Designing a basic rocket plane, the Y-1

Rocket science is fundamentally pretty simple. Big explosion go down, steel cylinder go up. A very simple implementation of Newton's third law: "For every action, there is an equal and opposite reaction".

Building aeroplanes is somewhat trickier. In ordinary (stock) Kerbal, building your first aeroplane is generally tougher than building, say, an orbital rocket, but the aerodynamic model is still pretty simple and forgiving. Something that is sort of like a plane will still fly.

RP-1, specifically the mod FAR, is not nearly so forgiving.

First, let's make it clear that we're not building a traditional plane. While RP-1 does let you build propeller planes of the era, I've decided to head straight to rocket-planes. Rocket planes are what they sound like, planes that are primarily (or solely) powered by rocket engines.

Historically, the most famous rocket plane is the Bell X-1, the spiritual analogue of the Y-1 we're about to build. In our fantasy timeline, the X-1 has already long since flown its famous supersonic first, achieving the speed of sound in level flight in 1947 and granting immortal fame to its test pilot, Captain Charles "Chuck" Yeager. We're playing catch up.

The Bell X-1 was powered by the Reaction Motors XLR11, a rocket engine which isn't that far removed from the basic principle of the V-2 rocket engine, just scaled down. We already used this engine on our last rocket, the second stage of our SR-2c. We'll be using two of these on our Y-1. While it is certainly achievable to get up to supersonic speeds with one such engine, it's much easier to achieve with twice the thrust! We can control the throttle to manage fuel levels, so we don't use any more fuel than is required.

In an ordinary plane, control is usually offered by three 'control surfaces': The ailerons (which control roll clockwise-anti-clockwise), the elevators (which control pitch up/down), and the rudder (which controls yaw left/right). Our first plane, the Y-1 will roughly follow this principle. It deviates from standard plane building in a couple of ways however.

 

Firstly, the design is shaped cylindrically. This is along the same lines of the X-1, which was intentionally modelled on a .50 browning bullet, an object known to be stable in supersonic flight.

Secondly, instead of simple elevators on the tailplanes at the back I have replaced them with all-moving tailplanes. I decided upon this configuration after some testing determined that elevators were not really doing the trick. Armed with modern knowledge of supersonic flight, I was able to establish that all-moving tailplanes are the preferred 'supersonic-friendly' alternative.

One important criterion of aeroplanes in general is also that the center of pressure (aka. center of lift) must be behind the center of gravity (aka. center of mass). One of our main sources of mass in our plane is rocket fuel, so we need to make sure that the main source of lift (our wings) are sufficiently behind our main source of mass (our fuel).

 

To optimise our lift from a runway (allowing us to take off with plenty of fuel!), I ended up settling on the following design to pump the fuel for our engine from the front of the plane in to a high-pressure chamber at the back:

This keeps most of the fuel mass at the front. It should be noted that the design can still support fully fuelled operation when not operating from a runway, as I'll explain lower down.

This design is quite expensive up-front. Purchasing all-moving aerodynamic surfaces capable of handling supersonic speeds without failing has cost our agency quite a bit of funds, but the ability to go supersonic will pay this back pretty handily through our supersonic contract. I've also put a camera on it so we can collect MOAR SCIENCE as we go:

 

The Y-1 design is pretty specialised. While it is capable of general flight, it's limited by its fuel capacity. To maximise its capability, I'm going to launch it up with fuel to be drop launched from a bomber. The US drop launched their X-Planes from massive strategic bombers, such as the B-52 (affectionately known by crews as the BUFF, Big Ugly Fat F*r). We've managed to get hold of an early Avro Vulcan prototype from the British, which will act as our launch platform.

Historical Aside: The Avro Vulcan wasn't really in service during this time period (the first test flight was give or take a couple of months) and its precursors were too small to really serve in this role. Still, it seems like the most likely candidate a hypothetical Council of Europe space agency would have used.

Addendum: Maybe a Handley Page Victor?

The Y-1 design is also somewhat fragile, although not unrealistically so. Pulling a hard manuever to try to adjust the pitch of the aircraft more than 5-10 degrees at a time while going at supersonic speeds will result in the wings falling off. This isn't at all unusual for an experimental rocket plane, so I'm just going to limit my urge to perform silly maneuvers rather than trying to design around it.

The Y-1 is a very, very efficient glider, so it takes a long time to slow down without some external assistance. Thus, it also has a drogue chute in the back to save time.

Historical aside: The X-1 was noted for spending far more time gliding then it ever did under powered flight.

In the next post, we'll be taking the Y-1 out for its first drop-launched flight, attempting to breaking the speed of sound and maintain it for 30+ seconds, then perform a glide landing.

Next Post: Y-1 Flight... IN VR