Surviving 8 km/s Duna aerocapture

[sevenperforce]

21 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.

There are two main ideas here. The primary idea is that you have your long-term life support systems and extended hab space in orbit permanently. You don't save dV, but you save fuel mass because instead of accelerating a whole extended-stay space station to a high-speed transfer, you're only sending up a small shuttle.

The other element is that they are Aldrin Cyclers, not just a single cycler. There are a series of orbits which swing from the Mars SOI to the Earth SOI very fast, then spend a couple of years in an eccentric solar orbit before repeating the same cycle. If you have 3 or 4 of these ships in play, then there will always be a ship approaching for an optimal rapid transit, while the rest are out in the long periods of their orbits.

Those long periods are the problem, though. The cyclers will need regular resupply and maintenance; each cycler is only "used" a few times every decade, so it can't really be expected to last very long. Each shuttle ends up needing to do a full resupply, so you don't actually end up saving so much mass.

The other problem is that it's not terribly safe. What if the shuttle has a problem, or fails to dock, or something like that? Will the shuttle have enough dV to abort back to where it came from? Probably not. Then the shuttle will also need extended life-support systems, which obviates the need for the cycler.

1 hour ago, Starman4308 said:

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.

If possible, I would aim for your Ike encounter on the back end of the pass, rather than the front end. If you come out of your aerocapture pass still at escape velocity but get a retrograde Ike swingby, it will likely be able to bend your trajectory into a high Duna orbit. From there it will only take a tiny burn to make the next pass take you back through Ike's SOI to bend you back into Duna's atmosphere for your subsequent easier braking passes.

On 1/20/2018 at 3:12 AM, ej89 said:

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?

If you have decent passive aerodynamic stability it shouldn't be a problem. 

[ej89]

10 hours ago, 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?

Unfortunately not. I don't have HyperEdit installed (perhaps I should?) and couldn't figure out how to set myself on an escape-velocity flyby with the Alt-F12 tool.

For the first experiment I ran, I put it in LKO with Alt-F12 (since I'm planning to build it there with EL in the real career game), and flew it the whole way to Duna, since I wanted to dry-run the whole mission. Since the reentry proved to be the difficult part, I've been rolling out new versions of the craft on the launch pad and manually editing a quicksave to switch its place with the one already on the Duna escape trajectory. It's clumsy and there's probably a better way to do it, but it works. :-)

Can anyone with more save-file-editing experience confirm if the game stores any "sensitive" information (e.g. about my computer, etc.) in savefiles? If not, then I can post the savefile here if anyone wants to play around with it.

2 hours ago, sevenperforce said:

If possible, I would aim for your Ike encounter on the back end of the pass, rather than the front end. If you come out of your aerocapture pass still at escape velocity but get a retrograde Ike swingby, it will likely be able to bend your trajectory into a high Duna orbit. From there it will only take a tiny burn to make the next pass take you back through Ike's SOI to bend you back into Duna's atmosphere for your subsequent easier braking passes.

Interesting. Sounds like a cool idea - will have to try that!

What kind of maneuvers do I need to do to get that Ike encounter, in such a way that it happens after skimming Duna's atmosphere? Is it a matter of timing, to ensure Ike is in the right place when I fly by Duna? How far out from Duna do I need to be to have the ability to make such maneuvers without costing an exorbitant amount of delta-v?

[ej89]

On 1/20/2018 at 3:36 AM, ej89 said:

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

For those curious to know how this turned out: I finally got around to testing it, and it worked!

For this test, I used a very simple craft optimized for safe reentry: a Mk1 command pod with a heat shield on the bottom and a parachute on top. (This way, I know that if I don't survive, it's because it simply isn't survivable, not because of a poor craft design.)

I found that aerocapture at 6128 m/s, with a 18km periapsis, is "just barely" survivable. The heat shield temperature maxed out at somewhere between 98-99% of its capacity (i.e., the "red temperature bar" was at 98-99%, per KER's precise readout).

This was the highest periapsis that the Trajectories mod said would achieve capture (in retrograde orientation, of course). It seems that Trajectories's estimate was a little pessimistic: it initially showed me getting into a highly elliptical, "almost escaping" orbit, but after the full aerobraking pass I ended up with a ~700km apoapsis. However, it was still very close: even a 19 km Pe was still too high to get captured. I suspect somewhere around 18.5 km is the limit. My experimentation would indicate that the true maximum periapsis would give me only marginally gentler heating: at 19 km, the heat shield's temperature bar stabilized at 97.98%. At 20 km, it stabilized at 96.7%. Clearly, we are indeed at the very edge of the heat shield's performance margins.

Perhaps most importantly, the usefulness of the "4600 m/s rule of thumb" I had theorized is confirmed: 6000 m/s is about 4600 m/s higher than Duna's escape velocity. just as 8000 m/s is about 4600 m/s higher than Kerbin's escape velocity (and that is the highest reported aerocapture speed people have reported surviving there).

Thus, when planning my delta-v, I should be able to safely pack about 4600 m/s less than what the Launch Window Planner says I need for the full trip with no aerobraking.

Of course, the numbers may turn out differently with a different craft using a bigger heat shield, but since I would guess that the various stock heat shields are well-balanced with respect to each other, I'm guessing this rule of thumb will still hold for the bigger shields. (That's next on my list of tests to run... :-))

[foamyesque]

I suspect that's just a coincidence -- I, for one, would not care to try aerocapturing at Eve at 9km/s.

[ej89]

On 1/27/2018 at 11:53 PM, foamyesque said:

I suspect that's just a coincidence -- I, for one, would not care to try aerocapturing at Eve at 9km/s.

Yeah, you may be right. I intend to try it (at some point) and find out. :-)

In other news, I confirmed that the 6000 m/s approach at Duna is just as survivable with 3.75m heat shields as with the 1.25m. In this case the periapsis was higher (20,942 m), because the vehicle was more blunt (this time I tried my big bulky lander instead of the Mk1 pod), but I aimed for the same criterion as before, namely, that the Trajectories mod showed this as the absolute highest periapsis which would get me below escape velocity (into a highly elliptical orbit). My understanding is that, by doing this, I am aiming for the same maximum dynamic pressure ("Max Q", as they say in the real-world launch webcasts), which I understand to be the real limiting factor on what the heat shield can handle.

See above screenshot for the proof. :-D (Note especially the 107d mission elapsed time demonstrating the fast transfer! That's for real - it started counting from low Kerbin orbit, where I debug-teleported the vessel from the launch pad. This simulates the real mission, since I'll construct it in LKO using EL.)

The screenshot doesn't show all the heat shields since I jettisoned the outer ones before landing to make way for the legs. There was a 3.75m shield below each of the four outboard engines. (There's a fifth shield on the center part of the craft which is still there in the picture.)

 

(P.S.: anyone know how to attach a picture on this forum? I've been uploading them to Imgur but I know I've seen people use attachments before...can't find the button though.)

(P.P.S.: Hmm, I think I'm using the "Embed Imgur album" button wrong. I only get a link to the album, saying "Album qVZ33ta will appear when post is submitted". It still looks like that after submitting the post. Is it looking right for other people?)

Edited January 29, 2018 by ej89
fix Imgur album embed (again)

[Cpt Kerbalkrunch]

Not sure why the album isn't linking properly but, as for the screenshots themselves, when you left-click on a certain pic you'll see a column of links on the right side. You wanna copy the second one down; the Direct Link. Then, when you paste it into your post, the screenshot itself will show up instead of just a link.

[Human Person]

On 20.1.2018 at 12:03 AM, ej89 said:

The idea is to shave time off the transfer (and depart a good ~80-100 days before the planets are "ideally" aligned)

If you want to shave off time, you should depart shortly after the „ideal“ window, not before. The distance you have to travel is much shorter then and you need much less speed for equal time of travel. You can see this quite easy in this plot: you want to be around where I put the cross in order to have the best fast transfer:

Edited January 28, 2018 by Physics Student