Fly­ing ki­wis no match for Santa

Physics mean noth­ing to a guy like Santa, writes Hay­den Walles.

The Press - The Box - - FRONT PAGE -

Noth­ing in the laws of physics pro­hibits Santa from mak­ing his rounds in time on Christ­mas Eve. In fact, Ein­stein’s rel­a­tiv­ity the­o­ries give him a help­ing hand.

But there are cer­tainly tech­no­log­i­cal chal­lenges, since the mis­sion de­mands that Santa travel close to the speed of light, where time and space be­have dif­fer­ently than we are used to. It would take some ef­fort to catch up with Santa – both lit­er­ally and fig­u­ra­tively.

We know, for ex­am­ple, that Santa’s sleigh doesn’t fly like a plane. Con­corde, the su­per­sonic pas­sen­ger jet, used to cruise at a lit­tle more than twice the speed of sound. Five or six times the speed of sound is about the up­per limit for air­craft, which is still more than 150,000 times slower than the speed of light.

For higher speeds you have to turn to rock­ets. Rock­ets oper­ate on the prin­ci­ple that ev­ery ac­tion has an equal and op­po­site re­ac­tion – so if you pro­pel some mat­ter out the back of your ve­hi­cle, you’ll move for­ward, just as a gun re­coils when it is fired.

Most rock­ets are chem­i­cal. Fuel is burned to pro­duce rapidly ex­pand­ing gas, which rushes out the noz­zle, gen­er­at­ing thrust in the op­po­site di­rec­tion. But Santa isn’t us­ing these. The New Hori­zons space­craft head­ing to­wards Pluto was pro­pelled to more than 57,000 kilo­me­tres per hour by chem­i­cal rock­ets. That’s the best yet and noth­ing to be ashamed of, but a chem­i­cal rocket that could reach any­thing like light speed would re­quire more fuel than ex­ists in the uni­verse.

Elec­tric rocket mo­tors have been get­ting a lot of at­ten­tion lately. They use an elec­tric field to hurl ions out the back. Un­for­tu­nately elec­tric mo­tors ac­cel­er­ate very grad­u­ally, and though they can reach high speeds, they take a long, long time to do it.

The cut­ting edge, as far as high-power rock­etry goes, is the nu­clear rocket mo­tor. Nu­clear rock­ets were orig­i­nally in­ves­ti­gated in the mid­dle of the 20th cen­tury as a way to power a crewed mis­sion to Mars. Rather iron­i­cally, given sub­se­quent po­lit­i­cal de­vel­op­ments, the first Amer­i­can pro­to­types were named Ki­wis be­cause, like our national bird, they were never meant to fly. A nu­clear rocket uses a nu­clear re­ac­tor to heat and ex­pel a pro­pel­lant like hy­dro­gen, gen­er­at­ing thrust.

By the early 1970s descen­dants of the Ki­wis were al­most ready to fly, but they never did be­cause in­ter­est in Mars had waned and dis­trust of nu­clear tech­nol­ogy had grown. Nev­er­the­less, nu­clear rock­ets out­per­form chem­i­cal ones, get­ting higher speeds from the same size rocket.

Even nu­clear rock­ets can’t com­pete with Santa, though. It seems most likely that he doesn’t use rock­ets at all, re­ly­ing in­stead on some­thing like anti-grav­ity, warped space or worm­holes, things that break the laws of physics as we know them. The rest of us can only dream of such things at present.

Santa seems happy to keep the se­cret to him­self. How­ever he zips around the globe, it’s pretty clear that he feeds his rein­deer some­thing very spe­cial in­deed.

On the web: Search for ‘‘His­tor­i­cal Per­spec­tive of the NERVA Nu­clear Rocket En­gine’’ to find a 16-page PDF doc­u­ment on the pro­gramme, writ­ten in 1991.

Im­age: NASA

Things ed­i­tors do: This story would nor­mally be il­lus­trated with a cute pic­ture of Santa’s sleigh, but this is The Box and we have a much cooler im­age – a nu­clear-pow­ered rocket en­gine. Some ver­sions were called ‘‘Kiwi’’.

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