Author Topic: Grasshopper Discussion (including Grasshopper 2, aka F9R-Dev1) Thread 1  (Read 312128 times)

Offline meekGee

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I've tried to find a discussion of this using the search utility but haven't found it.
Has anyone tried to estimate the effectiveness of the grid fins at various speeds? I'm curious to what extent they could change the angle of attack of a re-entering stage and therefore cause lift and possibly significantly reduce the need for such a fuel intensive boost back.
Presumably this is dependent on whether they can be deployed whilst the stage is hypersonic?

The mode you describe, in which their only job is to tilt the much larger rocket body ("trim mode") is the mode that requires the least effectiveness from them - so I have no doubt they can do that.

The other mode is in which they are the prime aerodynamic surface (as would be the case if the body was much smaller) and they'd have to provide the lift themselves.  I don't think that's the plan.

However I don't think they can reduce fuel consumption by much, since you still have to kill all the forward velocity, and give at least a good measure of backwards velocity.

What they do is guarantee that the rocket can hit the "box" from which the terminal burn can take over and land - even if encountering stronger unexpected winds on the way back through the atmosphere.  I think that's their principal function.
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Offline Lourens

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1000m F9R flight, new actuators, new software according to Reisman.  ~25:40 in the video.

http://new.livestream.com/AIAAvideo/space2014/videos/58462185

What are actuators? I know it has something to do with stability.

An actuator is an electronically controlled device that acts on a system. Compare with a sensor, which senses some aspect of a system.

A closed-loop control system typically runs in a loop:

1. Sense the current state of the system
2. Compare the sensed state with the desired state
3. Act to correct any deviations
4. Go back to 1.

I don't know how the control system for F9R works, but I'm guessing it has a GPS position sensor, inertial sensors (accelerometers), we know it has radar sensors on the legs to measure the distance to the ground, there will be a whole bunch of sensors associated with the engine(s), the grid fins will have sensors that measure their current position, and so on.

The flight computer will read out measurements from all these, and compare them to the planned trajectory. There will always be some deviation, for example, maybe on final approach the rocket is approaching the ground a little bit more rapidly than planned. The flight computer will notice this from the radar readings, calculate how big the difference is, and figure out how much additional thrust is needed to compensate and get back to (or at least closer to) to the planned position and velocity.

To actually get the rocket to slow down, the flight computer sends a signal to an actuator, in this case the throttle control, to throttle up a bit, thus creating the required extra thrust. Other actuators on F9R would include engine gimbals (which point the engines to the side for coarse steering), the small attitude control thrusters that are used to flip the stage over after stage separation and generally to keep it pointing in the right direction, and recently (and I suspect that that's what Reisman was talking about, although I haven't watched the talk yet) the grid fins.

From a control software perspective, the tricky thing with the grid fins is that you now have more ways of compensating for a deviation from the planned trajectory towards the landing location. So the software has to decide whether to compensate using the attitude control thrusters or the fins. You'll want to minimise thruster usage so you can optimise N2 consumption, but not to the point where the probability of actually getting to the right spot diminishes. The effectiveness of the grid fins will vary depending on the speed of the stage and the local air density, so the solution depends on where you are in the flight, too. Plenty of things for SpaceX to test during the test programme.

Wikipedia has an article on control theory for those interested in more on the subject.

Offline alang

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Thanks for the link deruch:

I've tried to find a discussion of this using the search utility but haven't found it.
Has anyone tried to estimate the effectiveness of the grid fins at various speeds? I'm curious to what extent they could change the angle of attack of a re-entering stage and therefore cause lift and possibly significantly reduce the need for such a fuel intensive boost back.
Presumably this is dependent on whether they can be deployed whilst the stage is hypersonic?

I remember when they were first seen, someone posted this link, which I thought did a great job of explaining a bit about grid fin utility.

I eventually started to wonder about this after ugordon's comment about angle of attack change (by thrust vectoring I guess) during the ASIASAT-8 launch :

It's not a dogleg, it's flying at an angle of attack during that phase. Vehicle is angled slightly more toward vertical than its velocity vector through the atmosphere. It can be seen in the Thaicom 6 launch (as well as a pre-MECO maneuver to get back to zero AoA), probably would also have been visible in SES-8 if the lighting angle was better.

Offline WindyCity

According to the FAA the F9R-Dev1 made a flight on the first of August.

Any confirmation that the flight took place? If so, was it successful? Why hasn't a video been released as usual?

Offline docmordrid

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IIRC it was Steve Jurvetson at SmallSat who confirmed the hop and said a video was forthcoming.
DM

Offline deruch

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Thanks for the link deruch:

I've tried to find a discussion of this using the search utility but haven't found it.
Has anyone tried to estimate the effectiveness of the grid fins at various speeds? I'm curious to what extent they could change the angle of attack of a re-entering stage and therefore cause lift and possibly significantly reduce the need for such a fuel intensive boost back.
Presumably this is dependent on whether they can be deployed whilst the stage is hypersonic?

I remember when they were first seen, someone posted this link, which I thought did a great job of explaining a bit about grid fin utility.

I eventually started to wonder about this after ugordon's comment about angle of attack change (by thrust vectoring I guess) during the ASIASAT-8 launch :

It's not a dogleg, it's flying at an angle of attack during that phase. Vehicle is angled slightly more toward vertical than its velocity vector through the atmosphere. It can be seen in the Thaicom 6 launch (as well as a pre-MECO maneuver to get back to zero AoA), probably would also have been visible in SES-8 if the lighting angle was better.

alang, no prob on the link.  As I said, it was someone else who originally posted it somewhere here, but I couldn't find it to give proper credit where it's due. 

NB: ugordan was talking about the launch AoA not the re-entry.  The grid fins won't be used on the upward portion of flight.
Shouldn't reality posts be in "Advanced concepts"?  --Nomadd

Online chrisking0997

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According to the FAA the F9R-Dev1 made a flight on the first of August.

Any confirmation that the flight took place? If so, was it successful? Why hasn't a video been released as usual?

IIRC it was Steve Jurvetson at SmallSat who confirmed the hop and said a video was forthcoming.

Have we seen anything (news, video, cow movement reports) from this hop yet?
Tried to tell you, we did.  Listen, you did not.  Now, screwed we all are.

Offline alang

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This may also have been discussed at length( can't find it), has anyone here looked at the implications of the lift effects of a rotating cylinder, if the returning stage can be modelled as such:
http://www.grc.nasa.gov/WWW/k-12/airplane/cyl.html
Clearly, by the time the legs open there is no such effect, but is it clear yet whether spin is an important feature at an earlier point? That link suggests that any such lift would both perpendicular to the direction of travel and perpendicular to the length of the tube. On the face of it that effect doesn't seem very useful and possibly problematic, but it does help  make me appreciate that trying to understand the aerodynamics of that returning stage is way beyond a high school education...

Offline guckyfan

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This may also have been discussed at length( can't find it), has anyone here looked at the implications of the lift effects of a rotating cylinder, if the returning stage can be modelled as such:
http://www.grc.nasa.gov/WWW/k-12/airplane/cyl.html
Clearly, by the time the legs open there is no such effect, but is it clear yet whether spin is an important feature at an earlier point? That link suggests that any such lift would both perpendicular to the direction of travel and perpendicular to the length of the tube. On the face of it that effect doesn't seem very useful and possibly problematic, but it does help  make me appreciate that trying to understand the aerodynamics of that returning stage is way beyond a high school education...

That picture shows a rotating cylinder with the airflow at a 90 angle to the long axis of the cylinder. The incoming stage will have airflow mostly parallel to the long axis. Attitude control will be done with grid fins which would become harder if not impossible with a rotating stage. So I assume the stage will not rotate and the grid fins will assure that.

A cylinder will produce lift if the grid fins will place it at a slight angle to the airflow.




Online llanitedave

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In the video of the incoming F9 stage, it didn't seem to be spinning much, if any, even at elevation.
"I've just abducted an alien -- now what?"

Offline JamesH

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This may also have been discussed at length( can't find it), has anyone here looked at the implications of the lift effects of a rotating cylinder, if the returning stage can be modelled as such:
http://www.grc.nasa.gov/WWW/k-12/airplane/cyl.html
Clearly, by the time the legs open there is no such effect, but is it clear yet whether spin is an important feature at an earlier point? That link suggests that any such lift would both perpendicular to the direction of travel and perpendicular to the length of the tube. On the face of it that effect doesn't seem very useful and possibly problematic, but it does help  make me appreciate that trying to understand the aerodynamics of that returning stage is way beyond a high school education...

This is worth a look if you are interested in rotating cylinders producing thrust.

http://en.wikipedia.org/wiki/Rotor_ship


Offline Kabloona

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A cylinder will produce lift if the grid fins will place it at a slight angle to the airflow.

This is the key point. The stage will not be rotating, so lift produced by a rotating cylinder is irrelevant.

But as guckyfan says, the grid fins will induce a small angle if attack, and dynamic pressure acting on the exposed side of the (non-rotating!) cylinder will induce a small sideways lift force that will cause the stage to move sideways. The key will be to see how much of a "glide slope" they can induce this way, and therefore how much cross-range distance they can achieve. I expect that to be one of the test objectives at Spaceport America.

Offline Lars_J

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A cylinder will produce lift if the grid fins will place it at a slight angle to the airflow.

This is the key point. The stage will not be rotating, so lift produced by a rotating cylinder is irrelevant.

But as guckyfan says, the grid fins will induce a small angle if attack, and dynamic pressure acting on the exposed side of the (non-rotating!) cylinder will induce a small sideways lift force that will cause the stage to move sideways. The key will be to see how much of a "glide slope" they can induce this way, and therefore how much cross-range distance they can achieve. I expect that to be one of the test objectives at Spaceport America.

Yes, maybe it won't be much, but should allow the stage to at least compensate for drift caused by wind. Perhaps it can do more.

Offline meekGee

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A cylinder will produce lift if the grid fins will place it at a slight angle to the airflow.

This is the key point. The stage will not be rotating, so lift produced by a rotating cylinder is irrelevant.

But as guckyfan says, the grid fins will induce a small angle if attack, and dynamic pressure acting on the exposed side of the (non-rotating!) cylinder will induce a small sideways lift force that will cause the stage to move sideways. The key will be to see how much of a "glide slope" they can induce this way, and therefore how much cross-range distance they can achieve. I expect that to be one of the test objectives at Spaceport America.

Yes, maybe it won't be much, but should allow the stage to at least compensate for drift caused by wind. Perhaps it can do more.

I think it will be significant - it is the purpose of the fins.

Whether it is used to counter winds or to bias the stage after intentionally short boost-back burns - that's a conops decision.

If you only wanted stability, you wouldn't put 2-DOF servo control on each one.
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Online douglas100

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Yes, agree. My first thought was that the grid fins would just deploy like a Soyuz abort, to a fixed position for extra stability. The fact that they are active strongly suggests they are used for some kind of terminal guidance. When they would deploy during an operational flight, and how much trajectory control they could effect makes for interesting speculation.
Douglas Clark


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