How to do a Duna aereobrake... a safe one

[diegog.idk.3112]

Im new in interplanetary travels and i sent multiple probes and satellites to Duna... and i will send my first crewed mission to the red planet.

I want to know if the aereobraking is possible in Duna and how to get a succesfully braking.

 

[Vanamonde]

It is possible, the air is so thin you might not even need heatshields, and also because it's so thin you have to go very low for it to have much effect. Low as in 10-15kms perigee, which can be scary and exhilarating at interplanetary transit speeds. Save the game as you get close and try different altitudes until you find one that works for your ship's drag and mass. Have fun.

[James M]

Remember to point your draggy side forward when you want to slow down faster and your less draggy side when you want to slow down less. Sometimes just changing the orientation of your ship's entry profile during an aerobrake can be the difference between achieving orbit and landing unexpectedly. (Or flying back into deep space..)

Edited August 16, 2022 by James M
A forgotten yet important detail.

[king of nowhere]

aerobraking is a matter of speed. if you go too fast, too shallow, you burn. but if you don't go shallow, you don't brake.

hencefore, what makes duna uniquely suited for aerobraking is not its thin air - which is actually no different from other upper atmospheres. duna is unique because it is small.

you arrive at a planet with your intercept speed. for a kerbin-duna transfer, it's generally 400 to 600 m/s. then you fall towards the planet, so you have to add the escape velocity from the planet to your intercept speed. on kerbin, this would result in speeds well over 3000 m/s, which incinerate most parts. eve is even worse, and laythe is only slightly better. but duna has a very low escape speed. you can reach it from interplanetary, and still be no faster than 2 km/s, which is totally survivable.

still, heat sensitive parts - like some solar panels - won't survive that. so make sure there aren't any; use retractable solar panels, those are very strong, or put your solar panels in shielded positions.

as for height, you can only figure it by trial and error. my experence is different from @Vanamonde, I generally get by with periapsis between 20 and 25 km. perhaps the difference is because i want to brake, but to stay in duna's orbit, while he wants to land directly. regardless, save the game before the encounter and try different periapsis until you find the right one.

[diegog.idk.3112]

On 16/8/2022 at 6:33, king of nowhere said:

aerofrenado es una cuestión de velocidad. si vas demasiado rápido, demasiado superficial, te quemas. pero si no vas poco profundo, no frenas.

por lo tanto, lo que hace que duna sea especialmente adecuada para el aerofrenado no es su aire enrarecido, que en realidad no es diferente de otras atmósferas superiores. duna es única porque es pequeña.

llegas a un planeta con tu velocidad de intercepción. para una transferencia kerbin-duna, generalmente es de 400 a 600 m/s. luego caes hacia el planeta, por lo que debes agregar la velocidad de escape del planeta a tu velocidad de intercepción. en el bordillo, esto daría lugar a velocidades muy por encima de los 3000 m/s, que incinerarían la mayoría de las piezas. eve es aún peor, y laythe es solo un poco mejor. pero duna tiene una velocidad de escape muy baja. puede alcanzarlo desde interplanetario y aún así no ser más rápido que 2 km / s, que es totalmente sobrevivible.

aún así, las partes sensibles al calor, como algunos paneles solares, no sobrevivirán a eso. así que asegúrate de que no haya ninguno; use paneles solares retráctiles, que son muy fuertes, o coloque sus paneles solares en posiciones protegidas.

en cuanto a la altura, solo puedes calcularla por ensayo y error. mi experiencia es diferente a@Vanamonde, generalmente me las arreglo con periapsis entre 20 y 25 km. quizás la diferencia sea porque yo quiero frenar, pero quedarme en la órbita de duna, mientras que él quiere aterrizar directamente. independientemente, guarde el juego antes del encuentro y pruebe diferentes periapsis hasta que encuentre el correcto.

I tried with periapsis of 10km but the resulting speed reaching the lower atmosphere was very high (it ended very badly) the lander is quite small and I thought it wouldn't end up going very fast more or less like 750m/s and the speed before the descent was around 1000m/s 

[Vanamonde]

So you loaded a save and tried again? 

[king of nowhere]

21 minutes ago, diegog.idk.3112 said:

I tried with periapsis of 10km but the resulting speed reaching the lower atmosphere was very high (it ended very badly) the lander is quite small and I thought it wouldn't end up going very fast more or less like 750m/s and the speed before the descent was around 1000m/s 

wait, wait, wait, those speeds make no sense.

escape speed from duna is somewhere between 1400 and 1500 m/s, I don't remember. so, if you come from outside, that's the speed you'll pick up falling towards duna. that's the minimum possible speed you can have.

To that you have to add your intercept speed (well, a bit less, for the way energy works), so a normal speed to enter duna atmosphere is 2000 m/s. I therefore have no idea what those 750/1000 m/s you mention are.

and 2000 m/s is an easily survivable speed. it's slower than you enter kerbin's atmosphere, and one routinely does that with unshielded crafts. So I see a feww options for what went wrong

1) you were not in an optimized trajectory to duna. You took a bad transfer, leading to much higher intercept speed (even a few hundred m/s can make the difference), leading to faster aerobraking, leading to death

2) your rover is fragile. maybe it has heat sensitive parts, like a science jr, in exposed positions. check maximum temperature of parts in the VAB. most parts have 2000°, that's the "standard", and that's what survives a 2000 m/s reentry. parts with lower heat tolerance are bad for aerobraking.

3) you just were too low. I didn't think it's possible to actually break a lander by aerobraking from 2 km/s, but maybe I'm wrong. Anyway, 10 km is very, very low. you get in the low atmosphere very fast.

 

i suppose it could be a combinaton of all those factors

[diegog.idk.3112]

Just now, Vanamonde said:

Entonces, ¿cargaste un guardado y lo intentaste de nuevo? 

Yes, I made a new orbit but I added more aerodynamic surfaces and set a periapsis of 20km. This time it went well but I have to improve the landing.

I did a burn at 1km height to reduce the speed since the motor I use is not very powerful and it can hardly lift-off

[diegog.idk.3112]

4 minutes ago, king of nowhere said:

espera, espera, espera, esas velocidades no tienen sentido.

la velocidad de escape de duna está entre 1400 y 1500 m/s, no lo recuerdo. entonces, si vienes de afuera, esa es la velocidad que tomarás cayendo hacia duna. esa es la velocidad mínima posible que puede tener.

A eso tienes que añadir tu velocidad de intercepción (bueno, un poco menos, por la forma en que funciona la energía), por lo que una velocidad normal para entrar en la atmósfera de las dunas es de 2000 m/s. Por lo tanto, no tengo idea de cuáles son esos 750/1000 m/s que mencionas.

y 2000 m/s es una velocidad fácilmente superable. es más lento de lo que entras en la atmósfera del kerbin, y uno lo hace rutinariamente con naves sin blindaje. Así que veo algunas opciones para lo que salió mal

1) no estabas en una trayectoria optimizada hacia duna. Hizo una mala transferencia, lo que llevó a una velocidad de intercepción mucho más alta (incluso unos pocos cientos de m/s pueden marcar la diferencia), lo que provocó un aerofrenado más rápido, lo que provocó la muerte.

2) su rover es frágil. tal vez tenga partes sensibles al calor, como una ciencia jr, en posiciones expuestas. compruebe la temperatura máxima de las piezas en el VAB . la mayoría de las partes tienen 2000 °, ese es el "estándar", y eso es lo que sobrevive a un reingreso de 2000 m / s. las piezas con menor tolerancia al calor son malas para el aerofrenado.

3) simplemente estabas demasiado bajo. No pensé que fuera posible romper un módulo de aterrizaje aerofrenando desde 2 km/s, pero tal vez me equivoque. De todos modos, 10 km es muy, muy poco. entras en la atmósfera baja muy rápido.

 

Supongo que podría ser una combinación de todos esos factores.

Basically what happened is that I braked to close an orbit in Dune and see if that made it easier for me to brake, when I started to have contact with the atmosphere the speed was around 1000ms. when I stopped having resistance I was going a little fast and neither the engines nor the parachutes (for emergencies) saved me and everything ended in an incredible explosion against the ground. I use "cheetah" motors for maximum efficiency during takeoff, the lander weighs about 30T in just necessities, plus it's the same one that should get me back to Kerbin

[king of nowhere]

1 minute ago, diegog.idk.3112 said:

Basically what happened is that I braked to close an orbit in Dune and see if that made it easier for me to brake, when I started to have contact with the atmosphere the speed was around 1000ms. when I stopped having resistance I was going a little fast and neither the engines nor the parachutes (for emergencies) saved me and everything ended in an incredible explosion against the ground. I use "cheetah" motors for maximum efficiency during takeoff, the lander weighs about 30T in just necessities, plus it's the same one that should get me back to Kerbin

oh, i see. then the problem is not the aerobraking process, but the landing itself.

yes, duna's atmosphere is thin, so it won't brake you much. parachutes would help, but you still need rockets. it doesn't matter the periapsis you set.

rocket braking down to 1000 m/s before entering atmosphere was exaggerated. you could have managed with a higher speed for sure.

well, it seems you managed anyway

[diegog.idk.3112]

11 minutes ago, king of nowhere said:

oh, i see. then the problem is not the aerobraking process, but the landing itself.

yes, duna's atmosphere is thin, so it won't brake you much. parachutes would help, but you still need rockets. it doesn't matter the periapsis you set.

rocket braking down to 1000 m/s before entering atmosphere was exaggerated. you could have managed with a higher speed for sure.

well, it seems you managed anyway

yes thank you all for your advice, on the third try i barely managed to land and i did a little jump which made my lander spin and crash (all kerbals are alive, well...i hope so) but i just have to practice and figure out where to start the engines to save deltav but still make a safe landing. There is only one other unknown: if I will be able to go back to Kerbin, the DeltaV is super tight and the image is very, very small, so if you can recommend an engine for this situation, that would be nice.

[king of nowhere]

3 hours ago, diegog.idk.3112 said:

There is only one other unknown: if I will be able to go back to Kerbin, the DeltaV is super tight and the image is very, very small, so if you can recommend an engine for this situation, that would be nice.

i can't recommend you an engine because it depends on the craft. but I can recommend three things:

1) don't rocket brake before duna's atmosphere. you said you brake to 1000 m/s, you arrived presumably at 2000 m/s, so you spent 1000 m/s. you can save that. worst case scenario, you can brake less and still save at least 500 m/s. that alone is enough if you're tight.

2) you mention having problems with low thrust, needing a long burn to stop. this is bad. you probably have an engine too small. a bigger engine would help. yes, a bigger engine is heavier and makes you lose deltaV, but it's a lander, you have gravity drag for low thrust, you waste more deltaV by that than you lose with a bigger engine.

3) include parachutes in your lander. without parachutes, you'll fall at over 100 m/s, maybe up to 300 m/s, depending on the shape and size of your lander. with parachutes, you'll fall between 30 and 50 m/s. you save a lot more deltaV than you lose by their mass. and you can remove them before leaving, either by eva construction, or by putting them on decouplers

[Vanamonde]

I wouldn't say that low thrust is bad in interplanetary missions. Big ships will require fuel efficiency, and that means long burns at low thrust. My large interplanetary craft normally operate at around 0.1 to 0.2 Gs on LVN engines. 

[king of nowhere]

2 hours ago, Vanamonde said:

I wouldn't say that low thrust is bad in interplanetary missions. Big ships will require fuel efficiency, and that means long burns at low thrust. My large interplanetary craft normally operate at around 0.1 to 0.2 Gs on LVN engines. 

I specifically said landers.

yes, my motherships also use nuclear engines and twr 0.1. But landers, I learned through harsh experience (in particular as detailed in my DREAM BIG mission report, part 5, Moho landing) that it's worth sacrificing some deltaV to get a bigger engine with better twr. besides the strict math, it's also easier to drive and more forgiving

[magnemoe]

Very fast and Kerbal I say. 

as king of nowhere pointed out and I tested this does not work other places than Duna as your velocity at Pe is to high for the inflatable shield to survive with an large base. 
Now at other bodies I say its best to brake into an high elliptical orbit and the use aerobrake to circulate to save fuel, This has the benefit that you can trade fuel for time. 

The downside is that its very hard to estimate this, even mechjeb has problems, you can use airbrakes for control but still.

Edited August 19, 2022 by magnemoe