Refuel

What Is Refuel?

You are flying a jet 34 metres behind a tanker, 22 metres below it, with a full tank and no plan. Somewhere ahead of you is a boom with a contact box around the end of it, and the only way to stay in the air is to get into that box and stay there long enough to take on fuel.

The box is small. Three metres of altitude either way, seven metres fore and aft, so roughly the footprint of a large car. Get inside it for two and a bit unbroken seconds and 22 units of fuel move down the hose. Get outside it and nothing happens at all, while the tanks keep draining at 2.4 units a second whether you are in contact or not.

That arithmetic is the whole clock. A full tank is good for a little over forty seconds of flight with no contact at all, and every successful contact buys you roughly seven seconds more than it costs. Fly well and the flight goes on indefinitely. Fly badly and it ends in well under a minute, which is exactly what happens if you take your hands off the controls.

How to Play

1. Press the button and you are in formation, behind and below. The tanker is the aeroplane ahead of you and the dashed rectangle behind its tail is where you need to be.
2. Climb into the box with the up key. Altitude is the easy axis and you will have it under control within a second, because the aeroplane answers the stick in about four tenths of a second and settles at a rate of five and a half metres a second.
3. Now the hard part. Use the left and right keys as throttle: left slows you down and lets the tanker pull away, right speeds you up and closes the gap. It is not a steering control. You cannot point yourself at the tanker and arrive.
4. Watch the fuel readout on the left of the stage. It falls constantly and it is the only thing that ends the flight, so the question is never whether you can hold the contact but how much of the time you are holding it.
5. Hold the box for 2.2 seconds. A green bar fills underneath the box while you are in contact, and when it fills you take on 22 units and the count starts again.
6. When the tanker changes altitude, follow it. It makes a manoeuvre every few seconds and it will move more than 24 metres from where it started over the course of a flight, so there is no such thing as a position that holds itself.
7. Fly until the tanks run dry. Your best range is kept in the browser.

Tips for Staying in Contact

• Do not chase the gap. Fly where it is going. This is the single most useful thing on this page. A pilot with half a second of reaction time who corrects against the current error swings 34.7 metres either side of the tanker and never settles. The same pilot correcting against where the gap will be in 2.2 seconds holds it to within a few metres. Reacting is not enough. You have to anticipate.
• Make the correction early and small. One nudge now is worth three later. Because the throttle takes over two seconds to arrive, a correction you make when the gap is one metre wide does its work while the gap is still small. The same correction made when the gap is eight metres wide arrives when it is twelve.
• Altitude first, then throttle. Fix one axis at a time. The vertical axis answers nearly six times faster than the throttle, so mixing the two inputs means the quick one finishes and the slow one is still running when you start something else. Get level, then close the gap.
• Use the trail behind your aeroplane. The dots trailing back from your jet are where you have been for the last few seconds, which is the only way to see an oscillation before it becomes a problem. If the trail is a smooth line you are flying. If it is a wave, you are already behind the aeroplane and should stop making inputs until it settles.
• Give up on a bad approach. If you are 30 metres out and swinging, falling back and resetting is cheaper than fighting. Fuel burns at the same rate either way, and a fight costs you the contact you were trying to make plus the one after it.
• Expect the tanker to move. It changes altitude every few seconds, on a random timer, and it does not warn you. Treat the box as something attached to a moving object rather than a place in the sky, because that is what it is.

The Two Axes, and Why One Is Easy

The aeroplane has four controls and two things to position. Each axis has its own response time, and the gap between the two numbers is the difficulty of the game.

Altitude

Answers in 0.39 seconds and settles at 5.625 metres a second of climb or descent. Quick enough to feel connected to the stick, which is why a pilot can hold altitude almost without thinking about it.

Fore and aft position

Answers in 2.22 seconds, by way of the throttle. Nearly six times slower than altitude, and slow enough that by the time the aeroplane does what you asked, you are looking at a gap that closed two seconds ago.

What the throttle is worth

26 metres a second, each way. Full fast settles you at 246 against a tanker doing 220, full slow at 194, so the whole range of fore-and-aft control is a 52 metre a second spread across a 14 metre box.

What that means in practice

You are using a control that is worth twice the width of the box, changed slowly, to position yourself inside it. Overshooting is not a mistake you make once and learn from. It is the default behaviour of the system, and avoiding it is a technique rather than a correction.

The box itself

Three metres of altitude and seven metres fore and aft, held for 2.2 unbroken seconds. Break contact for six tenths of a second and the hold falls from 2.00 to 1.03, so a contact cannot be assembled out of pieces.

What Makes This So Hard

A delay in a control loop is not the same kind of problem as a small control loop. It is not that you have less authority, or that the aeroplane is heavy. It is that the information you are acting on is stale, and no amount of skill fixes stale information. The only fix is to act on a prediction instead.

The measurement that makes it concrete: a pilot correcting against the current error, with half a second of reaction time, swings 34.7 metres either side of the tanker over thirty seconds and never converges. Half a second is not a slow pilot. It is a normal one. The same pilot, looking at where the gap will be in 2.2 seconds rather than where it is now, holds it steady. Nothing else changed.

That second pilot is doing something a test can measure and a person can learn. The gap is closing at some rate in metres a second, and the throttle takes 2.2 seconds to have any effect, so the number that matters is not the gap but the gap plus 2.2 seconds of closing rate. If that sum is negative you are going to arrive behind, and you should add power now. If it is positive you are going to arrive ahead, and you should take power off now. Correcting against the sum rather than the gap is what anticipation means, and this is one of the few places where you can watch it work.

The vertical axis has the same structure and none of the difficulty, and comparing the two is the clearest way to see why. It answers in 0.39 seconds, so the gap you are looking at is nearly the gap as it is now, and correcting against it works. You can fly altitude reactively. You cannot fly the throttle that way, and the only difference between them is a factor of 5.7 in response time.

The mistake everyone makes first

Thirty-four metres behind, hold the fast key, close the gap. That is the obvious plan and it does not work, but it is worth being precise about why, because the reason is not what it looks like.

The gap closes. Thirty-four metres takes 2.9 seconds, which feels about right and encourages you to keep holding. But the aeroplane is still accelerating when it arrives, because the throttle only got there a moment ago. It does not stop on the tanker. It goes past, and by the time you have released the key and the slow side of the throttle has begun to take effect, you are 60 metres in front of it.

Then you hold slow, and the same thing happens in reverse. This is not a failure of nerve or of precision. It is what a double integrator behind a first-order lag does when you drive it with a switch, and the fix is to stop driving it with a switch. Short inputs. Take the power off before you think you need to. Assume the aeroplane is going to keep doing what it is doing for another two seconds, because it is.

What the tanker is doing to you

The tanker is not a fixed point. It flies its own profile: a slow climb, then a step down, then level for a while, on a timer of somewhere between three and six seconds. Over a forty second flight it will be more than 24 metres from where it started, which is eight times the height of the contact box.

This matters more than it sounds, because it means the box is not a place. There is no position you can find once and hold. Every manoeuvre the tanker makes turns a perfect contact into a lost one unless you follow it, and the follow has to start before the manoeuvre finishes for the same reason everything else here does.

It is also the honest part of the design. A tanker that flew perfectly straight would turn this into a game about finding the box once and then doing nothing, which is not a game. The manoeuvres are what make the anticipation technique necessary rather than merely optimal.

Refuelling From a Phone

A flight is short, the controls are four buttons, and nothing in it needs a keyboard. It works on a phone for the same reason it works on a desktop, which is that you are making small corrections against a slow response rather than reacting quickly to anything.

On a keyboard, the up and down arrows or W and S work the altitude, and the left and right arrows or A and D work the throttle. Space starts a flight and P pauses it.

On a touchscreen the four buttons sit across the bottom of the screen: down and up on the left pair, slow and fast on the right. They are hold buttons rather than taps, so your thumb stays down for as long as you want the input applied and comes off when the correction is done. That matters here more than in most games, because the right technique is a short input followed by waiting, and a button that latches would make that impossible.

The one thing worth adjusting on a small screen is how you watch the contact. The dashed box is the thing to look at rather than your own aeroplane, because your aeroplane is somewhere inside it and the box is what moves. Watching your own jet means watching the one object on the screen that is not telling you anything.