Introduction
Overview
Development
Propulsion
Airframe
Avionics
Flights
Good People
Rocket Links
Steve Baughman
Web Updated Aug 15
© 1999
All Rights Reserved.
ROCKET DESIGN OVERVIEW
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Four key elements can ensure success regardless of size. Photo by Scott Ghiz |
What this implies, is that the best way to ensure long lasting success in rocketry is to avoid losing or damaging the rocket. And if that sounds easy, well, that's because you haven't tried it. It can take a lot of time and experience to design and construct anything which is truly reliable, and rocketry is no different. My strategy in design and construction involves four key elements: flight stability, structural integrity, adequate power, and reliable recovery. To elaborate, a successful rocket design incorporates appropriate methods to ensure the rocket's flight stability, it uses suitable materials and construction techniques to retain full structural integrity, it has enough power to get off the ground safely, and uses a recovery system that is reliable. With this methodology you can build rockets that will fly straight, stay together, and come back home to be flown again and again. In this section we explain the basics of these four key elements.
FLIGHT STABILITY
"It helps to put the fins on straight." - Tom Montemayor, regarding stability.
Getting a rocket like the XRV to fly straight for a mile or so high is a challenge, sure. You have to ensure that the vehicle flies in a stable manner so that it doesn't go out of control or peel out horizontally over the horizon. Failure to do so can result in a good long walk or worse. In practice, ensuring stability is really not that difficult. Unfortunately, some people do not take the time to do the calculations, contributing to a disasterous result.
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Stability depends on cg and cp. Photo by Jerry Webster |
Understanding the concept of center of pressure is slightly more difficult: it is defined as the point at which all aerodynamic forces are centered. To demonstrate this, look at figure 1-1 below, which represents a simple weather vane such as is commonly seen in rural areas. In diagram [a.], we see the side profile of the weather vane, with blue arrows depicting the force of a wind coming from the direction of you, the viewer. In [a.], notice that on the left side of the vane a large blue arrow shows the large wind resistance of the fins, compared to the small blue arrow on the right showing the small wind resistance of the tip. This represents the relative sizes of aerodynamic forces acting on either end of the weather vane. When we average these two forces together, the resulting equivalent force can be represented by a single blue arrow acting somewhere between the initial two, as represented in diagram [b.]. This is the point where all aerodynamic forces are centered, and as such it is what we call the center of pressure. For simplicity sake, diagram [b.] also shows the pivot point of the weather vane as the center of gravity since it is the point around which the vane rotates.
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Figure 1-1 A weather vane demonstrates aerodynamic stability in the wind. |
Basically, for a rocket to be stable it must always rotate upwind like the weather vane does in diagram [c.]. This upwind cocking happens because the center of pressure is located behind the rotation point (or center of gravity), which causes the tail to turn downwind and the tip to turn upwind, as represented by the green arrows. The simple relationship of the centers of pressure and gravity is the key to stability for fin-guided rockets. You'll learn more about how we have incorporated this knowledge into the XRV later on in the airframe section.
STRUCTURAL INTEGRITY
"Heads up, we have incoming!" - Range Safety Officer after a shredded flight.
The strength of internal and external forces acting on a powerful rocket during it's ascent can be formidible. The very high thrust of commercially available solid fuel rocket motors is what draws a lot of interest towards high power rocketry in the first place, but it would seem that many have found themselves and their rockets unprepared to handle the stresses. The resulting energetic dissassembly (a.k.a. shredding) of a poorly designed airframe at maximum velocity typically produces a spectacular shower of smoke and scraps accompanied by drifting confetti and incoming pieces of airframe and fin around a wide area. To see it happen live might cause you to consider getting the unlimited damage waiver on your rental car next time around, trust me.
To avoid this type of mishap the airframe must be constructed very well. Doing it properly depends on knowing about the types of stresses an airframe is subjected to during a flight, then selecting the appropriate high strength materials to withstand them. Figure 1-2 below demonstrates how internal and external forces play a major role on a rocket during ascent.
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Figure 1-2 Internal and external forces demand high structural integrity during flight. |
When the rocket motor first ignites, it can produce a level of thrust which is virtually explosive in its nature. As exhaust exits the rocket nozzle at the end of the motor, its force is opposed by an equal force pushing against the motor mount. The motor mount then in turn transfers the energy to the rocket airframe to create motion. This motion grows very quickly, and as velocity increases, aerodynamic drag begins to push very hard in the other direction against the nosecone, which transfers this force again into the airframe but in a direction opposite of the direction of motion. These two opposing airframe forces work together to try and compress the airframe like an aluminum can. They can also create a shearing effect at the internal joint where the motor mounts attach to the airframe. As if this weren't enough, drag also pushes hard against the leading edges of the fins, causing them to want to bend backwards or flutter. This force can rip the fins out of the airframe, or simply break them off at the root.
The lesson to be learned here is that the highest stresses experienced during the rocket's ascent are at the interface between the fins, the motor mount, and the airframe, so this is the area that will most benefit from any efforts at strengthening. Build the fins extra strong and stiff and they'll do a better job of resisting flutter, and remember that the airframe must be strong enough to resist compression. In the airframe and avionics sections, we'll discuss the materials and construction techniques used in the XRV to maximize structural integrity.
ADEQUATE POWER
"Oh, it should hit about mach 4." - Dr. Franklin Kosdon comments on a two-stage 'O' to 'M' rated rocket.
Issac Newton's third law of motion states that for every action, there is an equal an opposite reaction. Newton's formula F=ma shows the relationship between a force (F) when it is applied to a mass (m) to produce a rate of acceleration (a). To honor Issac Newton for this discovery, science has labeled the standard unit of force the Newton. One Newton is equal to the force required to accelerate a 1 kilogram mass at a rate of 1 meter per second per second. Per second per second - is that a misprint? No, remember that velocity is a change in distance over time, which is measured in meters per second. Acceleration is a change in velocity over time, so the units are now meters per second per second. Rinse, repeat. Now you've got it.
The key to propulsion lies in combustion. Inside the motor, the burning propellant releases hot gases which expand to thousands of times their original volume, creating immense pressure in the combustion chamber. This pressure is released through an opening in the motor casing called the nozzle, which is shaped to optimize the chamber pressure and exhaust gas flow. In essence, rocket motors generate force by accelerating masses of burning propellant gas out of their nozzles at very high acceration, according to the F=ma formula. Though the mass of propellant is small in relation to the rocket, the gases are expelled at supersonic speeds almost instantaneously, and this very high acceleration of a relatively small mass can result in surprisingly large magnitudes of force directed in the opposite direction. This is the thrust that pushes the rocket upwards, and as we mentioned previously, it is measured in Newtons.
| Motor Impulse | |||
| min Ns | max Ns | ||
| A | 0.00 | 2.50 | |
| B | 2.51 | 5.00 | |
| C | 5.01 | 10.00 | |
| D | 10.01 | 20.00 | |
| E | 20.01 | 40.00 | |
| F | 40.01 | 80.00 | |
| G | 80.01 | 160.00 | |
| H | 160.01 | 320.00 | |
| I | 320.01 | 640.00 | |
| J | 640.01 | 1280.00 | |
| K | 1280.01 | 2560.00 | |
| L | 2560.01 | 5120.00 | |
| M | 5120.01 | 10240.00 | |
| N | 10240.01 | 20480.00 | |
| O | 20480.01 | 40960.00 | |
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Table 1-1 Impulse ratings are based on Ns. | |||
Consumer rocket motors are rated alphanumerically by their total and average impulse. Total impulse is identified by a letter designation which represents a the total impulse. An 'A' rated motor is one with a total impulse of 2.5 Ns or less. Each successive letter designation represents a doubling of total impulse from the prior letter, so a B rated motor would have a total impulse of between 2.51 and 5.0 Ns, while a C rating would be between 5.01 and 10.0 Ns. This doubling can continue in this way for as far as you'd like to go, though Table 1-1 only shows up to the O rating.
Motor ratings also include a number which follows the alphabetic rating for total impulse. This number is the motor's average impulse in Newtons. When combined, they form an alphanumeric designation like J350 or K1100 as examples. Thankfully, the doubling of total impulse with each successive letter enables one to approach some serious power before one gets too deep into the alphabet. In the propulsion section we'll help you to futher understand the workings of the rocket motors used in the XRV.
RELIABLE RECOVERY
"If you're not walking, you're not flying." - Alan Davis, returning from a recovery hike.
If a rocket survives the trauma of ascent, it's not out of the woods yet. As the old saying goes, "it's not launching it that's the problem - it's getting it back." The reason is that there are so many things that can go wrong. Foremost among these is the fact that when the parachute is deployed a mile up in the air, upper level winds can cause the rocket to drift a long way before it eventually makes it to the ground. Higher altitudes make the problem even worse, and can make it a real chore to successfully recover a high flyer when it comes down over the distant treeline or horizon. I'll admit I've been on some decent hikes to go get my rocket but in the end I've aways been lucky enough to find it. Others are not so lucky. They wander the hinterlands for hours, searching, hoping, hiking. They return to the pads later that day near exhaustion, usually looking for motorized transportation.
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Nothing but fins? Grab a shovel. The rest is underground. Photo by John Powell |
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Figure 1-3 Dual deployment.
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To avoid the wind drift problem, we use a method called barometric dual-deployment recovery. It is important to understand the theory behind barometric dual-deployment before actually seeing how it is implemented in the XRV. The key here lies in a pressure sensor within a system of on-board avionics which uses varying air pressure to determine the altitude during flight. We'll get into details about the avionics later, but for now let's examine a typical barometric dual-deployment profile as shown in figure 1-3.
The launch begins with the boost phase [a]. When the motor burns out [b] the rocket coasts upwards until it runs out of momentum and begins to arc over [c]. When the on-board avionics determine that the rocket has reached peak altitude (called apogee), it sends a command which deploys a very small parachute, called a drogue [d]. The drogue opens and the rocket begins to quickly descend [e]. When the rocket gets closer to the ground, the avionics sends the appropriate command, and a larger main parachute is deployed [f]. The main parachute then opens and slows down the descending rocket further [g] until eventual touchdown.
Using a drogue and main parachute in this manner lets the rocket descend from very high altitudes before it is able to drift too far, but still have a relatively soft landing. Assuming that it didn't land in a the middle of a pond, you should easily be able to retrieve it, take a victory lap, and then refurbish it for another launch. It just doesn't get any better than that.
Now that you've been exposed to the key factors that make up a successful rocket design strategy, it's time to see how these factors have driven incremental improvements within the complete XRV development program. In the development section coming up next, we'll take a look at the entire engineering history of the XRV during its evolution into a crowd pleasing mile high scorcher.
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