4 ms·

Can anyone shed more light on how the "four hypersonic grid fins" work? Why are they grids and not very small fins?

Alain-lf 12y ago
They are low profile since they can be folded against the body of a missile/rocket. They also have good aerodynamic properties at hypersonic speeds. http://en.wikipedia.org/wiki/Grid_fin http://en.wikipedia.org/wiki/Grid_fin for more info
deleted 12y ago
[deleted]
ChuckMcM 12y ago
Watching their example video I'm guessing they act as steerable "fins" where the working area of the fin is spread across a number of "vanes". Not clear if there are two degrees of freedom or just one for each one. As the rocket travels through the air each one can be rotated on its axis perpendicular to the body of the rocket. If all are rotated the same way they would impart a spin, if two were rotated one way and other two rotated the opposite direction they would simply slow the rocket down. If two adjacent ones are counter rotated the provide drag on one side (imparting a pitch change). I was thoroughly impressed when I saw the video, that is a really cool technique.
- mitchty 12y ago
  On the second video they show them gimbaling towards the rocket and down as well as rotating so I think they have at least 2 degrees of freedom to aide in control alongside the engine gimbal.
colechristensen 12y ago
* Grid fins work well at subsonic and high super sonic speeds (but poorly around Mach 1) lowering drag at high mach numbers. * Grid fins require less structure to support / actuate them per unit control force * Grid fins are more easily stowed (unneeded during launch, would cause significant drag forces if not stowed)
VLM 12y ago
An interesting mental model is a biplane (well, a grid of 20 not just 2) of really small leading edge slats. That's obviously not exactly how they work, but for seven word summary, not bad. They're originally a "Russian thing", for a decade or so it seemed like every missile they designed had grid fins. They're an example of high optimization. Around certain speeds they work REALLY well and weigh practically nothing and the control forces are very low. On the other hand they're little more than speedbrakes around other airspeeds.
martythemaniak 12y ago
Here's a Falcon 9R flight using the grid fins: https://www.youtube.com/watch?v=DgLBIdVg3EM https://www.youtube.com/watch?v=DgLBIdVg3EM
- josefresco 12y ago
  On a completely unrelated side note; the cattle grazing (or running?) in the foreground was quite a contrast to the modern, huge rocket landing itself in the background.

They are low profile since they can be folded against the body of a missile/rocket. They also have good aerodynamic properties at hypersonic speeds. http://en.wikipedia.org/wiki/Grid_fin http://en.wikipedia.org/wiki/Grid_fin for more info

deleted 12y ago

[deleted]

ChuckMcM 12y ago

Watching their example video I'm guessing they act as steerable "fins" where the working area of the fin is spread across a number of "vanes". Not clear if there are two degrees of freedom or just one for each one. As the rocket travels through the air each one can be rotated on its axis perpendicular to the body of the rocket. If all are rotated the same way they would impart a spin, if two were rotated one way and other two rotated the opposite direction they would simply slow the rocket down. If two adjacent ones are counter rotated the provide drag on one side (imparting a pitch change). I was thoroughly impressed when I saw the video, that is a really cool technique.

mitchty 12y ago
On the second video they show them gimbaling towards the rocket and down as well as rotating so I think they have at least 2 degrees of freedom to aide in control alongside the engine gimbal.

colechristensen 12y ago

* Grid fins work well at subsonic and high super sonic speeds (but poorly around Mach 1) lowering drag at high mach numbers. * Grid fins require less structure to support / actuate them per unit control force * Grid fins are more easily stowed (unneeded during launch, would cause significant drag forces if not stowed)

VLM 12y ago

An interesting mental model is a biplane (well, a grid of 20 not just 2) of really small leading edge slats. That's obviously not exactly how they work, but for seven word summary, not bad. They're originally a "Russian thing", for a decade or so it seemed like every missile they designed had grid fins. They're an example of high optimization. Around certain speeds they work REALLY well and weigh practically nothing and the control forces are very low. On the other hand they're little more than speedbrakes around other airspeeds.

martythemaniak 12y ago

Here's a Falcon 9R flight using the grid fins: https://www.youtube.com/watch?v=DgLBIdVg3EM https://www.youtube.com/watch?v=DgLBIdVg3EM

josefresco 12y ago
On a completely unrelated side note; the cattle grazing (or running?) in the foreground was quite a contrast to the modern, huge rocket landing itself in the background.