Space X’s trans-lunar mission isn’t a new idea – but it’s cool

I don’t know whether to applaud or shudder at Elon Musk’s announcement that Space-X will launch a private ‘moon tourist’ mission late in 2018.

Free-return lunar trajectory - out of Earth orbit, a quick pass around the Moon, and home again. Public domain, via Wikipedia.
Free-return lunar trajectory – out of Earth orbit, a quick pass around the Moon, and home again. Click to enlarge. Public domain, via Wikipedia.

I want to applaud because it’s deeply cool, damned bold, and it’s what humanity needs to be doing if we’re to keep any kind of space dream alive. Good stuff. But I shudder because it’s also risky. Sure, they’ll have to get FAA approval – but as I write this, neither the spacecraft nor the booster have flown and the time-frame seems tight. I expect there will be slippage.

The mission has all sorts of appeal, though. A lunar flyby is the easiest deep-space mission to mount in terms of energy and complexity. All the spacecraft needs is enough fuel to push itself from low-Earth orbit into a trajectory that flies by the Moon. Gravity does the rest, looping the spacecraft around the Moon and swinging it on course for Earth, where the re-entry module aero-brakes to a landing.

Gemini-Centaur lunar mission. W. D. Graham,
Gemini-Centaur lunar mission. Note the ‘eyeballs out’ orientation – acceleration leaves the astronauts dangling in their seats. W. D. Graham,

I’ve seen suggestions that the craft might brake into lunar orbit, and – later – accelerate back towards Earth, but that’s a lot more complex, and there’s the problem of fuel. The Apollo CSM, for instance, required fuel to change velocity by 2,804 m/s to get into and out of lunar orbit, which meant the CSM massed 28,801 kg and had to be lofted by a full Saturn V booster. This rocket was capable of pushing 41,000 kg (and, later, 48,600 kg) to a trans-lunar insertion orbit. The Falcon Heavy – big though it is – can only put 16,000 kg into the same orbit, which as I understand it is about the mass of the Dragon 2 spacecraft that’ll carry the astronauts. However, that doesn’t leave anything much for fuel. To me that says a flyby mission is practical – but an orbital one isn’t.

It’s not the first time lunar flybys have been proposed. The ease with which even a medium-size booster could loft a flyby mission made it appealing in the early days of the space race. In June 1965, NASA astronaut Pete Conrad lobbied for a circumlunar mission using a modified Gemini spacecraft, launched into Earth orbit by a Titan III booster and then propelled on a lunar fly-by trajectory with two of the small ‘transtage’ boosters developed for the aborted X-20 Dynasoar spaceplane. He proposed an unmanned heat shield test by December 1966; a February 1967 manned Earth-orbital test; then an April 1967 manned lunar flyby. Gemini’s manufacturer, McDonnell Douglas, also proposed a lunar-capable Gemini, this time using a Centaur booster. However, the problem with both was that funds would have to be taken off Apollo, and that killed ‘lunar Gemini’ in all its forms.

The original Soyuz 7K design by Sergei Korolev. Public domain, via Wikipedia.
The original Soyuz 7K design by Sergei Korolev. Public domain, via Wikipedia.

The Soviets were doing the same thing on the other side of the Iron Curtain. Vladimir Chelomei’s OKB-52 design bureau was ordered to develop a circumlunar vehicle in 1961, independently of Kennedy’s challenge. The Kruschchev administration intended to use it for propaganda, flying the mission to mark the fiftieth anniversary of the Soviet revolution in 1967. However, the program was cancelled in 1965, after Kruschchev fell from grace.

By this time the moon race was ‘on’, and Sergei Korolev’s OKB-1 bureau had already designed their own lunar orbiter, the Soyuz 7K. It was too heavy for any single Soviet booster of the day to send moonwards, and he initially envisaged a complex arrangement of multiple launches and tankers to let it reach lunar orbit. However, a modified version – the Soyuz 7K-L1 – could be launched on a free-return flyby trajectory by a single UR-500K ‘Proton’ rocket fitted with a Block-D stage for the trans-lunar boost. By stripping the Soyuz of its habitat module and other gear, it was – just – within the capacity of the UR-500K to do so.

This system offered a way of flying around the Moon. There was only one problem. It didn’t work in time. The initial UR-500 series rockets were unreliable, but a test Soyuz 7K-L1, minus heat-shield and dubbed “Cosmos-146”, flew in March 1967. A year later the first operational 7K-L1 was sent on a deep-space flight in the opposite direction to the Moon. This was an even simpler mission than a flyby. To disguise its purpose, the Soviets gave it the cover name “Zond 4”, as if it was part of a quite different series of earlier “Zond” probes. The word meant, variously, ‘explorer’, ‘probe’ and ‘catheter’. The mission was successful until re-entry, when the descent module guidance system failed on re-entry and forced ground controllers to destroy the module before it landed in Africa. “Zond 5” followed on a lunar flyby trajectory with a payload of tortoises and other animals – again suffering guidance failure on re-entry and plopping into the Indian Ocean after a crushing 20-g ballistic descent. The tortoises survived. Luckily it was unmanned: any cosmonaut would have been injured or dead.

Stripped Soyuz: the
Stripped Soyuz: the 7K-L1 “Zond” circumlunar version with crew living in the cramped re-entry module (think ‘front seats of a Volkswagen Beetle’, and don’t ask about the toilet). Public domain, via Wikipedia.

“Zond 6” – an unmanned repeat – followed in late 1968. The CIA was aware of the true nature of the Zond, briefing NASA on Soviet plans to fly a crewed successor mission, which was one of the reasons why Apollo 8 was sent into lunar orbit that Christmas. However, the Soviet plans came adrift. On the return leg of the “Zond 6” mission a faulty gasket depressurised the cabin, killing the animals on board. If it had been manned, it would have killed the crew – at this stage, the Soviets weren’t using pressure suits. The re-entry module then crashed in Kazakhstan after another re-entry guidance failure.

Cosmonauts Alexei Leonov and Oleg Makarov were tipped to fly on the next mission, “Zond 7”, scheduled to launch on 9 December, two weeks ahead of Apollo 8. It would have been a simple lunar fly-by, whereas Apollo 8 was orbital – but you can imagine the way it would have made the ‘moon race’ look neck-and-neck, disguising the fact that the Soviet effort was faltering. Korolev died in 1966, his successor Vasily Mishin was uninspired; and the OKB-1 bureau’s moonship, the Soyuz 7K-LOK/LK (lander) complex, was well behind schedule. Worse, the huge N-1 booster required to launch it kept exploding.

Conceptual image of Zond in flight. Public domain, via Wikipedia, Ebs08 CC BY-SA 3.0,

In the event, the failure of “Zond 6” prompted the Soviets to delay the manned flight until January 1969 – by which time Apollo 8 had taken the lead with a lunar orbit, and there was no propaganda value in a flyby. Another booster failure delayed the launch further, and in the end, “Zond 7” flew without its crew. It was a success. “Zond 8” – originally intended to be flown by Valery Bykovsky and Nikolay Rukavishnikov – was delayed to October 1970 and flew unmanned for the same political reasons. But that saved the cosmonauts: “Zond 8” made another 20g ballistic re-entry. “Zond 9”, which was to have been flown by Pavel Popovich and Vitali Sevastyanov, was cancelled. And the Brezhnev administration began pretending they had never been in a moon race, which was what we call an ‘alternative fact’ these days. But that’s the Cold War for you.

The US flew its own flyby accidentally in 1970, when Apollo 13 suffered a catastrophic internal explosion during the outward leg and the crew switched to a free-return trajectory that brought them around the Moon and safely back to Earth.

The idea of a lunar flyby gained new life with the era of space tourism. In 2005, Space Adventures – who facilitated the first low-Earth orbital tourism – came up with DSE-Alpha. This tourist mission proposed using a modified Soyuz spacecraft, which would dock with a Block DM stage in orbit. This would propel the Soyuz into a free-return loop around the Moon. It was basically the Zond-style mission – already proven –  and there were people who could pony up the $100 million-a-seat price tag. However, nothing eventuated.

Now we have Musk’s proposals. We’ll see where they go. I do hope it works. Would you go, if you had the money?

Copyright © Matthew Wright 2017


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