Why do people put nosecones on the rappiers on SSTOs?

[Dr. Kerbal]

Why do people put nosecones on the rappiers on SSTOs?  Is this because KSP calculates drag with unused nodes? 

[swjr-swis]

53 minutes ago, Dr. Kerbal said:

because KSP calculates drag with unused nodes

 

[Lt_Duckweed]

21 hours ago, Dr. Kerbal said:

Is this because KSP calculates drag with unused nodes? 

Not quite.  Ksp calculates drag based on exposed part surface area.  There are a couple ways to reduce the exposed surface area.

1. Place the part in a service bay, cargo bay, or fairing.

2. Node attach parts.  When you node attach 2 parts, the two node atached faces have the area of the attached face subtracted from their exposed surface area. Thus, the smaller face has its area set to 0, and the larger face has the area of the smaller face subtracted from it.

There is an additional "streamlining" factor applied to each face of a part (each part has 6 faces) at play, the "pointier" and more aerodynamic a face is, the lower the factor is.  (The front face of a nosecone has a low streamlining factor. The sides, being less pointy, have a high factor).  Infinite streamlining has a factor of 0, a perfectly flat plate has a factor of 1.

Rapiers have a very flat back end, with a factor that is nearly 1.  Adding a nosecone removes some of the exposed area on the rapier and replaces it with the much more streamlined area of a nosecone.

Edited May 26, 2021 by Lt_Duckweed

[Dr. Kerbal]

11 minutes ago, Lt_Duckweed said:

Not quite.  Ksp calculates drag based on exposed part surface area.  There are a couple ways to reduce the exposed surface area.

1. Place the part in a service bay, cargo bay, or fairing.

2. Node attach parts.  When you node attach 2 parts, the two node atached faces have the area of the attached face subtracted from their exposed surface area. Thus, the smaller face has its area set to 0, and the larger face has the area of the smaller face subtracted from it.

There is an additional "streamlining" factor applied to each face of a part (each part has 6 faces) at play, the "pointier" and more aerodynamic a face is, the lower the factor is.  (The front face of a nosecone has a low streamlining factor. The sides, being less pointy, have a high factor).  Infinite streamlining has a factor of 0, a perfectly flat plate has a factor of 1.

Rapiers have a very flat back end, with a factor that is nearly 1.  Adding a nosecone removes some of the exposed area on the rapier and replaces it with the much more streamlined area of a nosecone.

Thank you.

[Fraktal]

...okay, I'm beginning to approach what territory again. Do open nodes cause drag or not? Because I keep seeing opposing answers every couple of months rotating between "yes, they do" and "not since 1.0".

[Guest]

For me, looks better than not using them.

[Lt_Duckweed]

1 hour ago, Fraktal said:

...okay, I'm beginning to approach what territory again. Do open nodes cause drag or not? Because I keep seeing opposing answers every couple of months rotating between "yes, they do" and "not since 1.0".

Exposed surface area of a part's drag cube causes drag.  Node attachment has a side effect of reducing this exposed area.  Thus reducing drag.

See my earlier post in the thread.  It's not about the node being occupied/unoccupied, it is about the relative sizes of the parts that are atached at the nodes (NOT node size, it is based on the actual visual cross section of the part)

Edited May 26, 2021 by Lt_Duckweed

[FleshJeb]

17 minutes ago, Lt_Duckweed said:

Exposed surface area of a part's drag cube causes drag.  Node attachment has a side effect of reducing this exposed area.  Thus reducing drag.

See my earlier post in the thread.  It's not about the node being occupied/unoccupied, it is about the relative sizes of the parts that are atached at the nodes (NOT node size, it is based on the actual visual cross section of the part)

Do you happen to know how this works with the cargo bay, etc parts? I know we should occupy the internal nodes, and I always assumed that one should use the next size down.

What about heatshields? They have three nodes, so which do you have to occupy? (I actually never use heatshields and just pull heat out of normal parts heat with high-heat transfer parts.)

[Lt_Duckweed]

2 hours ago, FleshJeb said:

Do you happen to know how this works with the cargo bay, etc parts? I know we should occupy the internal nodes, and I always assumed that one should use the next size down.

As long as the exterior nodes are occupied by parts that are as large, or larger, in cross sectional area as the cargo bays, there is no need, or indeed any benefit, to occupying the internal nodes.

2 hours ago, FleshJeb said:

What about heatshields? They have three nodes, so which do you have to occupy? (I actually never use heatshields and just pull heat out of normal parts heat with high-heat transfer parts.)

The rear node will subtract area from the rear face of the drag cube, and the front 2 nodes will both subtract area from the frontal face of the heat shield, and likewise the heat shield will subtract area from whichever face the attached parts nodes are tied to.

[herbal space program]

6 hours ago, Fraktal said:

...okay, I'm beginning to approach what territory again. Do open nodes cause drag or not? Because I keep seeing opposing answers every couple of months rotating between "yes, they do" and "not since 1.0".

It definitely makes a noticeable difference on engines with a big, flat back end like the Rapier. Clipping backwards-facing nosecones into them reduces their drag quite a bit. I used to do that all the time, but I've since found that enveloping such parts with a backwards-facing fairing seems to reduce drag even more, and also seems less cheaty to me since it doesn't involve part-clipping.

Edited May 27, 2021 by herbal space program