Your spacecraft has 5,200 m/s of delta-v and that is all it will ever have. Mars is currently 100° ahead of Earth, the cheap route needs it at 44°, and the angle closes by only about 14° a month. You have eighteen months to leave, cross, and stop — because arriving at Mars is not the same as staying there. Free in the browser, no install and no account.
Spaceflight is not about pointing at your destination and going. It is about spending a fixed budget of speed changes at exactly the right moments — and the right moment is decided by where the planets are, not by when you are ready. Win this and you will have used the idea of a launch window rather than just heard the phrase.
LAUNCH — the booster puts you in a parking orbit around Earth. That alone takes about 9,400 m/s, which is why it is the booster's job and not part of your tank. Waiting on the launch pad is free; waiting in orbit is not.
TRANSFER BURN — the departure burn that stretches your orbit out to meet Mars. At the ideal alignment it costs about 3,600 m/s. Leave early and you are buying a faster, more expensive path to catch a planet that has not arrived yet — the price climbs steeply for every degree you are off.
COURSE CORRECTION — coasting between planets is free, because you are simply falling around the Sun. But tiny errors in the departure burn grow into thousands of kilometres over a seven-month cruise, so a small 60 m/s nudge near arrival is far cheaper than a big one early.
CAPTURE BURN — the one everybody forgets. You arrive travelling several km/s faster than Mars, and gravity alone will not catch you; it just bends your path and lets you go. Slowing into orbit costs about 1,300 m/s, and if you spent it getting there, you fly past forever.
AEROBRAKE — Mars has a thin atmosphere, so you can dip into it once per orbit and lose speed to drag instead of fuel. It costs only 400 m/s but takes months of passes, and it only works if you arrived slowly — that is, if you left inside the window.
Earth laps Mars as both orbit the Sun, and the angle between them changes by only about 14° a month. That means the same good alignment returns roughly every 26 months — the synodic period. It is why real Mars missions launch in tight clusters a couple of years apart, and why missing a window means waiting more than two years for the next one.
One playthrough runs about twelve minutes. It needs no login, no install and no student accounts — a link is enough. Every ending explains itself: flying past Mars, stranding the ship in Earth orbit as the propellant boils away, or simply running the clock out while the window closes. The model is deterministic, so a class can replay the same mission and compare plans fairly. Suitable for middle-school science and space units covering orbits, gravity, and mission planning.