Quantcast

Steerable drogue chute?

The Rocketry Forum

Help Support The Rocketry Forum:

jadebox

Roger Smith
TRF Sponsor
Joined
Jan 18, 2009
Messages
5,711
Reaction score
203
Take that same 15,000ft flight where the rocket is a half mile downrange at apogee, and a strong wind normally could have pushed it another 1-2 miles during its time under the drogue and chute. If a steerable drogue would even just halve that downrange drift it's a significant improvement.
Math is suddenly taking a beating. :)

With a 1:10 glide ratio it is only going to reduce the recovery distance by 1500 feet which is significantly less than half a mile.

Plus, a drogue is only used for only part of the descent, you have to subtract time for the drogue to deploy and orient itself, and at typical drogue descent speeds it may be very challenging to make the rig stable and steerable.

That's why I was suggesting that a quick descent under a drogue parachute might not be compatible with a controlled descent.

The same issues, of course, would apply to a larger parachute and slower descent. But, assuming the rocket flew into the wind, the slower descent rate would allow it to travel farther back to the launch site and I would think the challenges of deploying and controlling the descent would be easier.
 

HHaase

Well-Known Member
Joined
Aug 3, 2014
Messages
740
Reaction score
163
I think we are on the same page actually. My 1-2 miles of drift is with how we do things right now, without any descent control.

This is one of those times a picture would be better, but I haven’t reached my computer yet this morning.
 

Dan Griffing

Well-Known Member
TRF Supporter
Joined
Aug 5, 2020
Messages
198
Reaction score
95
Location
Belgrade, MT
Okay, how did we get from thread title, “Steerable drogue chute?” To “Transonic Vectoring Canards”?

We aren’t even in the same phase of flight, we started with recovery post deployment phase, now we are in boost or coast phase.

Time to start a new thread.

Hey, what’s this yellow stuff on my cornflakes?
In my post at the beginning of the thread I had stated the context of the problem:

“Coming down from 20k feet a 4-axis controller — with a similar technology as 4-axis rocket canard rotation and vertical controller — could control the four shroud lines of a drogue chute as it descends at say, 70fps from 10k to the 1k main chute deployment.

Instead of coming down 3 miles away, 100 yards would make recovery much easier and observable by the spectators.”

I thought it was clear that the context I was addressing was the residual distance from using a canard fin controller. But it was a complex enough issue that I wanted to have a thread to address it.

I agree that we’ve beaten this dead horse enough and its time for a new thread.

Thanks to everyone for participating in the discussion.
 

Dan Griffing

Well-Known Member
TRF Supporter
Joined
Aug 5, 2020
Messages
198
Reaction score
95
Location
Belgrade, MT
Math is suddenly taking a beating. :)

With a 1:10 glide ratio it is only going to reduce the recovery distance by 1500 feet which is significantly less than half a mile.

Plus, a drogue is only used for only part of the descent, you have to subtract time for the drogue to deploy and orient itself, and at typical drogue descent speeds it may be very challenging to make the rig stable and steerable.

That's why I was suggesting that a quick descent under a drogue parachute might not be compatible with a controlled descent.

The same issues, of course, would apply to a larger parachute and slower descent. But, assuming the rocket flew into the wind, the slower descent rate would allow it to travel farther back to the launch site and I would think the challenges of deploying and controlling the descent would be easier.
What a great observation to make!

All ideas needed to have someone cut through the other obscuring issue and do a sanity check on the basic premises of the idea.

You’ve done this completely and have demolished the whole basis for any kind of a steerable drogue chute working to reduce a 3 mile distance significantly from a 15k altitude.

How could any of us have missed such a blunder. This is a demonstration of what such discussions are for.

Thanks!
 

kbRocket

Well-Known Member
TRF Supporter
Joined
Nov 14, 2017
Messages
123
Reaction score
133
Location
Portland, OR
I got out my pencil to see when it may or may not be practical to use a steerable drogue chute to get back to the launch position. I have come up with relationships for the forward drogue speed Vf and descent rate Vd as a function of rocket altitude, downwind distance, glide ratio, and wind speed. These are boxed in red. With the sign conventions, all variables should have a positive value.
1599937891954.png

With the forward velocity relationship if we achieve equality, we can get back to the launch site. If the velocity exceeds equality, we can get upwind of our launch site. In this case, I would program the drogue to spin in a loop then go upwind again.

Lets start with the denominator of both velocity equations. This must be positive. The glide ratio must be larger than D/H, the rocket distance/altitude ratio. The forward to downward gliding progress must exceed the rocket downwind/altitude ratio. If we have a glide ratio that is much larger than D/H, then the forward and downward velocities can be reduced.

The forward drogue speed relationship, upper red box, tells us that when the glide ratio gets really big, then then necessary forward velocity can be lower. Forward velocity always is greater than wind speed, which also makes sense.

The descent rate relationship, lower box, indicates that we will typically have a descent rate lower than the wind speed. This assumes that g is at least twice D/H, which is reasonable.

So now for some real world numbers. I know a lot about kites, but not parachutes so I googled for some values.

Wikipedia claims that "The glide ratio of paragliders ranges from 9.3 for recreational wings to about 11.3 for modern competition models,[16] reaching in some cases up to 13.[17] For comparison, a typical skydiving parachute will achieve about 3:1 glide. A hang glider ranges from 9.5 for recreational wings to about 16.5 for modern competition models. " Lets assume that we can make a steerable rocket drogue that has glide ratio g of 6: somewhere in between parachute and paraglider.

Lets assume that the rocket was downwind by a quarter of the altitude. D/H = 0.25.

Then to get the rocket to steer back we need at least Vf = (6/5.75) * Vw = 1.043 Vw. Vd = Vw/5.75 = 0.173 Vw.

How fast can a paraglider go? The same wikipedia link above says "The speed range of paragliders is typically 20–75 kilometres per hour (12–47 mph), from stall speed to maximum speed. Beginner wings will be in the lower part of this range, high-performance wings in the upper part of the range.[note 2] " Lets assume our drogue can go 30 mph.

Then the max wind speed we could tolerate would be 30/1.043 or 28.75 mph. The descent rate would be 5 mph. Put a rocket up 2.5 miles under these conditions and you will get it back in a half an hour. It would be really cool to watch.

I wonder if a better parachute/paraglider could be designed for this application that has a higher forward speed. It makes sense that people don't want to land too fast or they will break their legs.

Overall, it seems practical to use a steerable drogue chute with winds up to about 30 mph, which is not fast for high altitude. There could be some fun on a calm day flying up to maybe 15k' or so. I would love to watch a rocket come back for a half hour and land at the pad.
 
Last edited:

jadebox

Roger Smith
TRF Sponsor
Joined
Jan 18, 2009
Messages
5,711
Reaction score
203
.
The descent rate would be 5 mph. [...] it seems practical to use a steerable drogue chute with winds up to about 30 mph, which is not fast for high altitude.
That would be the equivalent of a larger main parachute, not a smaller drogue parachute which typically would have a descent rate of about 35 miles per hour. As I suggested, you need a larger 'chute with a slower descent speed than was considered earlier when discussing a steerable drogue.

It might also need some knowledge of the wind direction or a way to sense its movement in relation to the ground in order to be to turn into the wind before landing to reduce the speed relative to the ground.
 

Dan Griffing

Well-Known Member
TRF Supporter
Joined
Aug 5, 2020
Messages
198
Reaction score
95
Location
Belgrade, MT
I got out my pencil to see when it may or may not be practical to use a steerable drogue chute to get back to the launch position. I have come up with relationships for the forward drogue speed Vf and descent rate Vd as a function of rocket altitude, downwind distance, glide ratio, and wind speed. These are boxed in red. With the sign conventions, all variables should have a positive value.
View attachment 431420
With the forward velocity relationship if we achieve equality, we can get back to the launch site. If the velocity exceeds equality, we can get upwind of our launch site. In this case, I would program the drogue to spin in a loop then go upwind again.

Lets start with the denominator of both velocity equations. This must be positive. The glide ratio must be larger than D/H, the rocket distance/altitude ratio. The forward to downward gliding progress must exceed the rocket downwind/altitude ratio. If we have a glide ratio that is much larger than D/H, then the forward and downward velocities can be reduced.

The forward drogue speed relationship, upper red box, tells us that when the glide ratio gets really big, then then necessary forward velocity can be lower. Forward velocity always is greater than wind speed, which also makes sense.

The descent rate relationship, lower box, indicates that we will typically have a descent rate lower than the wind speed. This assumes that g is at least twice D/H, which is reasonable.

So now for some real world numbers. I know a lot about kites, but not parachutes so I googled for some values.

Wikipedia claims that "The glide ratio of paragliders ranges from 9.3 for recreational wings to about 11.3 for modern competition models,[16] reaching in some cases up to 13.[17] For comparison, a typical skydiving parachute will achieve about 3:1 glide. A hang glider ranges from 9.5 for recreational wings to about 16.5 for modern competition models. " Lets assume that we can make a steerable rocket drogue that has glide ratio g of 6: somewhere in between parachute and paraglider.

Lets assume that the rocket was downwind by a quarter of the altitude. D/H = 0.25.

Then to get the rocket to steer back we need at least Vf = (6/5.75) * Vw = 1.043 Vw. Vd = Vw/5.75 = 0.173 Vw.

How fast can a paraglider go? The same wikipedia link above says "The speed range of paragliders is typically 20–75 kilometres per hour (12–47 mph), from stall speed to maximum speed. Beginner wings will be in the lower part of this range, high-performance wings in the upper part of the range.[note 2] " Lets assume our drogue can go 30 mph.

Then the max wind speed we could tolerate would be 30/1.043 or 28.75 mph. The descent rate would be 5 mph. Put a rocket up 2.5 miles under these conditions and you will get it back in a half an hour. It would be really cool to watch.

I wonder if a better parachute/paraglider could be designed for this application that has a higher forward speed. It makes sense that people don't want to land too fast or they will break their legs.

Overall, it seems practical to use a steerable drogue chute with winds up to about 30 mph, which is not fast for high altitude. There could be some fun on a calm day flying up to maybe 15k' or so. I would love to watch a rocket come back for a half hour and land at the pad.
This is great work. I’m a retired engineer but it’s been decades since I’ve able to do this depthy an analysis.
 

kbRocket

Well-Known Member
TRF Supporter
Joined
Nov 14, 2017
Messages
123
Reaction score
133
Location
Portland, OR
If you get a 1:10 glide ratio (1 foot forward for every 10 feet down—— reeeeeeeallllyy bad for a typical glider, but maybe not for a steerable drogue) for a 15k altitude flight that gives you almost 3 miles lateral direction. Then again, you might waste the first 5 k just trying to get the glider oriented.....
With a 1:10 glide ratio it is only going to reduce the recovery distance by 1500 feet which is significantly less than half a mile.
You guys are correct that with a 1:10 glide ratio you get 1 forward for every foot down in still air, so you could only make up for 1500 feet. However the glide ratio for parachutes/paragliders is in the range of 2.5:1 to 10:1. Lots more forward to down.
 

Dan Griffing

Well-Known Member
TRF Supporter
Joined
Aug 5, 2020
Messages
198
Reaction score
95
Location
Belgrade, MT
Is the horse dead or not?
It might breath some life into the discussion and save this horse from the glue factory.

But since the development of a steerable drogue chute requires a similar technology that went into a 4-axis steerable canard fin control system — the limited envelope that you’ve shown where this will work might make its development not worth the effort.

However, since this is meant to be used on a rocket whose steerable canard fin control system already exists and has steered it to an vertical apogee with no horizontal velocity component — would a similar analysis be possible to determine the limits the same canard fin control system to eliminate these “3 milesbefore apogee while still eliminating the horizontal velocity component.
 

BABAR

Builds Rockets for NASA
TRF Lifetime Supporter
TRF Supporter
Joined
Aug 27, 2011
Messages
6,772
Reaction score
1,902
It might breath some life into the discussion and save this horse from the glue factory.

But since the development of a steerable drogue chute requires a similar technology that went into a 4-axis steerable canard fin control system — the limited envelope that you’ve shown where this will work might make its development not worth the effort.

However, since this is meant to be used on a rocket whose steerable canard fin control system already exists and has steered it to an vertical apogee with no horizontal velocity component — would a similar analysis be possible to determine the limits the same canard fin control system to eliminate these “3 milesbefore apogee while still eliminating the horizontal velocity component.
Reasonable to work the problem at both ends. From my very limited exploration of the HPR forum, seems like progress is being made but even just keeping the flight VERTICAL is challenging enough, TARGETTING a controlled ascent to a predetermined upwind location I expect would be tougher but still would be really nice.

I still don't quite understand the "gravity turn", when I google it it seems like an INTENTIONAL maneuver, using minimal energy to transition from vertical to horizontal velocity rather than an unavoidable accident that puts the rocket in an unpredictable location. Weathercocking I understand.

There was a low power recovery style called "backsliding" that seemed pretty cool, not sure if it could be modified for HPR nor sure if it would be STEERABLE, but might be a cool solution.

I can't get link to post on android, it's a pdf file

Google

backsliding model rocket recovery

Gorgerocketclub.com
 

kbRocket

Well-Known Member
TRF Supporter
Joined
Nov 14, 2017
Messages
123
Reaction score
133
Location
Portland, OR
I made a couple plots of speeds and descent time vs glide ratio to better understand the relationships.

For these plots I assume:
  • Apogee H is at 15,000 feet
  • Downwind distance D is 3000 feet, so D/H = 0.2
  • Maximum forward velocity Vfmax of 45 ft/second = 31 mph
  • Maximum descent rate Vdmax of 80 ft/sec = 55 mph
Maximum forward velocity limits the flight when the wind speed is high. A high minimum glide ratio results.
1599956527534.png

The downward velocity is limited by the max forward velocity divided by large glide ratio, resulting in a long descent time.

The graph below shows what happens when the wind speed is 30 mph, just a little under the drogue forward velocity of 31 mph. The green/blue circles are at the minimum acceptable glide ratio of 6.2. Otherwise there is not enough forward velocity. This rocket comes down at only 7 ft/second and takes 34 minutes to come down.
glide30.png

So, we want to maximize the ratio of forward velocity to wind speed. This makes sense to travel upwind and make up for downwind distance D.

In the case that wind speed is low compared to the maximum forward velocity, we can have maximum descent rate limiting the glide ratio. Lower glide ratios, even glide ratios < 1, can work.

1599956759873.png

Here is a plot of the same scenario with a 15 mph wind speed. The max descent rate of 80 ft/second means the glide ratio can be as low as 0.475. We could get the rocket down in 3.1 minutes and a forward velocity of 26 mph is sufficient.
glide15.png

This seems pretty reasonable, but it required a low wind speed/forward velocity relationship.

A little rearrangement of one of the above relationships gives good insight into the maximum downwind distance that could be fixed by a steerable drogue:
1599961744081.png

High glide ratio is helpful. A low ratio of wind speed to forward drogue speed is helpful.

Overall, there is some space in which a steerable drogue chute could bring back rockets. Strive for a high forward velocity drogue chute. Because forward velocity must be greater than the wind speed, the overall usefulness at high altitudes is limited. Maybe with optimization a forward velocity in the 40 to 60 mph could be achieved.

I think that a steerable drogue chute could be implemented with one servo/control line. Let the line out and it spins left. Pull the line in and it spins right. Another single servo option would drive a continuous line attached to each side of the kite. When the servo goes one way, the line is shifted from one side to the other. Think of it like a two line stunt kite, even if it has more than two lines. I think this could fit into a 54 mm tube with plenty of fun in the J to L motor range.

And, if everything gets tangled up, then it will probably behave like a descent with a small drogue or streamer. Main at 1k feet or so will happen.
 
Last edited:

boatgeek

Well-Known Member
Joined
Dec 27, 2014
Messages
2,913
Reaction score
1,346
Reasonable to work the problem at both ends. From my very limited exploration of the HPR forum, seems like progress is being made but even just keeping the flight VERTICAL is challenging enough, TARGETTING a controlled ascent to a predetermined upwind location I expect would be tougher but still would be really nice.

I still don't quite understand the "gravity turn", when I google it it seems like an INTENTIONAL maneuver, using minimal energy to transition from vertical to horizontal velocity rather than an unavoidable accident that puts the rocket in an unpredictable location. Weathercocking I understand.

There was a low power recovery style called "backsliding" that seemed pretty cool, not sure if it could be modified for HPR nor sure if it would be STEERABLE, but might be a cool solution.

I can't get link to post on android, it's a pdf file

Google

backsliding model rocket recovery

Gorgerocketclub.com
I think the "keep it vertical" systems have a few good working examples. In theory* there wouldn't be that much more effort to turn the rocket vertical, then turn it (say) 5 degrees off vertical in a direction you want for a while, then turn it vertical again.

Gravity turns are definitely intentional for orbital rockets, since they want lots of horizontal velocity at apogee. Their feature is our bug though, since we want minimum horizontal velocity at apogee to minimize deployment forces.

I've seen an HPR rocket backslide when its drogue failed to separate the rocket. It was the weirdest thing--it came down with the fin end down a few degrees below horizontal in a nice slow manner until the main blew at its programmed altitude. It blew my mind. I asked about it then, and it sounded like it's something that happens every so often when fortune supplies ju t the right combination of stability, spped at apogee, and some pixie dust.

* In theory, there's no difference between theory and practice.
 

Dan Griffing

Well-Known Member
TRF Supporter
Joined
Aug 5, 2020
Messages
198
Reaction score
95
Location
Belgrade, MT
I got out my pencil to see when it may or may not be practical to use a steerable drogue chute to get back to the launch position. I have come up with relationships for the forward drogue speed Vf and descent rate Vd as a function of rocket altitude, downwind distance, glide ratio, and wind speed. These are boxed in red. With the sign conventions, all variables should have a positive value.
View attachment 431420
With the forward velocity relationship if we achieve equality, we can get back to the launch site. If the velocity exceeds equality, we can get upwind of our launch site. In this case, I would program the drogue to spin in a loop then go upwind again.

Lets start with the denominator of both velocity equations. This must be positive. The glide ratio must be larger than D/H, the rocket distance/altitude ratio. The forward to downward gliding progress must exceed the rocket downwind/altitude ratio. If we have a glide ratio that is much larger than D/H, then the forward and downward velocities can be reduced.

The forward drogue speed relationship, upper red box, tells us that when the glide ratio gets really big, then then necessary forward velocity can be lower. Forward velocity always is greater than wind speed, which also makes sense.

The descent rate relationship, lower box, indicates that we will typically have a descent rate lower than the wind speed. This assumes that g is at least twice D/H, which is reasonable.

So now for some real world numbers. I know a lot about kites, but not parachutes so I googled for some values.

Wikipedia claims that "The glide ratio of paragliders ranges from 9.3 for recreational wings to about 11.3 for modern competition models,[16] reaching in some cases up to 13.[17] For comparison, a typical skydiving parachute will achieve about 3:1 glide. A hang glider ranges from 9.5 for recreational wings to about 16.5 for modern competition models. " Lets assume that we can make a steerable rocket drogue that has glide ratio g of 6: somewhere in between parachute and paraglider.

Lets assume that the rocket was downwind by a quarter of the altitude. D/H = 0.25.

Then to get the rocket to steer back we need at least Vf = (6/5.75) * Vw = 1.043 Vw. Vd = Vw/5.75 = 0.173 Vw.

How fast can a paraglider go? The same wikipedia link above says "The speed range of paragliders is typically 20–75 kilometres per hour (12–47 mph), from stall speed to maximum speed. Beginner wings will be in the lower part of this range, high-performance wings in the upper part of the range.[note 2] " Lets assume our drogue can go 30 mph.

Then the max wind speed we could tolerate would be 30/1.043 or 28.75 mph. The descent rate would be 5 mph. Put a rocket up 2.5 miles under these conditions and you will get it back in a half an hour. It would be really cool to watch.

I wonder if a better parachute/paraglider could be designed for this application that has a higher forward speed. It makes sense that people don't want to land too fast or they will break their legs.

Overall, it seems practical to use a steerable drogue chute with winds up to about 30 mph, which is not fast for high altitude. There could be some fun on a calm day flying up to maybe 15k' or so. I would love to watch a rocket come back for a half hour and land at the pad.
It looks like all of these equations have a constant drogue speed, Vd that's independent of altitude.

But coming down from 15,000 AGL involves a big change in air density, which changes relatively with the as the negative exponential of altitude. And since Vd is inversely proportional with the parachute's Cd, which itself is inverse with fluid density, shouldn't the Vd for the drogue chute be much higher at 15,000 AGL, and its ability to cause a drogue chute to glide be much lower? In other words, if your glide ratio is 0.1 near the ground, wouldn't it be much lower at 15,000 AGL?
 

BABAR

Builds Rockets for NASA
TRF Lifetime Supporter
TRF Supporter
Joined
Aug 27, 2011
Messages
6,772
Reaction score
1,902
Gravity turns are definitely intentional for orbital rockets, since they want lots of horizontal velocity at apogee. Their feature is our bug though, since we want minimum horizontal velocity at apogee to minimize deployment forces.
Since gravity turns are intentional for orbital rockets, I presume there is some input (likely a gimballed motor with a touch of thrust) to “nudge” the rocket off vertical toward the desired horizontal trajectory.

For conventional model rockets, where there is no intentional variable guidance (basically we aim the rocket with the rail and hope the fins keep it STRAIGHT on that heading once it departs the rail) what is the “force” that both initiates and once initiated causes progression (worsening or exaggeration) of the gravity turn? I am guessing it starts with weathercocking. Since the CG is ahead of the CP, is gravity acting to “rotate” the rocket around the CP? So once the rocket gets a little off vertical for whatever reasons gravity is preferentially and progressively pulling the “down side” of the rocket even more downward, rotating the nose toward the earth?
 

neil_w

Working on 20K
TRF Supporter
Joined
Jul 14, 2015
Messages
10,090
Reaction score
3,183
Location
Northern NJ
Since gravity turns are intentional for orbital rockets, I presume there is some input (likely a gimballed motor with a touch of thrust) to “nudge” the rocket off vertical toward the desired horizontal trajectory.
For a thrust-vectored rocket, every inch of flight trajectory is strictly controlled with thrust vectoring. Gravity may be used as an assist as needed, of course.
Since the CG is ahead of the CP, is gravity acting to “rotate” the rocket around the CP? So once the rocket gets a little off vertical for whatever reasons gravity is preferentially and progressively pulling the “down side” of the rocket even more downward, rotating the nose toward the earth?
Gravity acts on the CG (hence the name :)); it does not inherently apply any rotational force. Once the rocket is significantly off vertical, though, there will progressively be less and less vertical thrust to offset gravity, and so the rocket's trajectory will tend to arc. The wikipedia page is pretty good: https://en.wikipedia.org/wiki/Gravity_turn

Finally: "Gravity Turn" added to list of potentially good geeky band names. ;)
 

BABAR

Builds Rockets for NASA
TRF Lifetime Supporter
TRF Supporter
Joined
Aug 27, 2011
Messages
6,772
Reaction score
1,902
For a thrust-vectored rocket, every inch of flight trajectory is strictly controlled with thrust vectoring. Gravity may be used as an assist as needed, of course.

Gravity acts on the CG (hence the name :)); it does not inherently apply any rotational force. Once the rocket is significantly off vertical, though, there will progressively be less and less vertical thrust to offset gravity, and so the rocket's trajectory will tend to arc. The wikipedia page is pretty good: https://en.wikipedia.org/wiki/Gravity_turn

Finally: "Gravity Turn" added to list of potentially good geeky band names. ;)
Thanks, I think I get it.

Pertinent quote from above reference

After the pitchover, the rocket's flight path is no longer completely vertical, so gravity acts to turn the flight path back towards the ground. If the rocket were not producing thrust, the flight path would be a simple ellipse like a thrown ball (it's a common mistake to think it is a parabola: this is only true if it is assumed that the Earth is flat, and gravity always points in the same direction, which is a good approximation for short distances), leveling off and then falling back to the ground. The rocket is producing thrust though, and rather than leveling off and then descending again, by the time the rocket levels off, it has gained sufficient altitude and velocity to place it in a stable orbit.


Interesting. But if I throw a rock straight up (vector is perfectly vertical and there is no additional force on the rocket other than air resistance and gravity) it will fall straight down. If we add wind, the wind may impart some LATERAL velocity, but GRAVITY doesn’t. so I (being a biology major, although we had to do a bunch of engineering including aerodynamic and astronautical, but that was a loooooong time ago) am thinking that unless the rocket is under thrust DURING a turn started by some other process, there is no “force” to impart horizontal acceleration. So whatever horizontal acceleration has to be the result of the rocket being under thrust during the turn. True, at apogee when all vertical velocity is sapped by gravity and air resistance, there is still potentially significant horizontal velocity, but I theeeenk all this had to be imparted by the motor DURING the portion of the turn under thrust.

I am still struggling a bit to “intuit” WHY the rocket CONTINUES to rotate (goes progressively from vertical to horizontal) WHILE under thrust. If the motor acts effectively through the CG, AND gravity acts effectively through the CG, wouldn’t seem to cause a rotational force.

This pic helped

0A921AD9-4847-44B0-AFDB-85F8302254FA.png


There is NEVER a true horizontal vector from gravity, but once the rocket is minimally displaced off a vertical trajectory (intentionally for an orbital insertion flight space flight rocket, for a model rocket unintentional from weathercocking OR potentially from angling the rocket rod or rail into the wind), the thrust vector (along the current flight axis) combined with the gravity vector (which now has a small component which is NOT LATERAL TO TRUE VERTICAL but IS lateral to the current FLIGHT AXIS) combine to form a net acceleration laterally as long as the rocket is under thrust. I am a little lost as to why the rocket “rotates” but I am guess with the fins it has to rotate to maintain zero angle of attack.

I am kind of wondering whether this affects us low power guys too. Sometimes I angle the rod or rail a bit into the wind. I understand weathercocking, but sometimes it seems like the degree of rotation is a lot more than I would expect from just the wind, with the rocket turning almost horizontal only 100 feet up! Is this a mini-gravity turn? I still will angle SLIGHTLY into the wind at times (never toward spectators), but I always try to err on the side of UNDERESTIMATING the degree of angular ion rather than overestimating it.

Also may be why I like the C5-3 over the C6, for the C5 most of the Newtons are expended early, before the rocket has a chance to start keeling over from weathercocking or other forces. The cost I guess is higher drag, as the rocket’s initial velocity is higher, but at least it gets it up off the pad and well off the ground before Mother Nature’s other forces start screwing it up.
 

jderimig

Sponsor
TRF Sponsor
Joined
Jan 23, 2009
Messages
3,204
Reaction score
617
For those still struggling with gravity turn here is an hopefully easier visualization. From high school physics we should remember that any acceleration component perpendicular to the velocity vector results in a circular motion. As soon as the rocket deviates from vertical a small acceleration component, g, appears perpendicular to the motion.

This happens with a short burn, long burn, no burn or in a vacuum.
 

jderimig

Sponsor
TRF Sponsor
Joined
Jan 23, 2009
Messages
3,204
Reaction score
617
Also, this is a reference frame issue, this turn on the rocket cannot be sensed by an onboard accelerometer. In the rockets reference frame there is no lateral acceleration. However a gyro will sense this.
 

BABAR

Builds Rockets for NASA
TRF Lifetime Supporter
TRF Supporter
Joined
Aug 27, 2011
Messages
6,772
Reaction score
1,902
For those still struggling with gravity turn here is an hopefully easier visualization. From high school physics we should remember that any acceleration component perpendicular to the velocity vector results in a circular motion. As soon as the rocket deviates from vertical a small acceleration component, g, appears perpendicular to the motion.

This happens with a short burn, long burn, no burn or in a vacuum.
THAT helped! Thanks!
 

Richard Dierking

Well-Known Member
TRF Supporter
Joined
Nov 25, 2018
Messages
320
Reaction score
131
Location
Temecula, CA
Here's a couple images showing the GPS track of a rocket I launched on Saturday at FAR. First, the Perfectflite altimeter graph, then GPS street view, and directly overhead. The rocket went to 4K' and landed 3700' away. Wind on deck was 10 to 11 mph. Dual deploy with a main at 1100'. I intensionally used a larger drogue (24") so the descent would be slower. The main was a 30". Both standard nylon chutes with no spill-holes.
Anyway, shows why controlling the descent direction would certainly be a challenge.

Altimeter 09-19-20 copy.jpg



GPS Flight Profile.jpg

GPS Flight Profile Looking Down.jpg
 

jderimig

Sponsor
TRF Sponsor
Joined
Jan 23, 2009
Messages
3,204
Reaction score
617
Anyway, shows why controlling the descent direction would certainly be a challenge.
But....If you knew the windspeed and direction versus altitude (which is knowable), you can calculate a point in the sky for apogee that would bring the rocket down close to where you want it.
 

Richard Dierking

Well-Known Member
TRF Supporter
Joined
Nov 25, 2018
Messages
320
Reaction score
131
Location
Temecula, CA
Not to say that doesn't sound fun John. I enjoy challenges myself. But, my rockets are just going to come down fast through all that craziness. Hope I can slow down before the hard deck. ;-)
Take another look at the street-view profile and imagine coming down from apogee at like 150 ft/sec to main deployment. I would have shortened my recovery walk a lot. And, it was hot too!
 

jderimig

Sponsor
TRF Sponsor
Joined
Jan 23, 2009
Messages
3,204
Reaction score
617
This is a hobby of long walks and quests....
 

Richard Dierking

Well-Known Member
TRF Supporter
Joined
Nov 25, 2018
Messages
320
Reaction score
131
Location
Temecula, CA
Yes, I tell people that rocketry is better exercise than golf. And, the higher you go the better shape you're in. So far, my longest desert sojourn has been 8 miles round trip. And, also got to do a bit of running when a dog tried to attack me. Wow, what a fun hobby.
 

Dan Griffing

Well-Known Member
TRF Supporter
Joined
Aug 5, 2020
Messages
198
Reaction score
95
Location
Belgrade, MT
But....If you knew the windspeed and direction versus altitude (which is knowable), you can calculate a point in the sky for apogee that would bring the rocket down close to where you want it.
You don’t need to know windspeed versus altitude, but the convolution integral of windspeed and the amount of time that the rocket is subject to it both ascending and descending.

Without collecting detailed data and doing complex math calculations, this can be estimated by the distance and direction that other rockets have landed.

But its true that with knowledge about this overall effect you can determine the point in the sky at any altitude where apogee needs to occur for the rocket to land near the launch point.
 

Richard Dierking

Well-Known Member
TRF Supporter
Joined
Nov 25, 2018
Messages
320
Reaction score
131
Location
Temecula, CA
Kind of OT, but still guiding from apogee.
I just ordered Flying Without Wings by Milton Thompson. It's been over 20 years since I read it the first time and I was fascinated by the development of these kinds of aircraft. Also, I got to see the original "flying bathtub" at Dryden years ago.

I've decided not to pursue TVC gimbals and instead going to try and make a lifting body/glider. I would like to launch it either on a rocket or helium balloon at XPRS next year. If anyone following this thread is interested in being involved with this project please let me know.
 

Dan Griffing

Well-Known Member
TRF Supporter
Joined
Aug 5, 2020
Messages
198
Reaction score
95
Location
Belgrade, MT
Kind of OT, but still guiding from apogee.
I just ordered Flying Without Wings by Milton Thompson. It's been over 20 years since I read it the first time and I was fascinated by the development of these kinds of aircraft. Also, I got to see the original "flying bathtub" at Dryden years ago.

I've decided not to pursue TVC gimbals and instead going to try and make a lifting body/glider. I would like to launch it either on a rocket or helium balloon at XPRS next year. If anyone following this thread is interested in being involved with this project please let me know.
Hopefully that would that the place of a drogue and would be much more steerable. Sort of like the space shuttle?

I’ll follow it with interest.
 

Richard Dierking

Well-Known Member
TRF Supporter
Joined
Nov 25, 2018
Messages
320
Reaction score
131
Location
Temecula, CA
Hopefully that would that the place of a drogue and would be much more steerable. Sort of like the space shuttle?
Yes, and I'm thinking that perhaps it may be able to be maneuvered by moving the GC around. Will not turn on a dime, but maybe good enough. Still main deploy at some altitude. One of the main reasons I feel comfortable about even trying something like this is my good experiences with the FW GPS.
I'll post a separate thread after I do a bit of research and determine approximate size.
 

Dan Griffing

Well-Known Member
TRF Supporter
Joined
Aug 5, 2020
Messages
198
Reaction score
95
Location
Belgrade, MT
Yes, and I'm thinking that perhaps it may be able to be maneuvered by moving the GC around. Will not turn on a dime, but maybe good enough. Still main deploy at some altitude. One of the main reasons I feel comfortable about even trying something like this is my good experiences with the FW GPS.
I'll post a separate thread after I do a bit of research and determine approximate size.
Instead of being just a recovery system, your steerable falling object might grow in significance to be the majority of the project like the Space Shuttle did.

And I’m predicting that an inertial guidance system like one based on the MPU-6050 will play a greater role than the GPS.
 
Top