Duna Aerocapture

[Meithan]

Hi all! I thought I'd share my latest achievement in KSP: performing an aerocapture around Duna.

This was my first time outside the Kerbin system. I've landed Kerbals on the Mun three times and once on Minmus, plus having a sizeable space station / refueling station in orbit.

Because I had no experience in interplanetary transfers, and was (still am) unsure on how to land and takeoff on Duna with a heavy manned spacecraft (parachutes? delta-v for liftoff to orbit considering the atmo? etc.), my first trip to Duna was with a robotic spacecraft, which included two satellites (one for Duna, one for Ike) and a small non-mobile lander for Duna.

I had read about aerobraking and aerocapture and thought, "what the hell, let's try to do an aerocapture; no Kerbals will be harmed if it fails".

This is my spacecraft (I called it Duna Express):

And this is the launch rocket I used:

After the transfer (which was simpler than expected; I learned that the important thing is launching during the window), I entered Duna's sphere of influence with a periapsis around 1000 km. I had read on the forums that the aerocapture window for Duna is 12-15 km. I decided to aim for 13 km. So I burned radially to lower the periapsis into the atmosphere. The final periapsis was 12.7 km:

Duna's atmosphere is so thin! Atmospheric interface was at around 42 km altitude.

Then really tense moments followed. Would I overshoot? Or worse, would I brake too much and crash into Duna? My spacecraft carried more than enough fuel to perform an insertion burn in case I missed the capture, but overdoing it would be harder to correct. So I just crossed my fingers and watched the telemetry data (I picture a roomful of over-caffeinated Kerbal engineers nervously watching the data).

In less than 30 s, things went from not experiencing any aerobraking at all to more than 6 m/s2 of drag acceleration (it's twice Duna's surface gravity!). Eek! Then, the first milestone: my initial hyperbolic trajectory turned into an ellipse. Captured!

But I wasn't out of the atmosphere yet, and was still experiencing a lot of drag! I watched that ellipse get smaller and smaller and smaller ... but then, as the spacecraft climbed, it started to stabilize. And finally, the atmospheric density dropped to zero. Success!! This was the final orbit after the aerobraking pass:

All that remained now was waiting until apoapsis to increase the periapsis above the top of the atmosphere and voila, orbital insertion with zero fuel consumption! I was a

very happy mission controller.

[magnemoe]

Nice, first you did not overbuild, no need for an nerva engine with an small payload and I liked your satellites.

[Meithan]

Nice, first you did not overbuild, no need for an nerva engine with an small payload and I liked your satellites.

Thanks! Yeah, I wanted to keep this thing light and simple. I also used a smaller launch rocket than I usually do (no 2.5m parts or heavy Mainsails on that thing).

Here's a pic of the first satellite (the Duna orbiter) after being released from the main craft and deploying its comms/solar panels:

[Scotius]

Congratulations! Very well done. Aerobraking is tricky around Duna, because thin atmosphere doesn't leave much margin for error.

[NeoMorph]

Yeah... I really enjoy my aerobraking manoeuvres around Duna. Once or twice I've come close enough to count the boulders lol.

[Meithan]

Thanks, guys! It was much fun, to be honest. I really didn't know whether it would work. I'm writing a program to simulate orbits and aerobraking in KSP, and I hope I can soon get it to predict the 12-15 km window. We'll see.

So, the next step is a manned mission. I have a three-kerbal lander that is quite heavy (23 tons). Could you give me some pointers? Are parachutes safe, even if my craft is that heavy? I've heard of people having their craft ripped apart when they open them. What's a safe speed / altitude for that? I need to throw in some drogue chutes, right?

What about takeoff? How much delta-v do I need to get back to orbit (including atmo drag). What's a good altitude for the gravity turn? I'll appreciate your answers.

[Guest]

Nice work! Aerocapture around Duna is fun because you have to dip so damn low.

[Specialist290]

Thanks, guys! It was much fun, to be honest. I really didn't know whether it would work. I'm writing a program to simulate orbits and aerobraking in KSP, and I hope I can soon get it to predict the 12-15 km window. We'll see.

Nicely done! Getting plenty of practice at aerobraking should help you down the road on several missions.

Also, keep us posted on how that program turns out.

[Moar Boosters]

Thanks, guys! It was much fun, to be honest. I really didn't know whether it would work. I'm writing a program to simulate orbits and aerobraking in KSP, and I hope I can soon get it to predict the 12-15 km window. We'll see.

So, the next step is a manned mission. I have a three-kerbal lander that is quite heavy (23 tons). Could you give me some pointers? Are parachutes safe, even if my craft is that heavy? I've heard of people having their craft ripped apart when they open them. What's a safe speed / altitude for that? I need to throw in some drogue chutes, right?

What about takeoff? How much delta-v do I need to get back to orbit (including atmo drag). What's a good altitude for the gravity turn? I'll appreciate your answers.

Parachutes work well on Duna, the problems only come when people try and use them as their main method of braking. I found the easiest way to land was do a hybrid powered/parachute landing. The chutes will rip your craft apart if you're going much over 200m/s when they open, so just before 2500m above the surface (thats when the drogues open), I used my engines to slow down to about 175m/s and then opened the chutes. The drogues will open safely, then you drift down, the regular chutes open at 500m and then you'll probably be going around 20m/s, so you'll need another tiny little burn of the engines to touch down safely.

The ascent back to Duna orbit is one of the easier parts of the mission. You'll need about 1500m/s delta v to get back to orbit. It doesn't need that much engine power though. Something that will barely get off the ground on kerbin will fly nicely on Duna.

What type of craft have you built? Are you going to do a rendezvous with a command module in orbit, or is the lander also the return craft?

[Meithan]

Also, keep us posted on how that program turns out.

Sorry for not replying until now, I've had some busy days. I finally finished the aerobraking simulator. Don't know if you'll read this, but here it goes anyway.

I simulated a ship arriving to Duna on an interplanetary transfer trajectory from Kerbin (eccentricity: 1.94, hyperbolic excess speed: 921 m/s), and with periapsis set within Duna's atmosphere at altitudes of 10, 11, 12, 13, 14 and 15 km. Sadly, I'm short on time right now so I'll only post the result of the analysis, but I have some nice plots of the simulated trajectories which I'll post another day.

Duna Aerocapture Analysis

Periapsis altitude: 10 km

Result: crashes into surface before exiting atmosphere

Periapsis altitude: 11 km

Result: crashes into surface before exiting atmosphere

Periapsis altitude: 12 km

Result: exits atmosphere on grazing low-eccentricity orbit (which reenters quickly)

Final apoapsis altitude: 50.83 km

Final periapsis altitude: -5.94 km

Final orbit eccentricity: 0.083

Periapsis altitude: 13 km

Result: exits atmosphere on high elliptic orbit

Final apoapsis altitude: 871.62 km

Final periapsis altitude: 11.89 km

Final orbit eccentricity: 0.564

Periapsis altitude: 14 km

Result: exits atmosphere on very high elliptic orbit

Final apoapsis altitude: 8771.25 km

Final periapsis altitude: 13.50 km

Final orbit eccentricity: 0.929

Periapsis altitude: 15 km

Result: no capture, continues past Duna on a hyperbolic escape orbit

Final eccentricity: 1.202

Final excess speed: 426.13 m/s

The results of my simulations seem to be consistent with the 12-15 km window. I'd say something around 13 km is optimal, to have some room for error and depending on how high a final orbit you want

Testing the simulator

In order to validate the simulator, I also tried to reproduce the actual aerocapture I performed and reported in this thread. The simulation comes pretty close: it predicts a final elliptic orbit with a 534 km apoapsis altitude, while the actual final orbit ingame had an apoapsis of 518 km. Not bad.

I discovered that the slight discrepancy is due to a slight difference in atmospheric drag as computed by my program (vs. what's reported by MechJeb ingame). The cause is a slight discrepancy in the airspeed conversion from orbital speed. If your trajectory is not exactly equatorial, your airspeed has a slight dependence on your current latitude (and my program assumes an exactly equatorial trajectory). It's not a big difference, but since drag depends on speed quadratically, even a few m/s off can change your final apoapsis by a dozen kilometers. So I'd conclude that my program is reasonably accurate.

PS: Yes, I know Alterbaron's simulator already does all this, but I just couldn't resist the challenge of programming it myself .

[Meithan]

Parachutes work well on Duna, the problems only come when people try and use them as their main method of braking. I found the easiest way to land was do a hybrid powered/parachute landing. The chutes will rip your craft apart if you're going much over 200m/s when they open, so just before 2500m above the surface (thats when the drogues open), I used my engines to slow down to about 175m/s and then opened the chutes. The drogues will open safely, then you drift down, the regular chutes open at 500m and then you'll probably be going around 20m/s, so you'll need another tiny little burn of the engines to touch down safely.

The ascent back to Duna orbit is one of the easier parts of the mission. You'll need about 1500m/s delta v to get back to orbit. It doesn't need that much engine power though. Something that will barely get off the ground on kerbin will fly nicely on Duna.

What type of craft have you built? Are you going to do a rendezvous with a command module in orbit, or is the lander also the return craft?

Thanks for replying. Your tips will be tremendously useful in designing my manned mission to Duna. I'll go with the hybrid approach. Thanks for the tip on ascent delta-v. As for the gravity turn, just turn as soon as I exit the first layer of the atmosphere (marked below the altimeter), I guess?

As for my craft, it's still on the drawing board but it'll be similar to this:

http://meithan.x10.mx/Minmus/images/2_OrionPadTest.jpg

It will be coupled to an interplanetary transfer stage composed of a larger fuel tank and 2-4 LV-N engines. Something similar to this (probably with radially mounted LV-Ns; it was complicated to stack this design on the Kerbin launch rocket):

http://meithan.x10.mx/Minmus/images/B_Orion_IPS.jpg

The spacecraft is meant to undock from the transfer stage once in Duna orbit, land, take off, and rendezvous back before heading back to Kerbin. The command pod also functions as final reentry vehicle back on Kerbin.