd still be hurtling towards the ground like a missile. > Another thing is the crane mechanism, which is to protect the rover from the debris kicked up from the rocket motors. I was wondering why the rover couldn't just be encased in a simple lightweight protective box and then placed directly on the deck? Weight. Believe it or not, the sky crane delivers more payload to the surface. > I appreciate that their system probably is the best design as they are a smart bunch, however I haven't heard the reason's why some of the more conventional mechanisms weren't used. > > Does anyone have any links to the other designs that were considered and rejected? http://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/20090007730_2009006430.pdf http://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/2009000...

ballooney 14y ago
> They do exactly that. But Mars' atmosphere is so thin that terminal velocity of the capsule is still supersonic. Also, that kind of blunt body is stable hypersonically, but unstable supersonically. As soon as you get to a mach number where a parachute can deploy stably (about mach 1.8 and below) you deploy it to stabilise the thing, and of course the drag is much greater than entry body alone. >> Another thing is the crane mechanism, which is to protect the rover from the debris kicked up from the rocket motors. I was wondering why the rover couldn't just be encased in a simple lightweight protective box and then placed directly on the deck? The point of the sky crane is not just to avoid kicking up dust, this notion seems to have become the received wisdom in the media though. The reasons include: -Having rovers drive off from on top of things is difficult, it puts the CoG of the lander right up and if you have to drive off a sloped ramp having landed on a slope, the ramp can be really very sloped. -Rocket forces nearer the ground can be quite an interesting control problem, in terms of the forces generated when you're near the ground and where all the exhaust goes. -Having rocket fuel tanks landing on spikey rocks has the potential to really ruin your day. -Knowing when to turn the rocket engines off (i.e. successfully detecting landing) is actually quite a hard problem. It's a very noisy problem, and getting it wrong was what destroyed Mars Polar Lander (i.e. it thought it landed when it was still up in the air, and so fell out of the sky). The skycrane has the rocket motors on the whole time and goes off an crashes. Lots of this is explained in the video I linked to somewhere else in this family of comments.

ballooney 14y ago

Everybody just watch this. It's Tom Rivellini, one of the lead mechanical designers of the landing mechanism, explaining how it works and how it evolved and what was considered and what was rejected. It's not a pithy over-produced 4 minute video, it's the real stuff for the technically interested. It answers all your questions. http://www.jpl.nasa.gov/events/lectures_archive.cfm?year=2009&month=8 http://www.jpl.nasa.gov/events/lectures_archive.cfm?year=200... > I haven't heard the reason's why some of the more conventional mechanisms weren't used. What's a conventional mechanism when you're landing on another planet?