Landing on Duna w/o Drogue Chute

[AeroGav]

48 minutes ago, arkie87 said:

I landed safely with retroburn. I think indicator finally turned green at around 200 m/s. Still not sure why this speed is LESS than on Kerbin? Seems to me like it should be larger than on Kerbin if drag/stagnation pressure is to blame. 

Sounds like the 'chute is bound by mach number then, not absolute speed or dynamic pressure (or temperature).    Due to low air pressure and temperature, the speed of sound is less on Duna than on Kerbin.      Yep, a lot of stock parts aren't really set up with offworld use in mind.  We could do with some nuclear turbojets and jet engines that run off only oxidizer when in planets with a methane atmosphere, for starters.

[Snark]

I'm trying to understand why everyone keeps saying that chutes should work at higher speeds on Duna because the "atmosphere is thinner". Surely the planet is irrelevant? (Or, if it is relevant, it's because of some difference like temperature or molar mass, not absolute pressure. But I don't see anyone talking about that.)

It's meaningless to say that planet A has a different pressure than planet B. All planets have the same pressure... they just have those pressures at different altitudes. Pick me any altitude on Duna and there will be an altitude on Kerbin that has exactly the same pressure. It just happens to be at a higher altitude.

It's meaningful to say that one planet has a higher SURFACE pressure than another, since that affects landing. But that's not what we're talking about here. We're talking about how fast you can go without ripping off your chutes.

How fast you go is a function of air pressure. For your reentering ship to slow down to, say, 250 m/s will happen when the air pressure reaches a certain level. That will happen at roughly the same pressure whether you are on Kerbin or Duna, it just happens to be at a much lower altitude on Duna (possibly underground) But as long as it's above ground, why would one expect the speed tolerance of the chutes to be different on the two planets, if you're hitting the chutes at about the same pressure level in both places?

 

Edited December 2, 2015 by Snark

[arkie87]

13 minutes ago, Snark said:

I'm trying to understand why everyone keeps saying that chutes should work at higher speeds on Duna because the "atmosphere is thinner". Surely the planet is irrelevant? (Or, if it is relevant, it's because of some difference like temperature or molar mass, not absolute pressure. But I don't see anyone talking about that.)

It's meaningless to say that planet A has a different pressure than planet B. All planets have the same pressure... they just have those pressures at different altitudes. Pick me any altitude on Duna and there will be an altitude on Kerbin that has exactly the same pressure. It just happens to be at a higher altitude.

It's meaningful to say that one planet has a higher SURFACE pressure than another, since that affects landing. But that's not what we're talking about here. We're talking about how fast you can go without ripping off your chutes.

How fast you go is a function of air pressure. For your reentering ship to slow down to, say, 250 m/s will happen when the air pressure reaches a certain level. That will happen at roughly the same pressure whether you are on Kerbin or Duna, it just happens to be at a much lower altitude on Duna (possibly underground) But as long as it's above ground, why would one expect the speed tolerance of the chutes to be different on the two planets, if you're hitting the chutes at about the same pressure level in both places?

 

When people say Duna's atmosphere is thinner or the pressure is less, they mean the surface pressure is less or the scale height is smaller. But your implied point about density (which is what causes drag, not pressure) is dependent on molar mass and temperature as well as pressure. Surely, the temperature on Duna is less than on Kerbin, so the surface atmospheric density might not be that different (i'm not sure about the composition of Duna's atmosphere though). 

That said, i'm not sure why you say "how fast you go is a function of air pressure" when you implied above it is a function of density, not air pressure. 

36 minutes ago, AeroGav said:

Sounds like the 'chute is bound by mach number then, not absolute speed or dynamic pressure (or temperature).    Due to low air pressure and temperature, the speed of sound is less on Duna than on Kerbin.      Yep, a lot of stock parts aren't really set up with offworld use in mind.  We could do with some nuclear turbojets and jet engines that run off only oxidizer when in planets with a methane atmosphere, for starters.

AFAIK, speed of sound is only dependent on temperature c = sqrt(gamma*R*T); pressure does not play a role, which is slightly counter intuitive.

[Snark]

7 minutes ago, arkie87 said:

When people say Duna's atmosphere is thinner or the pressure is less, they mean the surface pressure is less or the scale height is smaller. But your implied point about density (which is what causes drag, not pressure) is dependent on molar mass and temperature as well as pressure. Surely, the temperature on Duna is less than on Kerbin, so the surface atmospheric density might not be that different (i'm not sure about the composition of Duna's atmosphere though). 

That said, i'm not sure why you say "how fast you go is a function of air pressure" when you implied above it is a function of density, not air pressure. 

Even if you take molar mass and temperature into account, I think it's still a wash.

Perhaps instead of "pressure" I should have said "dynamic pressure."  Your ship slows down because of air resistance.  The amount of air resistance is a combination of multiple factors, including some combination of static pressure, density, temperature, the shape of your ship, etc.

But the fact remains that whatever determines the air-resistance force on your ship is exactly the same "stuff" that determines the air-resistance force on your parachute.  There's nothing magical about a parachute, it's just a gizmo that happens to have N times the air resistance of your ship, where N is a constant for pretty much any set of conditions.

Your ship is going to slow down to 250 m/s when the air resistance reaches a certain level, and that level will be roughly the same regardless of whether you're on Duna or Kerbin.  So the minimum chute deployment speed shouldn't matter between the two places.

That's not to say that the chute physics modeling in KSP is perfect.  If my chutes can withstand a 250 m/s deployment at sea level on Kerbin, then they ought to be able to handle a deployment at considerably higher speeds if I'm up at 15 km where the air is much thinner, right?  But my observation is that the speed at which my chutes are red/yellow/green seems to be about the same regardless of altitude on Kerbin, which seems really wrong.

All I'm saying is that I think Kerbin-versus-Duna is a red herring, here.

Edited December 2, 2015 by Snark

[arkie87]

2 minutes ago, Snark said:

Even if you take molar mass and temperature into account, I think it's still a wash.

Perhaps instead of "pressure" I should have said "dynamic pressure."  Your ship slows down because of air resistance.  The amount of air resistance is a combination of multiple factors, including some combination of static pressure, density, temperature, the shape of your ship, etc.

But the fact remains that whatever determines the air-resistance force on your ship is exactly the same "stuff" that determines the air-resistance force on your parachute.  There's nothing magical about a parachute, it's just a gizmo that happens to have N times the air resistance of your ship, where N is a constant for pretty much any set of conditions.

Your ship is going to slow down to 250 m/s when the air resistance reaches a certain level, and that level will be roughly the same regardless of whether you're on Duna or Kerbin.  So the minimum chute deployment speed shouldn't matter between the two places.

That's not to say that the chute physics modeling in KSP is perfect.  If my chutes can withstand a 250 m/s deployment at sea level on Kerbin, then they ought to be able to handle a deployment at considerably higher speeds if I'm up at 15 km where there air is much thinner, right?  But my observation is that the speed at which my chutes are red/yellow/green seems to be about the same regardless of altitude on Kerbin, which seems really wrong.

All I'm saying is that I think Kerbin-versus-Duna is a red herring, here.

How does drag depend on static pressure and temperature if density is accounted for separately? dynamic pressure = 1/2 Density*Velocity*Velocity?

Your point about Duna vs. Kerbin being a red herring since there are other factors which complicate the comparison is well taken though. 

Either way, it seems like most people agree this is either a bug or an unrealistic simplification made with the limited time the devs have.

 

[Warzouz]

It seems this is a bug, there is few words on it in the Devnote

[Alshain]

20 minutes ago, Warzouz said:

It seems this is a bug, there is few words on it in the Devnote

I saw that, it's either one heck of a coincidence or incredibly fast service (considering this thread started hours before the devnote was posted).  You decide.

Edited December 2, 2015 by Alshain

[Warzouz]

I reported this bug in the support section last week.

here

Edited December 2, 2015 by Warzouz

[Snark]

1 hour ago, arkie87 said:

How does drag depend on static pressure and temperature if density is accounted for separately? dynamic pressure = 1/2 Density*Velocity*Velocity?

Your point about Duna vs. Kerbin being a red herring since there are other factors which complicate the comparison is well taken though. 

Either way, it seems like most people agree this is either a bug or an unrealistic simplification made with the limited time the devs have.

 

Because density is a function of static pressure and temperature, for a given molar mass.  My point was simply that it doesn't really matter what the factors are or how the air resistance is calculated-- just that "will it get shredded or not" is going to be determined by amount of air resistance, and the difference between planets is a difference of altitude rather than anything else.  A given ship going at a given speed will hit a certain amount of air resistance at a given altitude A1 on Kerbin.  It will hit the same air resistance on Duna, just at a much lower altitude A2.  The safe-deploy speed at A1 on Kerbin should be the same as the safe-deploy speed at altitude A2 on Duna.

In any case, yah, there's a bug here. 

[arkie87]

1 hour ago, Warzouz said:

I reported this bug in the support section last week.

here

I guess i was ninja'd

1 hour ago, Warzouz said:

It seems this is a bug, there is few words on it in the Devnote

" as well as tackling an issue where parachutes would overestimate mach effects in low-density situations. "

2 minutes ago, Snark said:

Because density is a function of static pressure and temperature, for a given molar mass. 

But you listed density as its own variable, so then you dont care about static pressure and temperature 

[wrcsubers]

2 hours ago, Snark said:

Your ship is going to slow down to 250 m/s when the air resistance reaches a certain level, and that level will be roughly the same regardless of whether you're on Duna or Kerbin.  So the minimum chute deployment speed shouldn't matter between the two places.

I agree with Snark.  The altitude that this happens at will obviously vary on planets with differing air density/pressure.  But all things being mostly equal (trajectory, ship orientation, etc), a ship being slowed to 250m/s should occur at the same same air pressure as anywhere else.  If you're chute works at 250m/s in one place, it should work in another.

Edited December 2, 2015 by wrcsubers

[Claw]

Yeah, I'm definitely not trying to say there isn't an issue going on. Just that I know for fact that the indicator isn't based on a fixed speed. But the overall "something isn't right" has been echoed elsewhere on the forum and probably merits a proper look.

[OhioBob]

12 hours ago, arkie87 said:

Is that setting not tweakable?

It's tweakable, but the lowest pressure it can be set to is 0.04 atm for main chutes and 0.02 atm for drogues.  That works out to the following altitudes (updated for 1.0.5):

	0.04 atm	0.02 atm
Eve	34 459 m	38 355 m
Kerbin	17 590 m	21 204 m
Duna	4 448 m	9 266 m
Jool	142 580 m	149 810 m
Laythe	21 482 m	27 471 m

If the pressure were set higher, then the maximum deployment altitude would be lower.

[OhioBob]

49 minutes ago, Snark said:

A given ship going at a given speed will hit a certain amount of air resistance at a given altitude A1 on Kerbin.  It will hit the same air resistance on Duna, just at a much lower altitude A2.  The safe-deploy speed at A1 on Kerbin should be the same as the safe-deploy speed at altitude A2 on Duna.

But to produce a given amount of air resistance at a given altitude, a ship will be traveling much faster on Duna than on Kerbin.  Therefore, if I'm going to deploy my parachute at an altitude of 4000 m, I can safely do so at a higher velocity on Duna than I can on Kerbin.

[arkie87]

22 minutes ago, OhioBob said:

It's tweakable, but the lowest pressure it can be set to is 0.04 atm for main chutes and 0.02 atm for drogues.  That works out to the following altitudes (updated for 1.0.5):

	0.04 atm	0.02 atm
Eve	34 459 m	38 355 m
Kerbin	17 590 m	21 204 m
Duna	4 448 m	9 266 m
Jool	142 580 m	149 810 m
Laythe	21 482 m	27 471 m

If the pressure were set higher, then the maximum deployment altitude would be lower.

17 km is pretty high still, and might even be below point of maximum Q.

It's possible the way the game is tweaked now, the lowest pressure limit for parachutes might prevent deployment before drag picks up above limit. However, in real life, this setting can be changed/lowered-- there is no physical reason for this lower limit (unless drag is so low it doesnt even pull the parachute out). 

46 minutes ago, wrcsubers said:

I agree with Snark.  The altitude that this happens at will obviously vary on planets with differing air density/pressure.  But all things being mostly equal (trajectory, ship orientation, etc), a ship being slowed to 250m/s should occur at the same same air pressure as anywhere else.  If you're chute works at 250m/s in one place, it should work in another.

It shouldnt be about pressure. It should be about air density only.

[arkie87]

6 minutes ago, OhioBob said:

But to produce a given amount of air resistance at a given altitude, a ship will be traveling much faster on Duna than on Kerbin.  Therefore, if I'm going to deploy my parachute at an altitude of 4000 m, I can safely do so at a higher velocity on Duna than I can on Kerbin.

Exactly, that's why im confused about @Snark's point. At the same altitude, drag should be less on Duna (unless differences in temperature and atmosphere composition contribute to make density the same).

Edited December 2, 2015 by arkie87

[OhioBob]

I just put together the following graph.  For each planet it gives, for altitudes up to 25000 m, the velocity at which the dynamic pressure is equal to 20000 Pa.  I'm using dynamic pressure because it should be approximately proportional to the tension generated in the parachute lines when the chute is deployed.  The higher the dynamic pressure, the greater the force generated and the more likely the parachute will tear lose on deployment.  I'm using 20000 Pa because that appears to match the conditions on Kerbin when a parachute can be safely deployed (i.e. 260 m/s at 5200 m).

Theoretically, and ignoring heat, it should be safe to deploy a parachute if the conditions place you in the part of the graph above the curve.  If you are below the curve, then the dynamic pressure is likely too high to safely deploy the parachute.  Of course heating should also be considered when determining when it is safe to deploy.

I remind you that this is not what the game does.  It is just an example of what the game could do to determine safe deployment speed.  The 20000 Pa number is an arbitrary threshold; some other similar criteria could be used as the determining factor.

[Snark]

7 hours ago, OhioBob said:

But to produce a given amount of air resistance at a given altitude, a ship will be traveling much faster on Duna than on Kerbin.  Therefore, if I'm going to deploy my parachute at an altitude of 4000 m, I can safely do so at a higher velocity on Duna than I can on Kerbin.

Absolutely-- if we're comparing same-altitude-on-Kerbin with same-altitude-on-Duna.  I think the source of confusion here is which thing is being held constant and which is being treated as a variable.

If you treat altitude as a constant:  yes, at a given altitude on Duna, you ought to be able to pop your chute at a higher speed than at the same altitude on Kerbin.

On the other hand, if you go by "soonest possible moment"-- i.e., not "I'm going to pop chutes at 4000m" but "I'm going to pop chutes as soon as the ship has slowed enough to do so without ripping them off"-- then that ought to be at around the same speed.  It'll just happen to be at a much lower altitude on Duna (possibly underground, meaning you will not be going back to space today).

 

[OhioBob]

2 hours ago, Snark said:

On the other hand, if you go by "soonest possible moment"-- i.e., not "I'm going to pop chutes at 4000m" but "I'm going to pop chutes as soon as the ship has slowed enough to do so without ripping them off"-- then that ought to be at around the same speed.  It'll just happen to be at a much lower altitude on Duna (possibly underground, meaning you will not be going back to space today).

I don't think that's true.  We have to slow the ship enough that we can pop the chutes without them ripping off, but the speed to which we have to slow the ship is that at which the drag on the chutes is low enough not to tear the lines.  Drag is proportional to the dynamic pressure, so we can probably say that it's safe to pop the chutes when the dynamic pressure is below some threshold value.  We have,

v = (2q/ρ)^1/2

where q is the dynamic pressure and ρ is the air density.  If q is a constant, then the velocity at which we can deploy the chutes is a function of air density.  The deployment velocity is not a constant, it depends on where we are.

Just to be clear, my argument is how I think it should be.  This is not necessarily how it currently works in the game.

[Snark]

10 hours ago, OhioBob said:

I don't think that's true.  We have to slow the ship enough that we can pop the chutes without them ripping off, but the speed to which we have to slow the ship is that at which the drag on the chutes is low enough not to tear the lines.  Drag is proportional to the dynamic pressure, so we can probably say that it's safe to pop the chutes when the dynamic pressure is below some threshold value.  We have,

v = (2q/ρ)^1/2

where q is the dynamic pressure and ρ is the air density.  If q is a constant, then the velocity at which we can deploy the chutes is a function of air density.  The deployment velocity is not a constant, it depends on where we are.

Just to be clear, my argument is how I think it should be.  This is not necessarily how it currently works in the game.

Except that I think it's a constant relative to where we are, if we take "first deployable moment" as our target.

Suppose I'm reentering, and I'm happily ripping through the atmosphere at 1200 m/s and decelerating.  At that point, it doesn't matter if the parachutes' safe deploy speed is 250 m/s, or 500 m/s, or even 1000 m/s.  I can't pop them, shredded is shredded, and their safe speed is irrelevant as long as it's less than the ship's.

So we wait for the ship to slow down enough, right?  As the ship descends, it slows down.  And also the atmosphere is getting thicker and the max safe chute speed is dropping, too, but not as fast as the ship's speed.  So if we graph the ship's speed and the max safe chute speed, they're both dropping, but the ship's is dropping faster.  Eventually the two lines will intersect, and that's where I can finally deploy the chutes.

And what I'm saying is, that's why I think it doesn't matter for Duna versus Kerbin in terms of max safe deploy speed.  That intersection will be at about the same velocity in both places.  It will be at a much lower altitude on Duna (possibly underground), but the speed will be about the same.

The thing that makes it not matter is that the forces that determine when you can deploy the chute are the same forces that determine how fast the ship is going as it aerobrakes.

[NathanKell]

Dynamic pressure doesn't take compressibility effects into account.

 

As to the main issue: It is indeed unintended behavior that parachutes have such a low tolerance on Duna. As mentioned in the dev notes, I'm fixing that for 1.1 (the extra stress from transonic/supersonic should fade out with density, but right now it doesn't). The problem here is that 1.0.5 corrected a different bug (one which allowed chutes to survive when they shouldn't) and that made chute deployment on Duna much nastier.

[arkie87]

6 hours ago, Snark said:

Except that I think it's a constant relative to where we are, if we take "first deployable moment" as our target.

Suppose I'm reentering, and I'm happily ripping through the atmosphere at 1200 m/s and decelerating.  At that point, it doesn't matter if the parachutes' safe deploy speed is 250 m/s, or 500 m/s, or even 1000 m/s.  I can't pop them, shredded is shredded, and their safe speed is irrelevant as long as it's less than the ship's.

So we wait for the ship to slow down enough, right?  As the ship descends, it slows down.  And also the atmosphere is getting thicker and the max safe chute speed is dropping, too, but not as fast as the ship's speed.  So if we graph the ship's speed and the max safe chute speed, they're both dropping, but the ship's is dropping faster.  Eventually the two lines will intersect, and that's where I can finally deploy the chutes.

And what I'm saying is, that's why I think it doesn't matter for Duna versus Kerbin in terms of max safe deploy speed.  That intersection will be at about the same velocity in both places.  It will be at a much lower altitude on Duna (possibly underground), but the speed will be about the same.

The thing that makes it not matter is that the forces that determine when you can deploy the chute are the same forces that determine how fast the ship is going as it aerobrakes.

I dont really see your point. At the same altitude above surface, one should be able to deploy parachutes at higher speeds at lower density because drag force at a given speed is lower. That's all that matters, and that is the point of this thread. 

[arkie87]

5 minutes ago, NathanKell said:

Dynamic pressure doesn't take compressibility effects into account.

By this, i assume you mean: 

q = 1/2 rho U^2

instead of

q=  p(1+(γ-1)/2 M^2)^(γ/(γ-1)) - p

Also, in KSP, is Mach number calculated from (γ*R*T)^0.5 i.e. does it depend on temperature and atmospheric composition?

 

 

[OhioBob]

5 hours ago, Snark said:

Except that I think it's a constant relative to where we are, if we take "first deployable moment" as our target.

Suppose I'm reentering, and I'm happily ripping through the atmosphere at 1200 m/s and decelerating.  At that point, it doesn't matter if the parachutes' safe deploy speed is 250 m/s, or 500 m/s, or even 1000 m/s.  I can't pop them, shredded is shredded, and their safe speed is irrelevant as long as it's less than the ship's.

So we wait for the ship to slow down enough, right?  As the ship descends, it slows down.  And also the atmosphere is getting thicker and the max safe chute speed is dropping, too, but not as fast as the ship's speed.  So if we graph the ship's speed and the max safe chute speed, they're both dropping, but the ship's is dropping faster.  Eventually the two lines will intersect, and that's where I can finally deploy the chutes.

And what I'm saying is, that's why I think it doesn't matter for Duna versus Kerbin in terms of max safe deploy speed.  That intersection will be at about the same velocity in both places.  It will be at a much lower altitude on Duna (possibly underground), but the speed will be about the same.

The thing that makes it not matter is that the forces that determine when you can deploy the chute are the same forces that determine how fast the ship is going as it aerobrakes.

OK, I see what you're saying.  I agree in principal, but I'm still not convinced that the intersection will occur at about the same velocity everywhere.

For example, on Kerbin we can typically deploy our parachutes at a velocity of about 260 m/s at an altitude of about 5-6 km.  Under these conditions the dynamic pressure is about 20 kPa.  Based on this, we should be able to say that our parachutes can withstand a dynamic pressure of about 20 kPa without failing.  During a typical entry at Duna, the dynamic pressure never reaches 20 kPa.  The curves don't cross because we are already below the theoretical safe deployment velocity.  We should be able to pop the chutes at anytime (ignoring heat) without them failing.

[arkie87]

34 minutes ago, OhioBob said:

OK, I see what you're saying.  I agree in principal, but I'm still not convinced that the intersection will occur at about the same velocity everywhere.

For example, on Kerbin we can typically deploy our parachutes at a velocity of about 260 m/s at an altitude of about 5-6 km.  Under these conditions the dynamic pressure is about 20 kPa.  Based on this, we should be able to say that our parachutes can withstand a dynamic pressure of about 20 kPa without failing.  During a typical entry at Duna, the dynamic pressure never reaches 20 kPa.  The curves don't cross because we are already below the theoretical safe deployment velocity.  We should be able to pop the chutes at anytime (ignoring heat) without them failing.

Exactly. Snark's logic only works if we start above the drag threshold, but on Duna, we are always below it (that's why we come in careening at 800 m/s at 3 km above the surface)