Quantcast

Altimeter 3, Open Rocket and Apogee

The Rocketry Forum

Help Support The Rocketry Forum:

dford

Tada
Joined
Apr 23, 2016
Messages
330
Reaction score
1
So last month at Roc I had a few comments my cert flight delay should have been shorter.
My rocket took off, bent over nearly horizontal at 'apogee' then deployed recovery.
At first I agreed with them and not with open rocket but then I recovered my altimeter and rocket. Sure enough, my data told me I was accurate within half a second of open rocket.
For this flight I drilled the delay to open rocket specs.
I flew again today with my club on a different rocket and Apogee accurring to open rocket would occur at 8 seconds, 2800'.
Again, same scenario and same commentary from different people.
What I am wondering is if I should be aiming for is a vertical recovery event, or a true apogee event.
Obviously the recovery event would be slower at apogee in a horizontal position but I keep hearing "shorter delay"

In my opinion, open rocket combined with altimeter data would be correct. Which would represent more of a slower horizontal deployment event.

Which is true?
 
Last edited:

Buckeye

Well-Known Member
TRF Supporter
Joined
Sep 6, 2009
Messages
2,511
Reaction score
360
Apogee is when maximum vertical height is achieved and vertical velocity become zero. The simulation is correct. Your rocket may still be moving horizontally, however.

Sound to me that you should strive for more vertical flights. Fly in less wind, avoid weathercocking and/or marginal stability, use motors with more oomph.
 

woferry

Well-Known Member
Joined
Oct 9, 2013
Messages
803
Reaction score
26
Location
San Jose, CA
Generally speaking, my goals are to (1) get as much altitude out of the flight as possible (why waste any), and (2) separate at the lowest speed possible (vertical and horizontal, least chance of airframe damage at least for cardboard rockets, though the horizontal speed is purely a function of how much off-vertical the flight was). So as close to apogee as possible is my goal, though I've had too many issues with (AT at least) motor ejects that I only fly electronic eject anymore. So the altimeter takes care of detecting apogee and firing at the right time.

Under the right circumstances, if things are pretty still and the rocket flight was pretty vertical the rocket may appear to sort of hang in the air for a while before it ejects, could it have been something like this that just made it look worse than it really was to spectators? Or if it was really arcing (not so vertical flight) it may have appeared to be past apogee and coming down even though it wasn't. So if your A3 said you were within a second of apogee you're doing pretty well I'd say (and if that's with motor eject you're getting pretty lucky IMO, certainly the opposite of my luck where I've had events 8s early to 4s late from the amount it should have been with the stock or drilled delay).
 

rharshberger

Well-Known Member
Joined
Oct 13, 2014
Messages
9,702
Reaction score
1,682
Location
Pasco, WA
So last month at Roc I had a few comments my cert flight delay should have been shorter.
My rocket took off, bent over nearly horizontal at 'apogee' then deployed recovery.
At first I agreed with them and not with open rocket but then I recovered my altimeter and rocket. Sure enough, my data told me I was accurate within half a second of open rocket.
For this flight I drilled the delay to open rocket specs.
I flew again today with my club on a different rocket and Apogee accurring to open rocket would occur at 8 seconds, 2800'.
Again, same scenario and same commentary from different people.
What I am wondering is if I should be aiming for is a vertical recovery event, or a true apogee event.
Obviously the recovery event would be slower at apogee in a horizontal position but I keep hearing "shorter delay"

In my opinion, open rocket combined with altimeter data would be correct. Which would represent more of a slower horizontal deployment event.

Which is true?
Was the rocket undamaged? Was the apogee event within 1 second of apogee regardless of the orientation of the rocket (horizontal or vertical)? If so you did it right. What was the velocity at the time of apogee? From my experience truly vertical apogee events are unusual (Frank Burkes Foamies are known for vertical apogee events with a slight backslide). Did the rocket weather cock off the pad? If so did you do anything to counteract the weathercocking by tipping the rail in the direction the wind is blowing (overstable rockets will weathercock back to near vertical if you do this most of the time). Long "heavy" rockets don't bleed speed as quickly as short fat or lightweight rockets so they tend to arc over at apogee rather than just stop going up.
 

Banzai88

Lvl 1,Wallet....Destroyed
TRF Supporter
Joined
Jul 15, 2015
Messages
2,375
Reaction score
559
Don't forget that the delay length is really 'notional' on timing. Spec is something like +/- 10%?? You can drill two delays back to back and fly them in the very same reload back to back, and get slightly different ejection events based on altimeter data.

Many rockets flop over at the top, especially top heavy ones. I try for an ejection event with as little vertical velocity as possible, but if it's a choice between early on the UP or a little late on the DOWN, I'd rather have it on the down. Early on when I started drilling delays, I started with a few flights where I chose the ejection on the UP and ended up flying up into and fouling my chute. Nose cone would pop open and unfurl the chute and begin slowing down while the heavier mass of the fuselage would retain enough energy to fly up into the unfurling chute.

Since I started picking the delay for the down, I haven't fouled a single one and have had 100% nominal recoveries. On the longer delay for the DOWN event, it's usually just after apogee, and the rocket doesn't have enough velocity and is usually nowhere near vertical, so the ejection event is off the line of travel for the fuselage.

Sounds to me like you're doing it right.
 

ksaves2

Lifetime Supporter
TRF Lifetime Supporter
Joined
Nov 25, 2009
Messages
5,981
Reaction score
294
Also remember if one has enough space at their launchsite to angle the rocket downwind especially if the rocket has a tendency to weathercock. How many degrees? Start jinking with it in a simulator. Over and over again to
get a feel for it with varying windspeeds.. It helps to get a low velocity at apogee for apogee only deployments. Kurt
 

John Beans

Founder, Jolly Logic
TRF Sponsor
TRF Supporter
Joined
Jun 5, 2010
Messages
827
Reaction score
148
Also don't forget that the rocket is moving the slowest at apogee, even if it's arcing over quite a bit. It's decelerating (not accelerating) when it's about to arc over.
So apogee's always the best time.
 

Coop

Well-Known Member
Joined
Dec 15, 2011
Messages
1,768
Reaction score
3
Cutting ejection delays takes a bit of experience and practice. Familiarity with the rocket and motor tends to help.

As was noted, above, if you recovered undamaged close to apogee, you're doing fine. Delay grains always have a bit of wiggle room with them... even if cut precisely, there's still a fair degree of variance.

If you want more precision... go electronic and forego the delay grain completely.


Later!

--Coop
 

cls

Well-Known Member
Joined
Jan 21, 2009
Messages
2,234
Reaction score
0
one more idea... the rocket needs to be moving to open the parachute!!

does your parachute open if you lay it on the floor in the house? no. so a little gentle motion is a good thing.

I just don't want the rocket to be going 300 MPH horizontally, that's too much apparent wind...
 

dford

Tada
Joined
Apr 23, 2016
Messages
330
Reaction score
1
Sorry for such a late reply. I've been busy in a different thread. I'll try to answer as many questions as I can.
Screenshot_2016-09-26-15-17-00.jpg
This image shows an early ejection on a G76 motor eject 10s delay. If I can get the .ork file up for this I will but I think I remember it to be pretty dang close to the optimal delay stated by the A3.
As far as this flight goes. I had to repair a zipper on it and extend the airframe to launch it. I also attached a camera without a hood to the side via electrical tape. She did a little cork screw dance at max velocity then flew straight.
This is a scratch build
Screenshot_2016-09-26-15-17-24.jpg
This is my L1 cert flight on a formula 75 with an H155St 14 second delay drilled down to an anticipated 12 seconds. This was my first time drilling a delay. The flight had a camera also, no hood. She took a slight arc at about 500'. (Now I look at the photo the arc occurred at far less altitude) but flew straight shortly afterwards.
20160813_115751_002.jpg

The wind was near zero AGL. Perfect day to launch. In my favor I'd like to think the camera played a factor in drag causing arc/spin in both launches. I haven't had a chance to launch again yet without camera.

Both rockets recovered entirely undamaged.

My cert flight recovery wasn't the prettiest. I don't like the twirling it did on the way down nor how fast it came down. A 30" chute was probably too small.
My home brew ripstop 18" is perfect for the scratch built.
If our club finishes the rail next month I'd like to fly an I245 in the F75 to compare more data.
 
Last edited:
Top