Surviving 8 km/s Duna aerocapture

[ej89]

Hi,

Using the oft-recommended KSP Launch Window Planner (http://alexmoon.github.io/ksp/), I'm attempting to set up a mission that will take a very aggressive (totally non-Hohmann) transfer to Duna. The idea is to shave time off the transfer (and depart a good ~80-100 days before the planets are "ideally" aligned) by spending a lot more delta-v than would be "optimal". I'm about to unlock the ISRU converter and build out a Mun mining base with Extraplanetary Launchpads, so my thinking is "fuel is cheaper than time". (I don't want to time-warp through the long coast because I've got lots of contracts and station stuff to keep me busy in the meantime back in the Kerbin system.)

I've built a ship with about 5300 m/s of delta-v, and have been doing dry runs of the mission in a sandbox save. Figuring I could use aerocapture to avoid the most expensive part of the trip (the braking burn at Duna periapsis to get into orbit), I put almost all of the delta-v (5000 m/s) into the ejection burn from Kerbin and used the rest to fine-tune my approach, getting a ~13km Duna periapsis on the flyby, which I've read should be enough to aerocapture.

However, my best-laid plans were shattered when I actually hit the atmosphere and my entire ship burned up almost instantly. :-) In retrospect this shouldn't have been a surprise, since the navball told me my orbital speed was well over 8000 m/s when I hit the atmosphere. (Yeah, this was probably a terrible idea.)

So I tweaked the design and practiced a few more aerocaptures. I found that if I use the engines to brake down to an orbital speed of 2600 m/s when I hit the atmosphere, my design can survive (even without any heatshields). But that needed a lot of delta-v, and the rocket was getting ridiculously huge; I wanted to do better. So I slapped one of the big 10m (stock) inflatable heatshields on the bottom of my lander and tried again. However, it still burned up almost instantly. It's like the heatshield wasn't even there. :-( (In fairness, some non-critical bits of the lander were clipping through the bottom of the heatshield, but if that were the only problem, I'd expect everything behind the heatshield to survive.)

What am I missing here? I was sure the heatshield would solve the problem, because I've seen a Scott Manley video where he was playing RSS and aerobraked at 10 km/s on Earth coming back from a lunar free-return; surely 8 km/s at Duna should be easier???

Thanks. :-)

[A_name]

I've never tried such an aggressive aerocapture myself but 8 Km/s does seem to be way too fast, and a periapsis of 13 Km seems too steep as well. What I can tell you is that there is indeed a certain speed where no heat shield will save you, even in Duna’s thin atmosphere. So there’s basically only two ways to solve your problem, slowing down before entering the atmosphere and/or going for a higher periapsis. Your solution will likely be a combination of both. You’ll probably need to do lots of trial and error in your “simulations” to find the maximum speed and minimum Pe where you won’t explode but will still capture. Just from memory, using a typical Hohmann transfer you would enter the atmo at about 5 Km/s so try shooting for a speed around that range and a slightly higher Pe of maybe 20-30 Km.

 

As to the Scott Manley video you mentioned, atmosphere and part physics are all tweaked for RSS so it’s normal that you could pull it off there on not in stock.

[Starman4308]

The very short version: because velocities in KSP are so much less than reality, KSP heatshields are very, very weak compared to reality. RSS, because it uses realistic velocities, buffs heatshields back up to realistic levels.

As such, slow down before hitting Duna, or you'll burn to a crisp. I don't know what the upper range is for stock heatshields, but it's definitely far below 10 km/sec.

[Cpt Kerbalkrunch]

I've actually survived an 8km/s free-return at Kerbin (and still have the savegame from it), but that was an Mk1 Command Module and heatshield only. With a larger ship to land on Duna, if you come in low enough to actually be captured at that speed, you'll be coming in too steep and will not survive. With the inflatable heatshield, at such a high speed, you are basically doing a belly flop onto a lake of lava. A normal heatshield has a smaller surface area and ablator. I think you'd have more luck with that. However, all the parts of your ship will need a high heat tolerance to survive. This is just speculation, though. I've aerocapured at Duna plenty of times but, as I said, I've only survived at that rate of speed on Kerbin.

Edited January 20, 2018 by Cpt Kerbalkrunch

[ej89]

4 hours ago, A_name said:

So there’s basically only two ways to solve your problem, slowing down before entering the atmosphere and/or going for a higher periapsis. Your solution will likely be a combination of both. You’ll probably need to do lots of trial and error in your “simulations” to find the maximum speed and minimum Pe where you won’t explode but will still capture. Just from memory, using a typical Hohmann transfer you would enter the atmo at about 5 Km/s so try shooting for a speed around that range and a slightly higher Pe of maybe 20-30 Km.

Yeah, my experimentation showed that I could survive at no more than 2600 m/s at a periapsis no lower than around ~18-19 km. That was before heat shielding, though, on a large, unaerodynamic vessel with a lot of surface area. Based on your experience, it sounds like I might be able to make it with a little higher speed if I add heat shielding, but I'll probably be limited by the fact that this vessel is frankly huge and ungainly. It was really designed to operate in vacuum and in Duna's thin atmosphere (at speeds where the dynamic pressure is proportionately low).

26 minutes ago, Starman4308 said:

The very short version: because velocities in KSP are so much less than reality, KSP heatshields are very, very weak compared to reality. RSS, because it uses realistic velocities, buffs heatshields back up to realistic levels.

As such, slow down before hitting Duna, or you'll burn to a crisp. I don't know what the upper range is for stock heatshields, but it's definitely far below 10 km/sec.

Ah, that explains it then. I was wondering if I was missing something like that. I figured "surely Realism Overhaul makes everything harder, so the stock heatshields can't be worse", but now I remember that stock KSP also nerfs engine performance considerably for the same reason, so it makes sense it'd do the same with heatshields.

Also, Scott Manley was using the (RO equivalent of) a Mk1 command pod and small heatshield, a much smaller craft (and with an ablative shield), which is more in line with Cpt Kerbalkrunch's experience...

24 minutes ago, Cpt Kerbalkrunch said:

I've actually survived an 8km/s free-return at Kerbin (and still have the savegame from it), but that was an Mk1 Command Module and heatshield only. With a larger ship to land on Duna, if you come in low enough to actually be captured at that speed, you'll be coming in too steep and will not survive. With the inflatable heatshield, at such a high speed, you are basically doing a belly flop onto a lake of lava. A normal heatshield has a smaller surface area and ablator. I think you'd have more luck with that. However, all the parts of your ship will need a high heat tolerance to survive. This is just speculation, though. I've aerocapured at Duna plenty of times but, as I said, I've only survived at that rate of speed on Kerbin.

I hadn't realized that the ablative heat shields had that kind of advantage over the inflatable one. It makes sense (from what I understand of how they work in real life), I just didn't expect KSP to model them as being fundamentally different from each other. Is KSP actually accounting for the way real ablative heat shields form a protective "barrier" of gas released as the heatshield burns, or does it do some more simplistic approximation?

How much does the size of the ship matter? Is it an issue of mass? (Mine has over 20 tonnes dry.) I confess I don't fully understand the relationship between vehicle mass, surface area, drag, and heat.

[Cpt Kerbalkrunch]

15 minutes ago, ej89 said:

How much does the size of the ship matter? Is it an issue of mass? (Mine has over 20 tonnes dry.) I confess I don't fully understand the relationship between vehicle mass, surface area, drag, and heat.

Well, I'm definitely not a math or physics guy (more like an experiment and see what works and what doesn't guy), but I think it's sorta like a bullet vs brick thing. I think size, shape, and mass all play a part. Velocity is obviously a huge factor, but I think angle of descent is probably every bit a s important. For my success on Kerbin, Pe was set at 35km. That's obviously half of Kerbin's atmospheric height. Again I'm no physics guy, so I don't know how significant that is, but without even trying, I would bet that if you came into Duna at 8km/s with a Pe of 25km (half of Duna atmospheric height) you would simply fly right through the atmosphere and back put again. And maybe not even be captured into the system; let alone the planet. You'd have to come in steeper. Which usually means you go "poof" and disappear.

This sounds like it's gonna provide some interesting opportunities for experimentation, though. It has certainly piqued my curiosity. I think there are some possibilities here. With traditional heatshields (and maybe a size larger than the fuselage they protect; for instance, a 2.5m shield on a 1.25 body to provide a larger cone of protection), you can at least test the boundaries. But you'll definitely need a high heat tolerance on every part you use. If even one part overheats, the whole thing will quickly come apart. I think it'll make for a pretty cool project. Hope all goes well.

[ej89]

35 minutes ago, Cpt Kerbalkrunch said:

Well, I'm definitely not a math or physics guy (more like an experiment and see what works and what doesn't guy), but I think it's sorta like a bullet vs brick thing. I think size, shape, and mass all play a part. Velocity is obviously a huge factor, but I think angle of descent is probably every bit a s important. For my success on Kerbin, Pe was set at 35km. That's obviously half of Kerbin's atmospheric height. Again I'm no physics guy, so I don't know how significant that is, but without even trying, I would bet that if you came into Duna at 8km/s with a Pe of 25km (half of Duna atmospheric height) you would simply fly right through the atmosphere and back put again. And maybe not even be captured into the system; let alone the planet. You'd have to come in steeper. Which usually means you go "poof" and disappear.

This sounds like it's gonna provide some interesting opportunities for experimentation, though. It has certainly piqued my curiosity. I think there are some possibilities here. With traditional heatshields (and maybe a size larger than the fuselage they protect; for instance, a 2.5m shield on a 1.25 body to provide a larger cone of protection), you can at least test the boundaries. But you'll definitely need a high heat tolerance on every part you use. If even one part overheats, the whole thing will quickly come apart. I think it'll make for a pretty cool project. Hope all goes well.

OK, I tried a few more experiments:

1. Used lots of 3.75m ablative heat shields instead of the one inflatable. (The design consists of a center "core" and four outboard tank/engine pods attached via girders; so for this, I put a 3.75m heat shield on each of the five sections.) This actually seemed promising...for about 10 seconds. :-) It lasted that much longer than the one with the inflatable, which was pretty good considering the inflatable burned up within 5 seconds of passing through the 50km atmosphere line.

2. Instead of the big awkward lander I'd designed, a simple craft consisting of a Mk1 command pod with a parachute on top and a 1.25m ablative heat shield on the bottom. This did about as well as test #1.

3. Same as #2, but with the 2.5m heat shield. Seemed to be about the same; it might have lasted a little longer but I'm not sure. In any case, the result was the same (boom).

4. Same as #3, but with 3.75m heat shield. Didn't seem to improve over #3.

5. Repeats of #4 and #2, but with the periapsis bumped up as far as I could take it and still get captured (according to the Trajectories mod's prediction for a retrograde reentry). With the 3.75m heat shield, I was able to get Pe up to about 23-24 km (versus the 13-16 km in the previous tests); it seemed to last a few seconds longer but still ultimately went boom. The 1.25m heat shield version had the opposite effect: I had to go with a lower Pe to ensure aerocapture, because there's less blunt surface slamming into the atmosphere to slow me down. This, of course, exploded about instantly.

What I observed through all the tests is that the heat shields actually did a great job insulating the rest of the craft from heat - there were no temperature bars except for the heat shields themselves (at least until they blew up :-)). The "only" real problem was that they were simply absorbing heat much faster than they were rejecting it. I noticed that the "Thermal" tab on KER's Flight Engineer shows "Convection Flux" and "Radiative Flux" readouts - for the best of these tests, the radiative flux (which I'm guessing is how fast the spacecraft is rejecting heat, since it was negative) was only half the magnitude of the convection flux (this was positive, guessing it's the rate at which I'm being cooked by the superheated plasma in front of me). If these numbers mean what I think they mean, then the problem is clear: these heat shields simply don't reject heat fast enough to offset the heat that's being pumped into them. Regardless of how small or lightweight the craft is, it just isn't enough.

As you noted, the same speed was survivable at Kerbin, but higher (relatively speaking) in the atmosphere. If I could pass through Duna's atmosphere less steeply, I could probably survive there too. However, I would then not get enough aerobraking to slow me down from escape velocity.

I think this is the real difference between the Kerbin-free-return and Duna-aerocapture situations: for Duna aerocapture, it's critical to get rid of all that excess velocity right away, because you've only got one shot at it. Whereas for just lowering an orbit, it's acceptable (assuming life support considerations don't prevent this :-)) to be patient and make multiple, gentler aerobraking passes. Indeed, even if you set the Kerbin periapsis low enough to come all the way down on a single pass, that one pass gives you a lot more time in the atmosphere to slowly shed velocity, since your trajectory is far less eccentric (i.e., you "hug" the planet more closely), and is becoming rapidly less so as you lose velocity.

Perhaps even a Kerbin aerocapture (coming in from escape velocity) would be easier to pull off than Duna's, at the same velocity, since the gravity well is deeper and thus escape velocity is lower, i.e. you don't need to shed as much velocity on that one pass (and thus can use a higher periapsis). Kerbin's escape velocity (3431.03 m/s) is a lot higher than Duna's (1372.41 m/s).

Oh well...guess I'll just have to use fuel to brake. Or do a less ludicrously fast transfer. :-) Maybe I'll try an aggressive aerocapture on the return trip to Kerbin! (Or save it for Jool or Eve - they're a lot easier to get captured by.)

[Laie]

Providing some data for @A_name's answer... The rescue from retrosolar challenge (search for "Burberry")  encouraged players to aerobrake as much and as hard as they could. It has been found that 8km/s is about the utmost that stock heatshields can take, at least on Kerbin. IIRC that required vastly oversized heatshields, but I may be wrong.

8 hours ago, ej89 said:

What am I missing here? I was sure the heatshield would solve the problem, because I've seen a Scott Manley video where he was playing RSS and aerobraked at 10 km/s on Earth coming back from a lunar free-return; surely 8 km/s at Duna should be easier???

The short answer is: different part stats. Realism Overhaul adjusts nearly ever part in the game to fit the different environment.

[foamyesque]

Have you tried layered heatshields?

[ej89]

7 minutes ago, foamyesque said:

Have you tried layered heatshields?

No, not yet...thought about it though. I presume by "layered" you mean stacking them so when they blow up due to overheating, the next heat shield in the stack is exposed? :-) (I imagine that would lead to a rather "wobbly" flight as the shields burn up slightly asymmetrically...probably not so much as to be a problem though.)

Does this approach actually tend to work out favorably in terms of mass, compared to spending the mass of the extra heat shields on more rocket fuel for "normal" braking?

(As it turns out, I'm planning to build the craft for the actual mission using Extraplanetary Launchpads in Kerbin orbit, so I only need to optimize for mass, not cost. RocketParts cost the same amount per kg no matter what you build with them...)

[foamyesque]

I have no idea, to be honest; I almost never use heatshields for anything. If something of my designs outside of early-career is hitting atmosphere, it's going to have wings on. I'll burn fuel to execute captures and even circularization (since shallow aerobraking takes ages and deep-aerobraking from highly-elliptical orbits has, as you are discovering, major heat management issues). It'd be interesting to find out, though.

Edited January 20, 2018 by foamyesque

[Hotel26]

Is it all worth it?

To paraphrase Dr Tyrell in Blade Runner: "if you are going to fly like a meteor, you are going to burn like a meteor."

Edited January 20, 2018 by Hotel26

[ej89]

29 minutes ago, Laie said:

Providing some data for @A_name's answer... The rescue from retrosolar challenge (search for "Burberry")  encouraged players to aerobrake as much and as hard as they could. It has been found that 8km/s is about the utmost that stock heatshields can take, at least on Kerbin. IIRC that required vastly oversized heatshields, but I may be wrong.

The short answer is: different part stats. Realism Overhaul adjusts nearly ever part in the game to fit the different environment.

Good to know, thanks!

Since Kerbin's escape velocity is 3431 m/s, that means that an 8 km/s aerocapture there needs to shed about 4600 m/s. As @A_name noted, a typical Hohmann transfer to Duna will have you aerobraking at ~5 km/s, which means you need to bleed off about 3700 m/s (escape velocity being 1372 m/s).

Taking the 8 km/s Kerbin aerocapture as an upper limit, that would suggest I can probably, with sufficient heat shielding, survive a Duna aerocapture in which I go in at roughly 6 km/s - and no more. (That is, such a trajectory would involve bleeding off about the same amount of speed as in the "upper limit" Kerbin example.)

Now I want to test this... :-D

[ej89]

3 minutes ago, Hotel26 said:

Is it all worth it?

If you are going to fly like a meteor, you are going to burn like a meteor.

Ah, you see, that is the challenge... ;-)

What I'm really hoping to learn with all this is just how hard I can push those suboptimal transfer windows to Duna. I'm planning to set up a base there, and am thinking about installing both the Tourism Plus and Bases and Stations mods, which will add lots of contracts for getting people to and from said base. Knowing in the back of my mind that I can do a really, really fast transfer between Kerbin and Duna if the need arises, without waiting for optimal planetary alignment, will simplify logistics planning a lot, especially if one of those "Emergency Medical Evacuation" scenarios comes up for one of my base personnel... :-)

[Laie]

13 minutes ago, ej89 said:

Taking the 8 km/s Kerbin aerocapture as an upper limit, that would suggest I can probably, with sufficient heat shielding, survive a Duna aerocapture in which I go in at roughly 6 km/s 

You're overthinking this in the wrong way. Heat comes basically from airspeed (plus a few other odds and ends that don't amount to much). If stock heatshields are tuned to ~8km/s, that's the speed you can afford on any body.

Edited January 20, 2018 by Laie

[Hotel26]

31 minutes ago, ej89 said:

Ah, you see, that is the challenge... ;-)

What I'm really hoping to learn with all this is just how hard I can push those suboptimal transfer windows to Duna. I'm planning to set up a base there, and am thinking about installing both the Tourism Plus and Bases and Stations mods, which will add lots of contracts for getting people to and from said base. Knowing in the back of my mind that I can do a really, really fast transfer between Kerbin and Duna if the need arises, without waiting for optimal planetary alignment, will simplify logistics planning a lot, especially if one of those "Emergency Medical Evacuation" scenarios comes up for one of my base personnel... :-)

There's a real-life need for this and someone (struggling to remember who) has proposed a rather brilliant mechanism.  Here it is: check out the Aldrin Cycler...

Also of interest is the Low Energy Transfer, particularly the Ballistic Capture, although this is about the opposite of what you want, as it takes longer.  It's useful to know about though.

Edited January 20, 2018 by Hotel26

[foamyesque]

8 minutes ago, Laie said:

You're overthinking this in the wrong way. Heat comes basically from airspeed (plus a few other odds and ends that don't amount to much). If stock heatshields are tuned to ~8km/s, that's the speed you can afford on any body.

 

Airspeed and *air density*. Particularly, while the max temp depends pretty much entirely on airspeed, the rate at which heat flows into any particular part is strongly influenced by the density of the air around it. This is why it's safe to fly at orbital speeds basically indefinitely in the extreme upper reaches of the atmosphere; despite the external temperature due to airspeed being very high, the air's so sparse that your stuff will radiate heat away faster than it gets warmed up.

[Laie]

Just now, foamyesque said:

Airspeed and *air density*.

I should have known better than to ignore that obvious objection. If you want to aerobrake, density falls out of the equation: if you stay too high, well, you won't brake. And if you can brake at all, you can also brake hard -- not sure if the latter is realistic, but it's how it works in KSP.

[ej89]

11 hours ago, Hotel26 said:

There's a real-life need for this and someone (struggling to remember who) has proposed a rather brilliant mechanism.  Here it is: check out the Aldrin Cycler...

Also of interest is the Low Energy Transfer, particularly the Ballistic Capture, although this is about the opposite of what you want, as it takes longer.  It's useful to know about though.

Have heard of these before, very interesting ideas. I'm having a hard time understanding how a cycler can save delta-v, though. Don't the "taxi" vehicles that shuttle people and cargo between Mars/Earth and the cycler have to burn just as much delta-v to rendezvous with the cycler as they would to put themselves into (and then out of, on the other end) its exact orbit?

Is the idea simply that you can save fuel by only putting the big, heavy, well-equipped cycler into its orbit once for good, and do the "taxi" flights with smaller and more efficient (but only comfortable/equipped for short durations) craft? I guess that would make it an indirect solution to the problem of how to get faster transfers efficiently: you're still spending the same delta-v to do an aggressive transfer, but it involves a lot less fuel since it's done by smaller craft.

Edited January 20, 2018 by ej89

[foamyesque]

You also don't have to streamline the cycler, nor does it require high-thrust engines, whereas a surface-to-orbit shuttle is going to need both.

[Hotel26]

5 hours ago, ej89 said:

Have heard of these before, very interesting ideas. I'm having a hard time understanding how a cycler can save delta-v, though. Don't the "taxi" vehicles that shuttle people and cargo between Mars/Earth and the cycler have to burn just as much delta-v to rendezvous with the cycler as they would to put themselves into (and then out of, on the other end) its exact orbit?

Is the idea simply that you can save fuel by only putting the big, heavy, well-equipped cycler into its orbit once for good, and do the "taxi" flights with smaller and more efficient (but only comfortable/equipped for short durations) craft? I guess that would make it an indirect solution to the problem of how to get faster transfers efficiently: you're still spending the same delta-v to do an aggressive transfer, but it involves a lot less fuel since it's done by smaller craft.

It certainly won't save delta-v but my recollection was that it was a way to open up more "transfer windows" -- but having read it again more closely, I doubt it does.

EDIT: I really wish I hadn't started thinking about your problem.  I have this picture in my mind now of two rather large asteroids dragged into a rather high orbit around Mars.  Dozens or even hundreds of kilometers apart, lightly revolving around a barycenter.  Connected by a rubber mesh looking a bit like a catcher's mitt...  and acting like an aircraft carrier arrester system. 

 

Edited January 21, 2018 by Hotel26

[Foxster]

I'd like to experiment with this. Do you have some Alt-F12 set-orbit or HyperEdit numbers that will put a craft on an 8km/s approach to Duna?

[foamyesque]

Just now, Foxster said:

I'd like to experiment with this. Do you have some Alt-F12 set-orbit or HyperEdit numbers that will put a craft on an 8km/s approach to Duna?

 

Could just use the velocity editor until you're falling fast enough and trim your angle from there. That'd get you most of that 8km/s in one shot.

[Foxster]

Don't forget you have a nice fat moon around Duna that is at a useful inclination. 

Tweak your Duna orbit so you pass just in front of Ike and you will lose a lot of velocity for the small dV cost of the maneuver node.  

Edited January 21, 2018 by Foxster

[Starman4308]

5 hours ago, Foxster said:

Don't forget you have a nice fat moon around Duna that is at a useful inclination. 

Tweak your Duna orbit so you pass just in front of Ike and you will lose a lot of velocity for the small dV cost of the maneuver node.  

To be fair, the effect of swinging by Ike will be reduced thanks to the tremendous velocity he's approaching Duna at, and the phasing is more of an issue thanks to the short travel time. It's still probably worth it, but the bulk of the braking will still be done at Duna itself. This is hardly a standard Hohmann transfer.