Note: This is Part 1 in a series of articles on the development of the RockeTiltometer...
When I first got back into model rocketry (January 2009), I was enamored with the high power rockets I saw at my first launch. However, I was less impressed by the 2-stage rockets that I saw. Most of them either did not ignite the second stage, or they flew in crazy arcs or blew up or did not fly at all. Most of the more successful ones were the smaller, sport-rocket types that seemed to work okay using simple instant, motor-to-motor ignition. Though the concept of staging seemed kind of interesting, I did not think too much about it since it seemed either too difficult, or conversely, too simplistic. Instead, I focused on working toward my TRA Level 3 with single stage rockets.
Somewhere around the time I was working on the build of my Level 2 project and reading like mad to try and assimilate all that I could to be sure things worked correctly, I came across G. Henry and Bill Steins’ book, Handbook of Model Rocketry. In it they discussed several aspects of staging, but again I did not pay much attention at the time, nor did I when, sometime later, I read Mark Canepa’s excellent book Modern High-Powered Rocketry 2, only glancing at the sections on staging.
After a successful Level 2 flight (March 2009), I rapidly worked toward my Level 3, all the while really enjoying the learning process and the excitement of each of our club’s monthly launch days. I was an avid reader of everything I could find about the hobby, and very grateful to have the power of the Internet to assist me in my research. When I first started out I thought I would be one of the rocketeers to always keep things in sight, but over time I became challenged by seeing others do higher altitude flights and recovery. I realized that I particularly love all the electronic gadgetry involved and the more technical mechanical aspects one must master in order to fly straight and recover high-power projects intact.
One day when I was searching for some information in Tim Van Milligan’s Apogee Newsletter archives, I stumbled across an article on staging. In it Tim discussed how to achieve maximum altitude. He was discussing a coasting period between booster burn out (BBO) and sustainer ignition…hmmm…what he was saying contrasted with what I remembered reading in Steins’ book. I remembered vaguely that Stein advocated performing sustainer ignition right after BBO, in this way you would be coupling the maximum velocity of the booster driven rocket with the maximum velocity of the sustainer rocket in order to achieve maximum altitude. However, when I went back and reread Stein, I realized he qualified his statement and indicated he was “neglecting aerodynamic drag”. I was a bit confused at this point so I went back and reread Canepa – he drew the distinction of attempting to achieve maximum velocity versus maximum altitude and that you needed two very different approaches.
Graphic from: Handbook of Model Rocketry - G. Harry Stine
Van Milligan pointed out that Stein’s instant ignition would achieve maximum altitude only if you were in a vacuum since parasitic drag is induced by our atmosphere. In fact, such drag is proportional to the square of the velocity, so the higher velocity gained by instant coupling is precisely the wrong approach to maximize altitude. What Canepa, Van Milligan and others point out is that minimizing velocity, and therefore drag, will generally achieve the higher altitude for the same total amount of thrust.
My simulations in RockSim show an average increase in altitude of about 30+% when the coast period is maximized verses igniting the sustainer motor immediately after booster motor burnout. That is a significant difference.
Since I was developing this urge to go high and realizing that staging was one way to accomplish this, I started thinking more about the coast period between sustainer ignition and BBO. I was intrigued. Just how would you go about maximizing the coast period? After a successful Level 3 certification (June 2009), I began to wonder just where to go next. Maybe I had the answer of how to maintain my interest in high power model rocketry. Maybe staging could prove interesting after all! It obviously was very challenging.
From what I could determine, most high power staging ignition is done with timers. But that seemed rather crude to me. If my goal is to achieve maximum altitude, how much time should I dial in to maximize the coast period? I could guess at how much time to allow after booster burn out. Or I could run simulations in a program such as Apogee’s RockSim, or, and probably the most expensive route, I could just experiment with test flights. However, to really attempt to maximize the coast period I would have to have something better than a good guess. Even if I were able to perform a very accurate estimate of the coast period, it would still be assuming the actual flight went exactly according to my assumptions. How would I allow for such things as inconsistent motor burns, erratic flight patterns, the weather and other assorted conditions that I could not reliably predict?
I thought about current day electronic altimeters. Prior to altimeters being adapted for use in high powered model rockets, most deployment was also based on timers, either the relatively simple motor ejection charge delay used as the igniter, or mechanical or electric timers. The basic problem with timers is that they are not dynamic in nature. That is, if everything goes according to plan in regards to a flight’s performance, then the timer works well. However, as we have often seen out on the launch site, things do not always go according to plan – maybe seldom is a better word! For example, if a motor’s burn is shorter than expected, or produces less thrust that expected, apogee will occur sooner than expected. Any delay that was designed into a timer to deploy at apogee will be later than desired and deployment will not occur at the slowest point in the rocket’s flight, but rather at some higher speed as it is accelerating on its downward path. However, if an altimeter was used in that same scenario, it would be looking for the apogee event in real-time, based upon its sensor readings, not a prescribed time – it is able to dynamically adjust the deployment point to suit the actual flight conditions incurred.
I wondered if I would be able to develop some sort of sensor system that was analogous to the altimeter to dynamically maximize the coast interval. Could I find something to replace a simple timer for staged ignition? The following article describes my trial–and-error journey to produce just such a coast sensing system!
So what factors or issues would I need to consider in order to replace the staging timer? What is it about the rocket’s flight that characterizes an ideal coast period anyway? Just how would you describe to someone what has to happen between booster burnout (BBO) and a point in the flight path that would be considered as the longest possible coasting interval that would result in the rocket achieving maximum altitude?
In the ideal world, after BBO, your sustainer rocket would fly perfectly vertical all the way to what would be apogee, but, just prior to it ”falling over”, you would ignite an “instant on” motor that would come up to pressure immediately and send the sustainer hurtling upward.
From a more realistic point-of-view, we want to initiate sustainer ignition just at the point that we have enough time to bring the motor up to pressure while the rocket is still flying fast enough to maintain its “relatively” vertical attitude. So, we need to be able to monitor both velocity and verticality on a real time basis and make decisions based upon that information.
Let’s go back to that ideal world picture and look at only at verticality aspect. Let’s assume that we’ve been able to design the rocket so it flies perfectly in the vertical all the way to apogee. And let’s assume we have a special sensor on board that is able to monitor the flight in real time and predict well ahead of time just how many seconds into the flight that the rocket will attain apogee. Then all we would have to do is ignite the motor at precisely the point in time in the vertical flight path that leaves us enough time to get the motor up to pressure before the rocket falls over at apogee. In other words, we simply subtract the time in seconds that it takes the motor to ignite and pressurize from the time it takes it to reach apogee.
Hmmm…that’s sounds straightforward enough. The problem is that nothing is that perfect. Aside from the fact that seldom does a rocket fly vertical all the way to apogee, we never know for sure ahead of time how long it will take a rocket to reach apogee due to variances in real performance versus specifications or simulations, or the current weather, booster motor performance, etc. If we could characterize and determine or predict what that value was however, such a predictive apogee sensor system would offer a dynamic trigger point, analogous to an altimeter.
Looking at it from the velocity aspect, if we knew that our magic rocket would fly vertical all the way to apogee, i.e., where it reaches zero velocity, we could pick a velocity point, e.g., decreasing through 200 MPH, prior to that which allowed us to pressurize the motor in time before the rocket arrived at apogee. Many of the popular commercial altimeters, or flight computers, out there today have the ability, either through on board accelerometers and their micro-controllers, to be calculating velocity in real-time. So, if we knew we would stay vertical throughout the coast period, we could fairly easily determine through flight test trial-and-error or simulation, a velocity-based ignition trigger point.
The problem remains that in the real world seldom do flights stay that vertical that long. So, the reality is that we can only get so close to that perfect flight path and therefore a coast maximization system needs to be a bit more complicated.
Adrian Adamson at Featherweight Altimeters has provided some access to verticality and velocity as a trigger with both his Parrot altimeter and the newer Raven. With the Parrot or Raven you can access a parameter in the Featherweight Interface Program (FIP) “decreasing through nnn MPH” as an event to control the output of any of the output/pyro channels. The particular velocity is fixed in the Parrot, but configurable in the Raven. Black Magic Missile Works’ UFC series also makes such an event readily available.
Featherweight Altimeters - RavenTM
Additionally, flight computers such as the Parrot/Raven and the UFC can offer a few more indicators that the rocket is probably still flying upward. Equipped with an accelerometer and timer, as well as a barometer, such flight computers can assure that the pressure is still decreasing (lesser air pressure as you ascend) and that the rocket is at a predictable/configurable minimum altitude after a certain amount of time – all indications that the rocket is still ascending. Assuming that the flight is perfectly vertical all the way to apogee, use of one of these “vertical-check” flight computers is basically all you need.
However, a more likely scenario is that the rocket will not fly perfectly straight and will be arcing over to some degree or another. What is missing from the vertical-check flight computer is a monitoring of the angle off the vertical that the rocket is experiencing.
The rocket may still be ascending per the vertical-check parameters, but do you want to trigger ignition if the rocket is flying at an angle of say, 30 degrees off vertical? What about 45, or 60? High altitude recovery is tough enough without setting yourself up for a very long retrieval or loss of the rocket, or even worse, a flight into the crowd. To be in even a more dynamic triggering environment, we need to be able to measure the angle off vertical in conjunction with the other information before deciding to trigger sustainer ignition.
How do you determine tilt? Sensors that may come to your mind might include mechanical sensors, accelerometers, gyros or magnetometers. Since I did not really know what I was doing at the time, nor much about the more sophisticated sensors, I initially only considered mechanical tilt meters, such as you might find on “tilt-over” shipping containers to record any damage or adverse handling during the shipping process or maybe recreational vehicle levelers. I had also seen some sort of electronic tilt meter in the tools catalog of MicroMark. Could one of these mechanical devices reliably indicate tilt when bolted inside a high power rocket? I thought so.
I looked for inexpensive tilt meters with some sort of useable output like a relay or switched contact and came across some from a company named Rieker. Rieker produces various commercial tilt meters for many different applications. These sensors are normally used to detect whether an article has been subject to tilt outside a prescribed range, for example, to detect shipment damage or as a heavy–equipment vehicle “tilt-over” safety device. They can be ordered with various options, including specifically what degree of tilt is required to trigger their output. They have both single-axis and dual-axis devices. A configurable dual-axis, X and Y, sensor seemed perfect for what I wanted to do.
To be continued in Part 2...