# How far can the supersonic parachute in the LDSD system for Mars be placed from the entry capsule?

In this interview on NASA Edge Ian Clark, principle investigator for the Low Density Supersonic Decelerator, talked about the environment the parachute part of the system has to operate in:

Parachutes are very fickle devices, particularly supersonic parachutes. When we have to use them like we do at Mars, it’s behind a very large, blunt vehicle. That vehicle is screaming through the atmosphere. It’s punching a hole in the atmosphere. All the air is rushing in behind it to fill the vacuum that it’s creating. That creates a very turbulent, very unsteady environment for the parachute to live in.

The farther the parachute is from the vehicle, the less turbulence there is. So what limits how far away it can be deployed?

If the above illustration shows the 6 m capsule with the SIAD deployed, it doesn't seem to be to scale - the parachute is 33.5 m. Is it of a different parachute and SIAD combination? Does it represent the proportions used in the real system accurately?

## 2 Answers

The diagram may not be exactly to scale, but it's closer than you think. The parachute is 30.5 meters (not 33.5), where that is the constructed diameter, referred to as D0. That is the diameter of, effectively, the parachute material laid flat on the ground. The diameter of the skirt when flying is about 83% of the constructed diameter, but varies as the parachute breathes. So it is not used as a specification for a parachute.

We like to keep the parachute about ten forebody diameters away to limit its effects. The further back you go, the more powerful the cannon has to be to shoot it out which has bad system impacts. We'd rather live with the behavior of the parachute back there, which produces nearly the full drag it would with no forebody, than try to put it back farther. But we need to understand that behavior in order to properly model it in simulations.

It all depends on the separation of the flow around the module; vortex formation is the most important contributor to turbulence, and if the flow has fully converged before it reaches the parachute, that's great! So it's not something that you can determine as a general rule, there's no rule of thumb other than "farther is better": it depends on what the fluid dynamics of flow around the module look like.

• Does it help if I limit the question only to the LDSD system for Mars? Jun 3, 2015 at 20:18
• @briligg: Certainly, but the best answer would be found from plugging this pancake-looking module into flow with the right Reynolds numbers and observing where separation seems to occur until. From my understanding there is no real rule of thumb you can use for such a problem as turbulence, especially in an atypical fluid setting as the Martian atmosphere at hypersonic reentry. It's a complex problem. See this paper for some interesting, somewhat related work: it's what I thought of when I read your question. Jun 3, 2015 at 20:24
• The section on Aerodynamics and Aerothermodynamics on page 11 seems to be the relevant part, if anyone else goes to look at it. Jun 3, 2015 at 20:33
• At any rate, I narrowed the scope of the question based on your input. I'm hoping more can be said regarding the design decisions on the LDSD as it relates to this. Jun 3, 2015 at 20:41