SpaceX Falcon 9 v1.1 - Dragon - CRS-5/SpX-5 -Jan. 10, 2015 - DISCUSSION

[AS-503]

No, it's not jettisoned. The grid fins are attached to the interstage.

Are the grid fins used during the reentry burn?  I thought they'd be used between the reentry burn and the landing burn (not sure when, but between).

The grid fins are aero surfaces, if you haven't reentered there's not yet any significant atmosphere for them to work. Your second comment is certainly accurate but that covers a large range of velocities. Most likely supersonic and transonic usage of grid fins but likely not hypersonic.

[archipeppe68]

I have a question: did the fins (so similiar to the Soyuz LV's ones) are used only to slow down the first stage or/and to stabilize it too?

[bombyx]

Here it is my contribution to such topic.

Ciao
Giuseppe

More informations :
During Cassiopée , the reentry burn began at  T + 6 min (approximately)
See here : http://forum.nasaspaceflight.com/index.php?topic=32946.120
During CRS3 , the reentry burn ended  at T + 8 min.  (it was 19:33) (approximately)
See here : http://forum.nasaspaceflight.com/index.php?topic=34502.105
During CRS3 , the stage touched water at 19:35  , so, 2 minutes later , ( T + 10 min)
See here : http://forum.nasaspaceflight.com/index.php?topic=34794.0

(BTW  Hello everyone !!    )

[butters]

I think the expectation is that the grin fins are deployed prior to reentry even though their primary role is to provide control authority between the reentry and landing burns. Deploying during/after reentry would put too much load on the mechanism.

[llanitedave]

... the grin fins are deployed...

Those are deployed after a successful touchdown.

[Alkan]

... the grin fins are deployed...

Those are deployed after a successful touchdown.

Yep, they're for steering the barge back to port.   

Really, the wait for this launch is killing me! Lol. The suspense. I really want to see it work.

[Lee Jay]

No, it's not jettisoned. The grid fins are attached to the interstage.

Are the grid fins used during the reentry burn?  I thought they'd be used between the reentry burn and the landing burn (not sure when, but between).

The grid fins are aero surfaces, if you haven't reentered there's not yet any significant atmosphere for them to work.

That's what I was thinking.  So, showing the grid fins deployed and the engines burning at the same time is inaccurate.

Your second comment is certainly accurate but that covers a large range of velocities. Most likely supersonic and transonic usage of grid fins but likely not hypersonic.

True, but for the graphic, that shouldn't matter.  They could just show deployed after engine cutoff of the reentry burn and before the landing burn.

[Hauerg]

I have a question: did the fins (so similiar to the Soyuz LV's ones) are used only to slow down the first stage or/and to stabilize it too?

Slowdown+stabilize +STEER.
IIRC Elon said that without the grid fins they could not hit the barge.

[cscott]

I don't think the grid fins will slow down the stage to any significant degree.  They are for attitude control.  Of course, keeping the stage in the proper orientation might lead to enhanced drag, but the grid fins themselves are not spoilers.

[Prober]

Here it is my contribution to such topic.

Ciao
Giuseppe

Giuseppe you do great work, always enjoy it 

[mme]

No, it's not jettisoned. The grid fins are attached to the interstage.

Are the grid fins used during the reentry burn?  I thought they'd be used between the reentry burn and the landing burn (not sure when, but between).

The grid fins are aero surfaces, if you haven't reentered there's not yet any significant atmosphere for them to work. Your second comment is certainly accurate but that covers a large range of velocities. Most likely supersonic and transonic usage of grid fins but likely not hypersonic.

From Wikipedia

Grid fins perform very well at subsonic and supersonic speeds, but poorly at transonic speeds; the flow causes a normal shockwave to form within the lattice, causing much of the airflow to pass completely around the fin instead of through it and generating significant wave drag. However, at high Mach numbers, grid fins flow fully supersonic and can provide lower drag and greater maneuverability than planar fins.

So they won't be used for control when transonic and since the are high drag when transonic my bet is they will be stowed.  They will definitely use them subsonic, that's how they plan to hit the barge.  I'm not sure if early flights will use them hypersonically.  I think eventually they will use them to improve cross range distance.  I'm not sure if they need them for this test.

[whitelancer64]

I don't think the grid fins will slow down the stage to any significant degree.  They are for attitude control.  Of course, keeping the stage in the proper orientation might lead to enhanced drag, but the grid fins themselves are not spoilers.

No, it's not jettisoned. The grid fins are attached to the interstage.

Are the grid fins used during the reentry burn?  I thought they'd be used between the reentry burn and the landing burn (not sure when, but between).

The grid fins are aero surfaces, if you haven't reentered there's not yet any significant atmosphere for them to work. Your second comment is certainly accurate but that covers a large range of velocities. Most likely supersonic and transonic usage of grid fins but likely not hypersonic.

From Wikipedia

Grid fins perform very well at subsonic and supersonic speeds, but poorly at transonic speeds; the flow causes a normal shockwave to form within the lattice, causing much of the airflow to pass completely around the fin instead of through it and generating significant wave drag. However, at high Mach numbers, grid fins flow fully supersonic and can provide lower drag and greater maneuverability than planar fins.

So they won't be used for control when transonic and since the are high drag when transonic my bet is they will be stowed.  They will definitely use them subsonic, that's how they plan to hit the barge.  I'm not sure if early flights will use them hypersonically.  I think eventually they will use them to improve cross range distance.  I'm not sure if they need them for this test.

in the trans-sonic regime grid fins will essentially act as drag flaps, allowing the stage to slow down. i doubt they will be stowed for this reason.

i think the grid fins will be deployed soon after they complete the retropropulsion burn. i don't think the stage will be hypersonic (speed > Mach 5) after that. the fins give guidance and can be used to improve cross-range distance, both of which will be needed to accurately arrive at the barge.

[Chris Bergin]

Remember to use the SpaceX Reusability section for ASDS content.

Live Launch Day thread tomorrow!!

[cscott]

It will be interesting to see how the grid fins will be used, that's for sure.  At hypersonic speeds they work, but might not generate enough force given the altitude. As the stage descends there's a tricky transition to transonic flow, where they aren't effective as fins but do generate spoiler drag -- and quite substantial forces on the actuators, presumably. Then there's another tricky transition to subsonic flow, where the fins are again low drag but can generate substantial aerodynamic forces with minimal actuator force.

Because of the difficulties with symmetrical transitions through flow regimes (this is what Carmack was referencing) and the potentially large actuator forces, most folks (me included) assume the fins will be stowed during transonic flight (and a comfortable margin of velocities around that transition).  This is why I mentioned that the fins are likely not being used for high drag.

But it is probable that they will be deployed in hypersonic flight, even though fin effectiveness may be low due to the altitude and thin atmosphere.  If they weren't interested in using them supersonic they wouldn't have bothered using grid fins in the first place.

And if SpaceX is really confident in their modeling (and it is true that grid fins are fairly well-understood analyticly) and in the strength of their actuators, then they may well keep them extended through the transonic transition region. It would be pretty gutsy to do so, considering the aerodynamic and mechanical unknowns, the risk of control inversion, etc.  Perhaps the actuators can be locked through transition. SpaceX do take risks, and the extra transonic drag might make it worth it.  Maybe not on the first flight, though?

It will be exciting to find out!

[whitelancer64]

It will be interesting to see how the grid fins will be used, that's for sure.  At hypersonic speeds they work, but might not generate enough force given the altitude. As the stage descends there's a tricky transition to transonic flow, where they aren't effective as fins but do generate spoiler drag -- and quite substantial forces on the actuators, presumably. Then there's another tricky transition to subsonic flow, where the fins are again low drag but can generate substantial aerodynamic forces with minimal actuator force.

Because of the difficulties with symmetrical transitions through flow regimes (this is what Carmack was referencing) and the potentially large actuator forces, most folks (me included) assume the fins will be stowed during transonic flight (and a comfortable margin of velocities around that transition).  This is why I mentioned that the fins are likely not being used for high drag.

But it is probable that they will be deployed in hypersonic flight, even though fin effectiveness may be low due to the altitude and thin atmosphere.  If they weren't interested in using them supersonic they wouldn't have bothered using grid fins in the first place.

And if SpaceX is really confident in their modeling (and it is true that grid fins are fairly well-understood analyticly) and in the strength of their actuators, then they may well keep them extended through the transonic transition region. It would be pretty gutsy to do so, considering the aerodynamic and mechanical unknowns, the risk of control inversion, etc.  Perhaps the actuators can be locked through transition. SpaceX do take risks, and the extra transonic drag might make it worth it.  Maybe not on the first flight, though?

It will be exciting to find out!

considering all the variables makes me really antsy for the start of high-altitude F9R tests at Spaceport America. SpaceX can do the risky testing there. i'm sure SpaceX has this all thought out of course, and it's not a tremendously great loss if they break off a grid fin and miss the barge, since the stage would have been splashed anyway. perhaps they are willing to try something more risky now, rather than lose another test flight vehicle later?

[deruch]

It will be interesting to see how the grid fins will be used, that's for sure.  At hypersonic speeds they work, but might not generate enough force given the altitude. As the stage descends there's a tricky transition to transonic flow, where they aren't effective as fins but do generate spoiler drag -- and quite substantial forces on the actuators, presumably. Then there's another tricky transition to subsonic flow, where the fins are again low drag but can generate substantial aerodynamic forces with minimal actuator force.

Because of the difficulties with symmetrical transitions through flow regimes (this is what Carmack was referencing) and the potentially large actuator forces, most folks (me included) assume the fins will be stowed during transonic flight (and a comfortable margin of velocities around that transition).  This is why I mentioned that the fins are likely not being used for high drag.

But it is probable that they will be deployed in hypersonic flight, even though fin effectiveness may be low due to the altitude and thin atmosphere.  If they weren't interested in using them supersonic they wouldn't have bothered using grid fins in the first place.

And if SpaceX is really confident in their modeling (and it is true that grid fins are fairly well-understood analyticly) and in the strength of their actuators, then they may well keep them extended through the transonic transition region. It would be pretty gutsy to do so, considering the aerodynamic and mechanical unknowns, the risk of control inversion, etc.  Perhaps the actuators can be locked through transition. SpaceX do take risks, and the extra transonic drag might make it worth it.  Maybe not on the first flight, though?

It will be exciting to find out!

Because each of the fins are independently controllable, they could theoretically also be used for drag braking even in the subsonic regime.  You just need to balance out the forces.  But I don't think this method of operation is at all likely on early tests.  It may however be one of the techniques eventually used to decrease terminal velocity of the returning stage and thereby reduce deltaV needed for the landing burn.

[IslandPlaya]

Maybe they could take the stage over the launch site and then go back to land on the barge.

[cscott]

@deruch Sure they could, but the subsonic cross sectional area of splayed grid fins is just not that great.  Most of the drag is coming from the stage cross section, which is much larger.

 (In the hypersonic regime the grid fins are essentially solid, which increases their effectiveness as drag surfaces.)

[meekGee]

The diagram above shows a re-entry burn, but no boost-back burn.

For forward recovery, this makes sense.

However, I think this test includes some boost-back.  I say that, because on the nav charts, the intended landing zone is #3 out of 4.  It makes sense to me that the furthest zone (#4) is a contingency zone in case the boost-back burn does not happen, and zone #2 is in case the re-entry burn does not happen, and zone #3 is where the X is.

Why are they doing boost-back when they can just park the barge further out?  Maybe it's part of practicing RTLS, so going through all the motions.  When it's time to do FH center cores, they might skip the boost back.

[rpapo]

Because each of the fins are independently controllable, they could theoretically also be used for drag braking even in the subsonic regime.  You just need to balance out the forces.  But I don't think this method of operation is at all likely on early tests.  It may however be one of the techniques eventually used to decrease terminal velocity of the returning stage and thereby reduce deltaV needed for the landing burn.

Anything which reduces the terminal velocity also, potentially, improves the cross-range and accuracy in landing.