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Richard Dierking

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I wasn't really sure what to title this thread. It's not active stabilization, because I want to include rockets that are aerodynamically stable. And, it's not just the up part because I'm interested about controlling recovery. Anyway, hopefully the title is appropriate.

I have some fundamental questions about controlling amateur rocket flight. For starters, I believe we all know you shouldn't use any amateur aircraft as a weapon. Also, our codes (TRA & NAR) say that you shouldn't aim a rocket at a target. If there's something else that applies please let me know.

A number of people are developing types of active stabilization systems. Some, are using things like Thrust Vector Control (TVC) to stabilize rocket flight without fins, or in my case with fins and ground launched, and some are using control surfaces to guide rockets for stable vertical flight. This could be very important for staged rockets and high alt flights; stay in the launch & recovery area. Also, I've heard of projects that would use something like active controlled paragliding to help in recovery. So, the rocket goes up where ever, and instead of drifting off, it glides back.

Here's the first question: What if you are trying to stay above the launch pad/location? Say, you use a GPS coordinate (the pad) in an active program that controls the rocket so it stays vertical and over the pad. Is that OK?

Question 2: How about guiding the rocket to a specific recovery location?
Are the laws different for rockets than say amateur drones in this regard?

Question 3 is more of a technical question: I often hear of roll, pitch, and yaw applied to stabilization systems on amateur rockets. And, I think about every system I've seen uses 4 fins for control surfaces. I'm interested why. Seems to me that a rocket is roll axis symmetrical (like if you were looking down), and really, there's only roll and pitch. So, if you control roll orientation, you could control pitch. And, a rocket can roll a lot more quickly than it can pitch, particularly a rocket that is already aerodynamically stable. Hopefully, you see what I'm trying to describe (I'm not an aeronautical engineer) and can help me understand the control issues.

Thank you,
Richard
 

Antares JS

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To answer question 3, controlling in 3 axes is a lot easier when you have control surfaces aligned with those axes. It's probably possible to control pitch with fewer than 4 fins, but 4 fins is easier to control with than three.

Let's say you want to pitch your rocket around an axis that one of the fins is aligned with. If you have three fins, you only have one fin aligned with the axis. When its control surface activates and tilts, it's going to create an asymmetrical torque that is going to induce roll along with the pitch. It's probably possible to cancel that roll out with the other two fins, but it's much easier to just have another fin directly across from it that can move its control surface the same way to give you a symmetrical pitch force on each side.
 

boatgeek

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Question 1: In that instance, you're guiding it away from a target, so it should be OK.

Q2: I'd like to know the answer to this as well. I presume that the designated landing point should be on the range area that's closed when rockets are flying and also well clear of the area where rockets are going up. There's some side logistics here for launch tempo. At my club, the LCO generally doesn't launch the next rocket until the last one is obviously clear of the crowd. That makes it so you aren't trying to track two objects in the sky at once. If the last rocket is coming down on a paraglider, it will take a while to land, and might hold up flights if you aren't sufficiently considerate about how close it stays to its desired LZ.
 

Yukon@K-9 Rocket Tech

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My friends and I have interest and even have projects for guide recovery to do Return To Launch Site (RTLS). Seems like it's legal because rather than point to point based guidance, it's basically trying to reduce drift so it falls back to where it started from.

Having a hard time understanding the 3rd question, but based on what your saying, a lot of people like myself in the TVC community refer pitch and yaw at the X and Y axis of the vehicle which I assume your referring for pitch and roll. In the TVC community we refer roll as the body of the vehicle rotating in relation to it's central axis. In a 4 canard system we can control pitch and yaw using a pair of fins on the X and Y axis and moving them at the same angle and roll by rotating all 4 fins in the opposite direction
 

Funkworks

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... and really, there's only roll and pitch. So, if you control roll orientation, you could control pitch. And, a rocket can roll a lot more quickly than it can pitch, particularly a rocket that is already aerodynamically stable. Hopefully, you see what I'm trying to describe ..
You could call them "pitch 1" and "pitch 2", or "pitch x" and "pitch y", but you need both (pitch and yaw) for fine control. If you only have 1 pitching direction, you need to roll into it first and that takes too much time.
 

Richard Dierking

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Thank you all. For Q1 and Q2 these are nagging questions I have. Frequently, when I see a TVC or other kind of stabilization project come up, there's someone that says something like, "well, you better watch out because the feds would really be concerned about what you are doing, and you could get in trouble." Do you know what I mean? I would like this be put to rest. I have searched and found nothing that would indicate TVC or any other active guidance system would be a problem, unless it's intended to be a weapon. And, I think that is it.
And, I think there's a big difference between Guidance, Aiming, and Target. A Target is a destination. But, a target is a lot different than a destination; do we agree?

And, I believe there is an implicit restriction with the word Target. I will not aim at a target. The intent is prohibiting amateur rockets as being weapons. Aiming and guiding are OK if they are to make an amateur rocket flight safer.

And, if you don't think rockets are aimed, think about when rockets are leaned away on the pads from the flight line and spectators. They lean to the unoccupied recovery area. You are aiming them away from you.

And, thank you Yukon. Yes, for Q3, that's what I meant. Correction during rolling when the rocket was going out of vertical. It's interesting because the axes are determined by the construction of the rocket before launch, like the yaw and pitch of a spacecraft based on the position of the astronauts inside.
But, if you consider just roll and pitch, the axis can change based on the position in the roll. Anyway, I'm not convinced that the roll couldn't be rapid enough to correct the deviation from the intended vertical trajectory. A rocket that is rolling at 1 Hz is rolling at 360 degrees a second. Pitch would change a lot slower. If two control surfaces "bumped" to control vertical twice a second, why wouldn't this work? I know it probably wouldn't work for TVC alone, because that's like balancing a pencil on the end of your finger. But, how about an aerodynamically stable rocket (well, at least marginally stable)?

Again, please remember I'm not an AE. Just trying to figure some stuff out for a couple projects and hope it helps others too.
 

les

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For aerodynamics, a body in flight actually has 6 degree of freedom (6DOF).
It will be easier to discuss with respect to an airplane.
There are three translational axis - forward/rear, left/right, up/down and
three rotational axis - pitch, roll, and yaw.

1591149083761.png


To clarify the differences, a plane may be pitched up (nose higher than tail) and go up - such as during takeoff. But it can also be pitched up while going down - watch a video of a plane landing with the nose up so the main gear touches first, yet the plane is still moving downward to the runway.

Similar, there are videos of planes landing that are crabbed to the side (left or right) to battle a cross wind but stay lined up with the runway.

A rocket may rotate around it CG to point into the wind (weathercock), but have you ever seen a rocket just translate to the side due to a strong wind - such as a large but light weight rocket (for the older folks, think about the Estes Dude).

A rocket can be somewhat simplified due to its symmetry, but it is still subject to the same 6DOF. To say "go straight up", if there is a cross wind it requires more than to just keep pointing up (preventing weathercocking), you may actually need to direct partially into the wind (similar to a plane crabbing or setting the launch angle) to compensate for the wind trying to blow the entire rocket to the side.

Bottom line, you need to consider more than just the 2 pitch/roll axis
 

OverTheTop

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Seems to me that a rocket is roll axis symmetrical (like if you were looking down), and really, there's only roll and pitch. So, if you control roll orientation, you could control pitch. And, a rocket can roll a lot more quickly than it can pitch, particularly a rocket that is already aerodynamically stable.
Assuming roll is stabilised (or not, for that matter), the only way you can control your direction in the other two dimensions is to control both pitch and yaw. If you are only controlling pitch, how do you control the intended direction of the rocket if it doesn't happen to be in the current pitch plane? You don't. Both directions are needed.

Yes, roll control needs much smaller control moments than pitch and yaw due to a very low moment of inertia about the roll axis. It is also easy to have too much gain in this control axis, resulting in oscillatory behaviour.

An oldie but a goodie. Might be useful if your maths is up to speed.
41QNkNxoitL._SX342_BO1,204,203,200_.jpg


You might be able to find a pdf on the interweb if you are lucky.

FYI, here is a link to a VTS I am working on: https://forum.ausrocketry.com/viewtopic.php?f=56&t=5324

Three fins or four doesn't really matter. It all comes out in the maths.
 

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Assuming roll is stabilised (or not, for that matter), the only way you can control your direction in the other two dimensions is to control both pitch and yaw. If you are only controlling pitch, how do you control the intended direction of the rocket if it doesn't happen to be in the current pitch plane? You don't. Both directions are needed.

Yes, roll control needs much smaller control moments than pitch and yaw due to a very low moment of inertia about the roll axis. It is also easy to have too much gain in this control axis, resulting in oscillatory behaviour.

An oldie but a goodie. Might be useful if your maths is up to speed.


You might be able to find a pdf on the interweb if you are lucky.

FYI, here is a link to a VTS I am working on: https://forum.ausrocketry.com/viewtopic.php?f=56&t=5324

Three fins or four doesn't really matter. It all comes out in the maths.
If you have a stable rocket with decent moment of inertia you can do with it with 2 control surfaces. You can roll the rocket to the desired pitch axis and pitch with a coordinated action of the canards. Fpr vertical stabilization this may be all that is necessary.
 

Not Quite Nominal

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Assuming roll is stabilised (or not, for that matter), the only way you can control your direction in the other two dimensions is to control both pitch and yaw. If you are only controlling pitch, how do you control the intended direction of the rocket if it doesn't happen to be in the current pitch plane? You don't. Both directions are needed.
Two control surfaces, 180 degrees apart from each other.

Turn them opposite to each other to roll, turn them in the same direction to pitch.

You can have full directional control by rolling to the direction of maximum pitch deviation, pitching the nose back in, and repeating. If you have a limited mass/drag budget and excess computational power, it might be the best solution.
 

georgegassaway

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Two control surfaces, 180 degrees apart from each other.

Turn them opposite to each other to roll, turn them in the same direction to pitch.

You can have full directional control by rolling to the direction of maximum pitch deviation, pitching the nose back in, and repeating. If you have a limited mass/drag budget and excess computational power, it might be the best solution.
Theoretically works, and IIRC the old Redeye Missile sort of did it, but not really (not like that anyway).

FAR simpler to go with pitch-yaw control. Also, faster response time. A lot of time lost in rolling to the "right" direction before using a single-axis pitch correction. By which time that is done it may need a correction at 90 degrees to the current roll position, and so on. A "drunk" kind of flight path.

What the Redeye did, with a single axis of flight path control, IIRC, was to have the rear fins angled to make it roll constantly. It only made a course correction while it happened to be rolled in the correct position. It's noteworthy that kind of single-axis steering was never repeated (That I know of) by any other US missiles (Redeye replaced by Stinger which has pitch-yaw control).

If someone wants to do that for the sake of doing that, fine. Otherwise, again, pitch and yaw control is simpler and easier and faster and better. :)
 

Richard Dierking

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Thank you Les for the explanation, and OTT for mentioning the smaller control moment for roll. That's what I'm counting on.
Additionally, I have a lot of confidence that John knows about roll & pitch, and understands the difficulty of even measuring pitch/yaw while a rocket is rolling.
I've heard from the TVC folks that roll isn't a big problem using that method, but that doesn't seem to be what I'm seeing; it's like coning, but much slower.
I'm inclined to think roll & pitch control will work a lot easier for control surfaces on a aerodynamically stabilized rocket than a strictly TVC rocket. But, maybe it could actually work. Two small roll control surfaces with a TVC that only moves in one plane.
Ah... maybe not, but I'm not an AE. ;-)

Answers to Q1, Q2, and Q3 are important and related to future development - and not just for me!
Here's what I envision: Say you place your rocket on the pad leaning a bit away from the flight line. It could be a single or multiple stage rocket. When you launch it, it gets some distance from the folks on the ground, and actively controls roll and orients so it then begins to pitch vertical. Continues to orient roll and control pitch. It reaches apogee, starts to fall and actually continues to come down with roll controlled without a chute until some altitude. Then, drogues deploy to slow it to a couple hundred fps. Then a main deploys, and maybe even a pre-main shortly before that.
Most of what I think is based on observation and some experience. For example, I've placed small fin tabs (1/2" x 1/4" and only 1/32" thick) on a couple opposing fins and saw the affect, which was substantial for their size. Also, because of great GPS tracking equipment from places like Real Flight Systems and Featherweight, I can visualize tracks of rockets ascending and recovering. You just can't get that kind of perspective by just watching video. Anyway, what I think I'm seeing is that 2 control surfaces will work. Just wish I had more capability. But, I'm not just talking/dreaming either! Here's a couple photos of what I'm working on, although I don't presently believe my control system will work that well. I started on this before I had an understanding of servos and an appreciation for what they can do.
IMG_2676.jpg

Rocket is 3" airframe with 54 mm mount. Not much room for a control mechanism.
Black hatch is the compartment for one of 9" drogues to slow the rocket before the pre-main and main parachute.


IMG_2675.jpg

The control surface for testing is 15 mm by 70 mm long. 1/8" aluminum. I still need to bevel the aluminum.
 

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boatgeek

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If I was the RSO, I would have a *lot* of questions about a flight profile that came down hot until some specified altitude and even more so if the rocket was actively trying to stay vertical.

You would have to have at least two levels of drogues, one tiny one to slow to a few hundred FPS and one to slow it to 80-100 FPS for the main to open safely. Maybe that's where main and pre-main come in, but it sounds like a lot of shocks to the system and associated worries about zippering.
 

Not Quite Nominal

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Theoretically works, and IIRC the old Redeye Missile sort of did it, but not really (not like that anyway).

FAR simpler to go with pitch-yaw control. Also, faster response time. A lot of time lost in rolling to the "right" direction before using a single-axis pitch correction. By which time that is done it may need a correction at 90 degrees to the current roll position, and so on. A "drunk" kind of flight path.

What the Redeye did, with a single axis of flight path control, IIRC, was to have the rear fins angled to make it roll constantly. It only made a course correction while it happened to be rolled in the correct position. It's noteworthy that kind of single-axis steering was never repeated (That I know of) by any other US missiles (Redeye replaced by Stinger which has pitch-yaw control).
I can't think of another case of fixed roll, variable pitch trajectory control.

On the other hand, I suppose you could say Gemini and Apollo are fixed pitch, variable roll, with a single axis of control as well. Fly with a fixed slight angle of attack, roll to point that lift vector in the most useful direction.

I don't think I'd use either of these steering systems for a rocket going up, but they do seem interesting for a simple system for steering a parachute coming down. If you only have pitch control, rig the chute for a slow roll, dive the chute when drifting away, flare the chute when drifting towards. If you only have roll control, roll faster if your velocity made good increases, and slower when your VMG decreases.

Thank you Les for the explanation, and OTT for mentioning the smaller control moment for roll. That's what I'm counting on.
Additionally, I have a lot of confidence that John knows about roll & pitch, and understands the difficulty of even measuring pitch/yaw while a rocket is rolling.
I've heard from the TVC folks that roll isn't a big problem using that method, but that doesn't seem to be what I'm seeing; it's like coning, but much slower.
I'm inclined to think roll & pitch control will work a lot easier for control surfaces on a aerodynamically stabilized rocket than a strictly TVC rocket. But, maybe it could actually work. Two small roll control surfaces with a TVC that only moves in one plane.
Single nozzle TVC roll control is difficult!

How useful is TVC for atmospheric rockets? Our rockets are always in the atmosphere, and only sometimes under thrust (unlike space launch rockets, which are often under thrust while out of the atmosphere) That leads me to believe aerodynamic control is superior.

(Not that TVC isn't cool. It is!)
 

Richard Dierking

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TVC is cool. Many people just enjoy the realistic look of a slowly launching/flying rocket without fins. I totally understand. And, it's a challenge for sure.

So, I understand "boatgeek's" concerns. I've had 8 flights (includes a couple 2 stage flights) using 2 side deployment hatches with 9" to 12" chutes for drogues. Descent velocities from 100 to 200 fps and all but one successful. For some, I had pilot chutes ahead of the main and some with a pre-main that slowed and turned the bird around for the main to deploy. I had one with a zipper about 2" long. I think that's when I switched from just having a pilot chute to the pre-main thing.
To give an example, I flew using an AT K270W to 12,130'. The drogue part with the two 9" chutes was 115 fps descent. The main at about 1000' took it down to 21 fps. The whole flight took 174 seconds and it landed 1500' away from the pad.
For the one unsuccessful flight, it was my fault because I launched a two stage with a long burn booster (underpowered). It gravity turned almost immediately, and there was also a wind above the deck I could have anticipated but didn't. It was Black Rock and from the post flight data, the upper wind was about 75 mph.
Anyway, the side hatches deployed the 9" parachutes at about 400 mph. Yeah. The cool thing was that the chutes held and that part of the rocket was undamaged. The not so cool thing was the drag separation of the pre-main and main sections. That part was very ugly.
However, I learned that I could deploy the side drogues at high velocity, and that's when I started considering the "hot rocket" recovery. It really could make the recovery much closer. But, of course I wouldn't like to try this without having some control over where it was descending at high velocity.
There's more to the story, but I don't want this to get totally boring. I do tend to go on a bit.

I appreciate all the comments and suggestions. Standing by and listening.
 

OverTheTop

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If you do go with only two control surfaces and roll the bird to provide control in three dimensions you are complicating things for yourself somewhat. A rolling missile will have a phase shift added to any of the control surface movements. If you have a really good "super duper" ;) control system you will be ok. If you are working at a fairly basic programming level you will find it easier to treat roll, pitch and yaw essentially as separate systems.

If you look at how a helicopter works, with rotating control surfaces, there can be a phase shift of up to nearly 90 degrees in the control actuation. The phase shift varies dynamically during flight. So pitch is controlled by the blades when they roughly abeam of the craft, poking out either side. Having rolling control surfaces complicates things!

So yes, you can control a rocket with only two control surfaces, but do you really want to do it that way?

As for flying a rocket during recovery back to the pad, I would be targeting a point a bit further downrange for safety. If you aim for the pad you are inadvertently also aiming to get closer to the flightline and the associated people and vehicles. Remember to design for off-nominal outcomes, not just nominal :). Nobody want to win the "closest to the RSO" competition.
 

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A 2 surface control system would need a lot of help with an airframe that is nominally very stable and has slow response to turning moments that would occur in flight. Many large projects easily fit this description. Large 2 stage Balls projects that are going for >100K feet are tall and heavy at boost with large moments of inertia in the yaw-pitch axes.

But a 4 control surface system can be tuned to any airframe.

A 2-surface system has the advantage that is it relatively easy to tune. Fly the rocket, do a quick step input to pitch, measure the response, do another quick step to roll, measure the response. Analyze the data on the ground with Matlab and you have the dynamic response model of the rocket. Then your optimal gains and control law can be easily determined.
 

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Question 2: How about guiding the rocket to a specific recovery location?
Are the laws different for rockets than say amateur drones in this regard?
Richard,

Sorry I can't answer your question about laws directly, but I'd like to remind you that the ARLISS projects do this (from arliss.org):

ARLISS missions not only include flight data collection and telemetry, but integrate autonomous robots to direct satellites back to designated locations at the launch site. And not only returning autonomously, but flying sophisticated science missions.

Mike
 

Richard Dierking

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I mentioned stabilizing a flight using the pad as a reference in my first post:

"Here's the first question: What if you are trying to stay above the launch pad/location? Say, you use a GPS coordinate (the pad) in an active program that controls the rocket so it stays vertical and over the pad. Is that OK?"

However, the main point I think is using GPS in a stabilization system. It would make it more of a guidance system than just a stabilization system. And, yes, I think having the launch pad as the reference in any higher-alt flight fraught with peril. btw, the RSO seeking comment made me LOL. But, having the launch point as the reference in lower power and low-alt flights might be very useful. Consider when the TVC method is used and there may be no rod or rail to lean to compensate for wind.

John, what you said is very interesting. I'm thinking through my next test flight. And, following flights (because I'm an optimist). I appreciate your input.

Thank you for posting about ARLISS Mike. I was not familiar with that competition.
 

jderimig

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Hitting a point in the sky near apogee to keep your rocket in a given cylinder for recovery is a much easier job than controlling the rocket flight to adhere to a vector (vertical or otherwise) during ascent. The latter requires a control system that is stable over a very wide dynamic range.

The former just requires a control system that is optimized over a narrower speed range. Let the rocket go where it wants (within reason) open loop. When the speed drops to the optimal control zone engage the control and execute 2 direction control maneuvers to point the rocket where it should be at apogee.
 

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This is one of the kind of posts that I really find frustrating. There are a large number of myths, misconceptions, and half-truths that are accepted by rocket flyers.

Show me any law or regulation against using active guidance on a hobby rocket!

Show me any law or regulation against programming a rocket to navigate to a desired landing site!

For that matter, show me any law or regulation that says I can't make my rocket do loops, rolls, or other aerobatics in a controlled and safe manner!

You cannot use your rocket as a missile. A missile is a flying object intended to cause damage or harm.
You cannot aim your rocket at a target. A target is an object or person you intend to damage or injure.
The regulations are against using a rocket as a weapon.

Nothing says you can't use active guidance for constructive purposes.
 

cerving

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This is one of the kind of posts that I really find frustrating. There are a large number of myths, misconceptions, and half-truths that are accepted by rocket flyers.

Show me any law or regulation against using active guidance on a hobby rocket!

Show me any law or regulation against programming a rocket to navigate to a desired landing site!

For that matter, show me any law or regulation that says I can't make my rocket do loops, rolls, or other aerobatics in a controlled and safe manner!

You cannot use your rocket as a missile. A missile is a flying object intended to cause damage or harm.
You cannot aim your rocket at a target. A target is an object or person you intend to damage or injure.
The regulations are against using a rocket as a weapon.

Nothing says you can't use active guidance for constructive purposes.
It's at least implied by the NAR safety code... you're supposed to launch "straight up" and you can't aim at a "target".. implying a fixed point on the ground somewhere, whether it's ballistic or active.

#6 I will launch my rocket from a launch rod, tower, or rail that is pointed to within 30 degrees of the vertical to ensure that the rocket flies nearly straight up...

#8 I will not launch my rocket at targets...

Those rules do not preclude the use of active stabilization, to keep your rocket going "straight up". Going "straight up" is a good thing...

The interesting thing is that "targeting" contests have been around for as long as hobby rocketry... there was actually one held at NSL last year. https://www.nar.org/site/nsl-2019/points_of_interest/land-the-eagle-event/ So it would seem that there is significant room for interpretation of this rule...
 

DaveW6DPS

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#6 I will launch my rocket from a launch rod, tower, or rail that is pointed to within 30 degrees of the vertical to ensure that the rocket flies nearly straight up...

#8 I will not launch my rocket at targets...

Those rules do not preclude the use of active stabilization, to keep your rocket going "straight up". Going "straight up" is a good thing...

The interesting thing is that "targeting" contests have been around for as long as hobby rocketry... there was actually one held at NSL last year. https://www.nar.org/site/nsl-2019/points_of_interest/land-the-eagle-event/ So it would seem that there is significant room for interpretation of this rule...
In the first place, the NAR safety codes are not laws or regulations.

Please refer to my comment about what a target is.

Yes, I sponsored a precision landing competition at NSL last year, and it was completely within the rules. No "interpretation" required. There have been precision landing contests as long as I can remember, and this has nothing to do with launching at a target.

You reply is, however, a perfect example of the myths and misconceptions rampant in our hobby. People who "interpret" rules and invent regulations.
 

georgegassaway

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Yes, far too often some "expert" will pipe up with "that's not legal" when it comes to rocket guidance systems. But those "experts" prove to have no expertise in researching the laws or application in this hobby. There's been successful guidance aboard hobby rockets since 1988.

That's 32 years of onboard automated guidance in this hobby. It's not a "new thing". Heck, even TVC in this hobby is not a new thing.... THAT's 31 years old (1989).

"Targeting" is just that. Trying to hit a target. Whether on the ground, or in the air. The safety codes do not get into verbose verbiage for fine detail, but the ban on targets on the ground is for ballistic impact, or otherwise intentional high speed impact against some planned target. SPOT LANDING is not that, at all. It's LAND your model as close to a designated spot ("target"), SAFELY, with a recovery system to reduce the landing velocity sufficiently. To claim Spot Landing is "targeting banned by the safety code" is ridiculous.

There's been work using GPS for steered glide recovery for many years. I think first I heard of was around 15 years ago. Some for rocket gliders, effectively doing the same thing as "Return To Home" model planes with GPS autopilots.

And also experimenting with steered parachutes, using a flight controller with GPS to command the servo(s) to steer.

Here's a video by Apogee:

Here's a long video of a presentation about the development of the system. It was presented at NARCON.

If there's something "illegal" with that, I guess every multicopter with GPS onboard is illegal then. With some of my multicopter models, I've had the software capability to program them to fly autonomously to any location it could reach while battery power lasted, coming in on any flight path/descent angle (avoiding obstacles), and land on a designated spot (or hover over it at a set altitude). BTW - same can be done with R/C planes using similar GPS autopilots.

Now, the key thing to definitely avoid, is anything that can have a high speed impact against a target. In the 1990's, some fool (or worse) on RMR, posted an idea he wanted help with. GPS was not practical for modelers at the time. For the sake of "nearby recovery", he wanted to have a model that flew ballistically upwards, but at apogee, he did not want to deploy a chute and drift. Ad he didn't want to do dual deploy. He wanted the rocket to have a laser-sensor system in the nose, so he could point a laser at a "spot on the ground" nearby to make the model steer towards that spot during its high speed nose-first descent from apogee. So it would fall way way way down pointed at the laser designated target, and then a few hundred feet up press a button to deploy the chute. Total madness!!! At best, he was a fool. At worse, he was a budding terrorist who was looking for help in developing a homemade leaser designated missile to hit a real target. I jumped all over his posts pointing out how dangerous that was even if he had no bad intentions (as well as how other bad actors reading about any such development could go about doing it) and that NOBODY should try to help him with that project. Fortunately, most heeded those warnings, and eventually that thread and that guy disappeared (IIRC that guy only ever posted on RMR for his laser-designated "landing" thread. Which made his ultimate intentions even more curious).

So, we should all be cautious.

But that does not mean people should be declaring guidance is banned (it's not). NAR and Tripoli have embraced it.

There is a lot of neat fun stuff that can be done. Whether using TVC for low speed, or more easily using aerodynamic controls for higher speeds.
 

Richard Dierking

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Thank you all for your replies and comments. I think it's good to give some time for people that have comments and maybe concerns about this to speak up. But, this thread will probably end with some kind of conclusion. I'll pick up on my particular situation mentioned in question 3 a bit later with a post specific to that project. I probably should have started with a separate post for Q3 but I was trying to save electrons.

We all follow the rules and regulations from a number of sources including any AHJ that concerns our launches. When people start out, they first learn the safety codes and promise to follow them when they become members. Then, they usually begin to learn laws like FAA regulations, state regs, and NFPA codes, etc. And, where are their sources? Well, probably you, and other experienced people in your club, so we need to get it right.

It's normal for them to have questions and interpretations are subjective. But, frankly there are good and bad interpretations, and if a code is clear it doesn't need to be refined. On the other hand if something isn't too clear and being misinterpreted constantly, it's perhaps good to look at defining that code. I think the intention of the existing safety codes and the word target is pretty clear, but maybe not. And, of course, please don't even unintentionally imply laws that don't exist.

Anyway it's important for us to know the codes and laws and provide good information for people new to our hobby. And, in this particular case, people that are interested in developing some cool and really useful technologies.
 

OverTheTop

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I think you will find you don't need much in the way of control surface movements. Mine are hard-stop limited to plus/minus 6 degrees and could pull some severe turns. You can probably use smaller canards if you want.

Yours looks similar in scale to mine, so possibly similar results I suspect. Let us know how it goes :).
 

cerving

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In the first place, the NAR safety codes are not laws or regulations.

Please refer to my comment about what a target is.

Yes, I sponsored a precision landing competition at NSL last year, and it was completely within the rules. No "interpretation" required. There have been precision landing contests as long as I can remember, and this has nothing to do with launching at a target.

You reply is, however, a perfect example of the myths and misconceptions rampant in our hobby. People who "interpret" rules and invent regulations.
I think the NAR could have maybe done a little better job of specifying what they mean by "target", but I also think they don't want to bog the safety code down with too many specifics. It's meant to be understood by kids as well as by well-educated and over-thinking adults like us. I think Rule #6 which specifies the <30 degree launch angle takes care of the "targeting" idea for a normal launch, but does that mean that it would be OK if you launched straight up and then your guidance system sent the rocket at an angle towards a fixed point somewhere? You'd have to ask the NAR's safety committee. I have a pretty good idea of what they'd say... I think THAT would be construed as "targeting". NAR has already ruled that once you have a controlled recovery (at a reasonable descent rate) that steering it to a fixed point somewhere is acceptable.

Active stabilization and guided recoveries are definitely goods thing to pursue, which is why there have been several NAR presentations on the subjects. It would be cool if somebody could figure out a way to have your rocket go straight up and come right back to you 100% of the time without having to build your entire rocket around some kind of complex electromechanical system... you would think that the HPR guys flying to 50K and above would be all over this.
 

jqavins

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Jumping in late here, and I confess I haven't read all the posts; please forgive me if I'm repeating something.

One point of confusion I think may be in play here is the reference frame. Pitch, roll, and yaw are defined relative to the vehicle, not the intended course, the actual heading, or the ground. Similar axes relative to the ground have different names, which I don't know. In post #7, les referred to up/down, left/right, and forward/aft. But those terms are not entirely clear (at least to me) as to which reference frame they are in. Is the up/down axis relative to the ground or relative to the vehicle?

The terms pitch, roll, and yaw come originally from ships. Pitch is when the bow and stern go up and down, roll is when the port and starboard sides go up and down, and yaw is when the craft turns. Is up/down relative to the (average) sea surface, or relative to the deck? Application of these terms to airplanes is obvious. Because of the symmetry about the long axis in most rockets, roll is rotation about said long axis, but pitch and yaw are chosen arbitrarily.

Notice that I wrote "long axis", not "vertical axis". Vertical is defined in the earth reference frame, not the rocket frame. The flight path is also defined in the earth frame; you want the rocket going vertically, which means you usually want the roll axis (rocket frame) aligned with the vertical axis (earth frame). Usually, because sometimes you need to compensate for horizontal translation, as les explained.

For the ground frame I'll refer to vertical, east/west, and north/south, though I doubt those are the official terms.

So, suppose you have only pitch and roll control. If your nose is pointed, let's say, north, then you might think that you can roll until your pitch axis is set to compensate for that. But rolling may very well cause rotation in the horizontal plane, so it's not obvious where your roll will actually need to end. But yes, it can be done. I wouldn't.

Performing simultaneous pitch and yaw control with three fins is more complicated on paper than doing it with four. However, once you work out the math on paper you can code the results in your microcontroller and it's not really any worse in the air. For myself, I'd be concerned about getting the math wrong in some subtle way that isn't apparent in static testing, so I'd go with four fins. (If this were a critical application I would go with five (or six) fins; then if the controller detects failure of one (or two) fin's steering capability it can fall back to a modified algorithm and still have control. But I digress.)

Thank you all. For Q1 and Q2 these are nagging questions I have. Frequently, when I see a TVC or other kind of stabilization project come up, there's someone that says something like, "well, you better watch out because the feds would really be concerned about what you are doing, and you could get in trouble." Do you know what I mean? I would like this be put to rest. I have searched and found nothing that would indicate TVC or any other active guidance system would be a problem, unless it's intended to be a weapon. And, I think that is it.
And, I think there's a big difference between Guidance, Aiming, and Target. A Target is a destination. But, a target is a lot different than a destination; do we agree?
I'm afraid not. A target is the spot you want the rocket to end up. A destination is the place you want the rocket to end up. Come right down to it, I don't see any difference at all. I don't know what the law actually says, and I too would like to be able to use guidance toward a landing zone, and I too have always been told that it would be illegal.

The intent of the law is, undoubtedly to avoid weaponizing rockets. Think, however, of what one would do who wants to weaponize them. One would test the guidance system before putting a warhead on, and an HPR club launch would be the perfect place to do that without attracting attention. So the law should be written to prevent development of weapons, as well as launching them, and it would make sense to ban guidance during descent in all cases. I hope that's not what the law says, as I'd love to try it, yet that is probably what it should say.

And, if you don't think rockets are aimed, think about when rockets are leaned away on the pads from the flight line and spectators. They lean to the unoccupied recovery area. You are aiming them away from you.
Sure, "aim (verb)" is not the same as "target (noun)". You can aim away from something or some area without having a specific target you are aiming at. The distinction you draw between "aim" and "target" shoots your own argument in the foot.

EDIT: I've now read all the way through. DaveW6DPS and George, I'd like to respectfully as for a written source. As one of you wrote, every now and then some "expert" pops up and says it's illegal. When someone "pops up" to say it's not illegal, someone like me is hard pressed to tell the difference. Admittedly, the existence of many sponsored events strongly tends to back up your position. On the other hand, none of the spot landing events I've heard of involve active control (which is not to say that some don't; I know far from everything).

I maintain that "target", in general terms, means a spot you want to reach, regardless of what is or isn't there and regardless of your speed when you get there. Your statement that it means trying to hit a thing and/or hit fast is a very reasonable interpretation, but you really can't simultaneously say it and denounce interpretations.

The key, narrow point in question is about active control on the way down. You're probably right that it's not forbidden by law or regulation. And I hope you're right. Still, when it comes to federal law and regulation that one even might be ready to violate, it pays to be absolutely certain, with a reliable, expert legal source in hand e.g. a statement from the relevant regulatory agency or a lawyer familiar with the subject.

Will be a fun low cost anti drone platform hence the temporary nickname "Interceptor"
Which would be A) despicable, B) a violation of the safety code "target" clause no matter how one interprets it, and C) vandalism or worse legally.
 
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boatgeek

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NAR has already ruled that once you have a controlled recovery (at a reasonable descent rate) that steering it to a fixed point somewhere is acceptable.
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Do you know if this ruling is written down somewhere? It would be really helpful for these arguments to be able to point to a ruling made by NAR or TRA (or better yet, both) of the point above and that guidance intended to keep the rocket vertical/within the waiver/away from the flight line is OK. Then we could just pull that out whenever this thread pops up again.
 
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