Speaking of which, a commercial Unix might actually be the best choice for these kinds of systems. The Royal Navy has the budget and bargaining power to sign a

Speaking of which, a commercial Unix might actually be the best choice for these kinds of systems. The Royal Navy has the budget and bargaining power to sign a multi-decade support contract from the likes of IBM, HP, and Oracle. The latter already offers 24 years of support for the latest series of Solaris; maybe they can add another dozen years on top of that if you throw enough money in their general direction. By contrast, no version of Windows has been supported for more than 13 years (though they might also offer special long-term contracts), and most flavors of Linux only get 5-10 years.