Aerobraking Question.

[Sr Alvero]

Hello everyone,
I am new to this forum and it is the first time I write here.
I am a physics student and I am doing my final project.
I am trying to carry out a mission to Titan and Enceladus. So far, I have reached Titan, but now I need to reduce the speed and the orbit. I have seen that you have used aerobraking and aerocapture techniques in this forum. Could you help me?
Thank you very much in advance

[Vanamonde]

Hello Sr Alvero. Since your question appears to be about actual spaceflight rather than in the game, your question has been moved to the real spaceflight subforum. 

[Gargamel]

8 hours ago, Sr Alvero said:

Could you help me?

We're going to need more specifics, those are really vague statements.  

Aerobraking is simple.  You use atmospheric drag to slow your velocity.

Now, I'm assuming this is all on paper, as 1) You would have mentioned using the RSS mod if you were playing KSP or 2) We would have heard of a independently launched flight to Saturn, we're kinda obsessive like that. 

You are going to have to provide us with the data on your orbital characteristics, the design of your ship, how it's drag changes over various AOA's in atmosphere, atmosphere density curves for the moons, etc.   

These are a lot of the info you will need to do the calculations.  We can help you along, but we won't do the work for you. 

[Wjolcz]

Are we talking KSP or some sort of realistic n-body physics simulation?

If KSP then either carry a thick heat shield and/or try to use Titan's orbital motion. Then from there aim for the moon.

If simulation then check what Cassini and Galileo did to slow down around Saturn and Jupiter. IIRC Galileo used a retrograde slingshot and a retroburn around Io to insert itself into orbit. IIRC Cassini did some funky n-body manouvering.

[Xd the great]

Basically, use atmosphere to slow down. High speed, plenty of friction, KE becomes heat and dissipated. You will need a heat shield, a low enough periphasis that is in the upper atmosphere. Like really really up. And take note that the longer you are in the atmosphere, the more you slow down. The deeper you are, the more you slow down. So, i would suggest extremely accurate calculations.

Good luck.

Or, if you are landing, just crash into the surface. The Mars landers/rovers style.

[K^2]

Aerobraking is hard to estimate with high accuracy, but at the speeds involved, you usually just scoop up the atmosphere with your craft, so a drag coefficient of 2 tends to be a sensible approximation. In other words,

F_D = Aρv²

Naturally, A - the cross-section presented, depends on orientation of the aircraft, and ρ - atmospheric density, will vary primarily with altitude. By integrating this force along trajectory, you can estimate amount of work done against your spacecraft, which will give you the effective delta-V.

Usually, unless you are doing direct descent, aerobraking will not alter trajectory through atmosphere too much. So you can get away with picking a periapsis and integrating the drag. In other words, you don't need to solve differential equation. The integral itself, however, is ugly, so you'll have to carry it out numerically.

In terms of finding correct periapsis, procedure is the same as for finding a root. Pick a periapsis that's definitely too low and one that's definitely too high. Take a point in between, compute delta-V, and see if it's too low or two high. Discard high or low starting point based on result, and keep repeating the procedure with remaining two points until they get close together.

[YNM]

On 7/6/2018 at 3:54 PM, Sr Alvero said:

I am doing my final project.

I'm pretty sure many of the members here are not really qualified for that ! We have a few though.

[Gargamel]

6 hours ago, YNM said:

I'm pretty sure many of the members here are not really qualified for that ! We have a few though.

It always bother's me when somebody comes onto a forum and posts their Senior project, and basically asks an XY question, often with no idea on how to do it on their own. 

EDIT: Not that this is exactly what is happening here, it might be, but I've seen it too many times on other technical forums.  But we'll need a lot more details in order to help out. 

Edited July 7, 2018 by Gargamel

[Vanamonde]

Guys, if you don't want to help OP, that's fine. But please don't stop by the thread just to grouse at him/her. 

[NSEP]

Are you playing KSP or Orbiter?

If you are playing KSP, you should have a look at this mod:

 

[YNM]

9 hours ago, Gargamel said:

we'll need a lot more details in order to help out.

And some beefy textbooks...

[Gargamel]

8 hours ago, NSEP said:

Are you playing KSP or Orbiter?

Given the OP said it was his final project, I'm guessing they are planning an actual mission, not a game. 

[Xd the great]

My suggestion: play ksp. Do interplanetary travel and use the trajectories mod.

[YNM]

On 7/6/2018 at 3:54 PM, Sr Alvero said:

I am a physics student and I am doing my final project.

 

16 hours ago, NSEP said:

Are you playing KSP or Orbiter?

 

10 minutes ago, Xd the great said:

My suggestion: play ksp.

 

Alright, jokes aside...

With my rather limited knowledge of aerodynamic physics (I hope I won't have to do them unless it's project-critical), the way out is for a differential equation.

I guess K^2 just explained the easiest method for it. I tried finding non-linear ones but... yeah.

Edited July 8, 2018 by YNM

[Bill Phil]

You've already reached Titan?

Enceladus has no atmosphere. Thus you would need to aerobrake in Saturn's atmosphere. The total Delta-V cost would likely still be the same as a minimum energy Hohmann orbit.

It might actually be better to escape Titan and then use it for gravity assists to get to Enceladus, using rocket thrust to brake into orbit around Enceladus. Aerobraking over Saturn could be useful in that situation. 

Unless you've reached Titan for a flyby and you want to set up an aerobrake?

Edited July 8, 2018 by Bill Phil

[wumpus]

Would an aluminum foil parachute work?  Or would a more supple fabric have better density and a workable melting point?  All the aerobraking suggestions I've seen tended to assume that the spacecraft was the aero-brakes, which puts a lot of thermal and mechnical stress on an item for a single use.

One *big* catch is heat management.  Remember that re-entry heating doesn't primarily come from friction but from adiabatic (air compression) heating, so expect extra large holes in the center to allow the compressed air to miss the parachute as it flows by.  NASA appears to have already discarded this idea, and sticks with the blunt shape used since Mercury via their "inflatable heat shield".

[K^2]

5 hours ago, wumpus said:

Would an aluminum foil parachute work?  Or would a more supple fabric have better density and a workable melting point?  All the aerobraking suggestions I've seen tended to assume that the spacecraft was the aero-brakes, which puts a lot of thermal and mechnical stress on an item for a single use.

When aerobraking against something like Jupiter or Saturn for aerocapture, heat generation is slightly less of an issue. Because the radius of the planet is larger, you get to spend longer in atmosphere, which means significantly less force, and the heat is deposited over longer time, allowing for it to dissipate naturally. This means that you can usually get away with using your craft and solar panels as your aerobrake, without needing a heat shield or dedicated drogue.