The goal of my project is to fly a model rocket that, upon reaching apogee, flies itself back down as a glider, using control surfaces operated by an onboard autopilot computer to guide itself back to the launch/landing field and a targeted, soft, horizontal landing.
As far as I know, this hasn’t been done before.
This post started as an email letter to my 41-year-old son, a software developer and a space and astronomy geek, because he had expressed interest in my rocket project a few months ago. After I’d been typing for a few hours (!), I realized I had more to say than I could fit into a reasonable-length email. The topic needed something like chapters.
That sounds like a series of blog posts to me.
So I copy/pasted the unfinished email into WordPress, massaging it only slightly to create this post.
Future posts will start getting into the aerodynamics, the flight computer, and the design process, but first we need to cover some basics, which is what this post does. Here I’ll
- define the goal;
- lay out some of the constraints—regulatory, legal, safety, and a couple of self-imposed constraints; and
- offer a couple of personal notes, including the (mildly embarrassing) story of how I came to be interested in rocketry.
|The goal is simply stated: to fly a model rocket that, upon reaching apogee, flies back down as a glider, using control surfaces operated by an onboard autopilot computer to guide itself back to the launch/landing field and a targeted, soft, horizontal landing.|
Simple to state, but difficult enough to do that no one has done it yet, to my knowledge.
This “gliding descent” is similar to spaceplanes like the Space Shuttle — except without a pilot — or the X-37B. That is to say, “gliding descent” is a euphemism for “barely controlled diagonal plummet terminating in a high-speed skid,” as seen in the linked video clips.
Nail-biting, but it works.
There’s a problem with both of these spaceplanes, though: as good as they are at being gliders on the way down, they’re actually not very good rockets on the way up. That’s because their wings are designed to provide lift, that is, a force that pushes from the bottom surface of the wings to the top whenever the aircraft is moving. That’s great when the Shuttle (or X-37B) is gliding down, parallel to the ground, but not when it’s pointing straight up, as at launch.
At launch, with the Shuttle (or X-37B) pointing straight up, that force of “lift” is actually directed sideways, because it’s still pushing from the bottom surface of the wings to the top. Obviously, you don’t want a sideways force when you’re trying to go straight up, so the Shuttle engines have to compensate: they direct part of their thrust sideways in the opposite direction to counteract the “lift” . This sideways thrust, and the fuel to provide it, are basically wasted. Pushing sideways doesn’t help the Shuttle get where it’s going; it just keeps it from tipping over as it ascends.
In fact, this was such a problem on the early X-37B flights that they now put the spaceplane inside the booster rocket for launch; it doesn’t come out until it reaches the edge of space.
|This leads to one of my self-imposed constraints: I want my rocket to be not only a good glider coming down; I also want it to be a good rocket going up.|
And for a reason — the more efficient the rocket is, the higher it flies; and the higher it flies, the longer the glider flight coming down. A longer descent from a higher altitude is more challenging, which, to me, makes it more interesting.
|This constraint becomes a design requirement: my rocket has to be symmetrical.|
What I’m doing is also different from other model rocket glider projects that I’ve seen online, because of a second self-imposed constraint:
|I want my descent glider to be the same aircraft as my ascent rocket.|
That rules out one approach I’ve seen: reconfiguring the fins / wings at apogee. One design I saw, for example, had long, slender glider wings that folded, spring-loaded, into the body at launch, where they wouldn’t interact with the air during ascent; then an electronic switch would release them at apogee, and the springs would pop these long, slender wings out for the glide down. That’s very cool and very clever, but that’s not what I want to make.
This presents a tricky design problem:
|The part that works as a rocket fin on the way up, also has to work as a glider wing on the way down.|
The reason this is tricky is because the best rocket fins are small, thin, triangular shaped, and mounted at the rear; while the best glider wings have a wide wingspan and a thick cross-section; they’re shaped like long skinny rectangles; and they’re mounted near the center of the glider.
As I said, more challenging is more interesting.
|The other big set of self-imposed constraints is implicit in the phrase “model rocket”, which has a precise definition in aeronautical law, in the national fire code, and in the amateur rocketry organizations’ safety codes.|
- You can’t shoot a rocket from one place to another.
- You have to launch straight up.
- Your rocket has to make a soft landing. That’s for safety reasons—you don’t want your rocket coming down like a 100-mph lawn dart!
For normal model rockets, this means a parachute descent. For my rocket, this means a gliding descent to a targeted, soft, horizontal landing.
- No explosives on board (except a tiny black powder charge is allowed to pop the parachute). Model rockets are not fireworks.
- The fully loaded rocket is limited to 1500 grams, about 3 lbs. Fuel mass is limited to 125 grams, about 4½ ounces.
- Motors must be certified, commercially available model rocket motors — no home-brewed fuel or hand-assembled motors.
- Total impulse of the motor(s) can’t exceed 160 newton-seconds.
- There are rules for the size of the launch field based on the power of the motor.
- Of course there are rules about fire safety, like don’t set the grass on fire in the launch field.
- And so on.
Why would I work within this restrictive safety code? In a word, insurance. As a member of the National Association of Rocketry (nar.org), I’m covered by a five million dollar liability policy for any damage caused by my rocketry activities, as long as my rocketry activities are within the safety code. So, major motivation.
That’s (more than) enough about the goal and the constraints.
Here are a couple of personal tidbits.
This is an early concept sketch from my home whiteboard (everyone has a home whiteboard, right? No? Just me?) You can see I’m still figuring out the geometry of the fins/wings.
Why the name “
JR-X1“? All my model rockets have a name and a number (they’ve all been simple kits so far). The number is
JR-1 for the first one,
JR-2 for the second one, and so on. Why
"J" for Jenny’s, and
"R" for Rocket. The
X is for eXperimental, just like NASA and the Air Force. I don’t know which
JR number this one will be by the time it’s built, so it’s just
X1 for now. And though I’ll have to lay eyes on it to know its name for sure, it’s likely to be named after my ancient Greek mythological spirit guide, Cassandra.
|I did promise a mildly embarrassing personal note. It’s this: I got interested in model rockets for some very wrong reasons.|
It started around the time those idiots were flying drones over Heathrow a few years ago, even shutting down the airport on a couple of days. I thought at the time, “If the military can make a surface-to-air missile that can take down an F-16, how hard can it be to make one to shoot down a 25-mph drone?”
So my engineering brain teamed up with my anger fantasy of shooting the damn things out of the sky, and I started planning how to make it work. Later, I got the idea of using air-to-air “missiles” (model rockets) instead of surface-to-air; they would be launched from an “anti-drone drone”. So I had to start studying the physics of flight, a lengthy undertaking.
At some point, Common Sense whispered in my ear, suggesting I at least take a look online, to find out if what I wanted to do was even legal. Unsurprisingly, it turned out that it would be very illegal, in so many ways. Like a $2,000,000 fine and life imprisonment illegal.
So I decided to let go of my dreams of righteous revenge. But the whole rockets / aerodynamics / engineering topic was so fascinating, I couldn’t let it go. So instead of dropping the whole concept, I morphed it little by little until I’d stripped out all the inconveniently illegal bits, and I ended up with the JR-X1 project. I’m OK with that. This project is challenging enough.
Stay tuned for my next post, which will cover the overall architecture of
JR-X1, and future posts, which will get into the design process!