Composing a suite vehicle
Let’s get to it! What are we actually working on here?
The project goal is to launch a vehicle vertically using a model rocket engine, then descend and land under rotor power. The vehicle itself will be one, combined piece, but we can think of the different components separately, as things that can we can figure out independently, and then put together to form the working whole.
So what are the pieces? The project goal tells us what each component is supposed to do:
Ascend using a model rocket engine
Descend and land under rotor power
This is a bit of a cheat; we’ve already constrained what we can use to solve the problem. If we hadn’t already decided to merge, specifically, a model rocket and a drone, we might propose various solutions, compare them, and then pick the ones that seem to work best. As it is, there’s mostly one kind of thing that uses a model rocket engine—a model rocket—and one kind of thing that uses rotors to move vertically—a multicopter (or less generically, a quadcopter, aka drone). In fact, let’s start by removing the rotor requirement:
2. Descend and land under power
Hmm, these requirements are pretty vague. How will we know if we’ve succeeded? Sure, if our vehicle pops up two feet on a tiny explosion, then turns on rotors and falls without hurting itself, that might technically meet the requirements. But it’s not what we really want. Let’s set some more specific goals:
Ascend to 1000 feet in altitude in under 10 seconds
Land with a final vertical velocity of under 2 feet per second, within 5 feet of a target point
Don’t separate into pieces
This is much better. Now we’ll be able to check whether the choices we make fit well with our goals. Note that the numbers we’ve selected are pretty arbitrary; after all, this isn’t a government plane or rocket whose contract is written with the vehicle’s use in mind. So these numbers might change, and that’s ok! If they do, we can reevaluate our choices in light of the new requirements. It’s pretty easy to make mistakes when that happens, though; some earlier decisions might be carried forward into the updated design without checking that they’re still appropriate, and that can cause problems. So we’ll try not to change things too much.
Admittedly, we’ve chosen these numbers so we pretty much have to use a rocket engine to ascend, and rotors to descend. This is another cheat, so to make amends, let’s at least propose alternative solutions and see how they do.
Ascent
The first requirement is to reach 1000 feet in under 10 seconds.
Solutions for ascent:
rotors
rocket engine
one of those stompy rocket things for five-year-olds
Based on what I can find online, most drones are going to max out at something below 70 mph, which would just get us to 1000 feet in 10 seconds, if we didn’t have to first accelerate. I’m also guessing that this max speed is for moving horizontally; going upward, against gravity, might be slower.
Can model rockets reach this height? Some definitely can, but many of them are very light. We’re going to have the added weight of the quadcopter pieces, which model rockets typically (obviously) don’t have. I think it should be possible, but I haven’t done the math yet! It could be that this simply isn’t possible without making a really big rocket. This is a part of the problem I’m going to set aside for now. If we find out that 1000 feet in 10 seconds isn’t doable, we’ll change the requirements, and we’ll be sad about it 😞 but we will persevere 💪.
Can we use a stomp rocket? No. No we can’t. Duh. But now I know what I’m getting my goddaughter for Chanukah!
Descent
The second requirement is to land softly, and near to a target.
Solutions for descent:
rocket engine
rotors
parachute
Could we use another rocket engine, like SpaceX does? The short answer is no. The long answer is hell no. I think what SpaceX and Blue Origin are doing is really cool, and they’re part of the inspiration for this project. But model rocket engines can’t throttle or steer, and they don’t always ignite reliably or in a reliable amount of time. So it simply isn’t possible to use a model rocket engine. And I am not qualified to use something more complex. Let’s keep it safe!
Rotors are a good bet. Drones are capable of controlled vertical and horizontal movement, and of making minor changes in velocity. A good approach would be to make sure that the vehicle can hover, and ideally maintain altitude while moving horizontally.
How about a parachute? Well, that’s the solution we’re trying to improve on, so we already suspect that it can’t slow the rocket enough. Apogee Rockets suggests that if you want to descend on a parachute, you should shoot for falling at 3.5-4.5 meters per second, or about 11.5-14.5 feet per second. So the parachute won’t work. Additionally, a parachute wouldn’t allow the vehicle to aim for a particular landing site; instead, it would be at the mercy of the wind. Finally, a parachute would violate the third requirement, because in order to expose the parachute, a model rocket has to pop its nose off. This would mean separating into pieces. The pieces are connected by a cord, but they’re still distinct pieces in my book.
Conclusion
The two pieces, then, are
model rocket—our up-goer
quadcopter drone—our down-comer
Separating these components conceptually means that we don’t have to complete one before working on the other. There are many ways that decisions in one component will impact the design of the other, but we don’t have to know exactly what that decision will be in order to make good progress elsewhere.
I think this is a good place to leave things for now! I have more stuff written about aerodynamics, and identifying the front of the vehicle (not so simple!) but I’d like to keep these posts reasonably bite-sized, so they don’t overwhelm the reader. Not overwhelming people with detail upon detail of in-depth explanation is something I’m working on 🙂.
Until next time! Have a goodun!
