Voyage - Stephen Baxter

Скачать книгу

section. The rockets were mounted at the hub of the rotor, and the whole thing was designed to spin about the hub, to provide artificial gravity. It would be, Dana thought, like a great metal snowflake, spinning toward Mars. It was a terrific concept, and utterly impractical.

      Next up was a project manager from General Dynamics. He got to his feet with a broad grin gleaming from out of a Californian tan. ‘I got to tell you,’ he told the audience deadpan, ‘that I can beat you NERVA folks hands-down. With two million pounds in Earth orbit I can get to Mars and back in just two hundred and fifty days – not much more than half your time – and taking no fewer than twenty guys. Gentlemen, I give you Project Put-Put.’

      The idea was to throw one-kiloton nuclear bombs out of the back of the spacecraft – thirty devices every second – and let them off, a thousand feet behind the ship. The shocks would be absorbed through water-cooled springs, and the ship would be driven forward. ‘Like setting off firecrackers behind a tin can. Am I right?’

      The concept seemed ridiculous, but General Dynamics had done some preliminary studies, called ‘Project Orion,’ in the early 1960s, and the presenter was able to show photographs of a small flight-test model which had used high explosives to hurl itself a few hundred feet into the air.

      The technical problems were all around the high temperature flux on the rocket’s back end structure, which would have to radiate away excess heat between explosions. And of course the system had one major drawback, the General Dynamics man said, and that was the radioactive exhaust. But that hadn’t seemed such an obstacle back in 1960 when the first Orion studies had been initiated. Then, it was thought that the unscrupulous Soviets might use this quick-and-dirty method to short-cut to space, so we had to look at it too.

      The General Dynamics man joshed and wisecracked his way through his talk. When he sat down he got the biggest hand of the day.

      Dana felt himself shrink into his seat. How the hell do I follow that?

      When he got to the podium Dana shuffled with his notes and foils, trying to avoid looking out over the sea of sleek suits before him. There was a spotlight on him; it seemed to impale him. It was already four thirty, and after the General Dynamics pitch the delegates had lost concentration; they were still laughing, talking.

      Dana began to read from his notes. ‘Manned Mars stopover missions of duration twelve to twenty-four months are characterized by Earth return velocities of up to seventy thousand feet per second, over the cycle of mission opportunities. A promising mode for reducing Earth entry velocities to forty to fifty thousand feet per second, without increasing spacecraft gross weight, is the swingby through the gravitational field of Venus. Studies indicate that this technique can be applied to all Mars mission opportunities, and in one-third of them, the propulsion requirements actually can be reduced below minimum direct mode requirements …’

      There was a ripple of reaction in the audience, a restless shifting. Dana ploughed on. He felt sweat start over his brow, around his collar.

      He hurried through the idea of gravity assist. He tried to emphasize the history and intellectual weight of the idea, showing that his own computations had built on the work of others. ‘The concept within NASA of using a Venus swingby to reach Mars dates back to Hollister and Sohn, working independently, who published in 1963 and 1964. This was further elaborated by Sohn, and by Deerwester, who presented exhaustive results graphically in a format compatible with the direct flight curves in the NASA Planetary Flight Handbook …’

      It was a little like a game of interplanetary pool, he said. A spacecraft would dive in so close to a planet that its path would be altered by that world’s gravitational field. In the swingby – the bounce off the planet – the spacecraft would extract energy from the planet’s revolution around the sun, and so speed up; in exchange, the planet’s year would be minutely changed.

      In practical terms, bouncing off a planet’s gravity well was like enjoying the benefit of an additional rocket stage at no extra cost, if your navigation was good enough.

      ‘We have already studied the Mariner Mercury mission, which would have swung by Venus en route to Mercury. A direct journey would have been possible, using for example a Titan IIIC booster; but the gravity assist would have allowed the use of the cheaper Atlas-Centaur launch system …’

      ‘Yeah,’ a voice called from the audience, ‘but Mariner Mercury got canned. And there were no men on it anyhow!’

      Laughter.

      Dana pressed on, brushing the sweat from his eyes. There were two ways Venus could be used to get to Mars, he said. The spacecraft could swing by Venus outbound, and use Venus’s gravity to accelerate it toward Mars. Or Venus could be used to decelerate the craft, on its way back to Earth.

      ‘First estimates show a mass in Earth orbit of two million pounds would be required for a mission duration of six hundred and forty days.’ Same weight as nuclear; two-thirds the trip time of chemical. ‘Thus a mission profile close to optimal is delivered, without the need for ambitious new technologies, and hence significantly reduced development costs compared to other candidate modes …’

      And it’s elegant. Don’t you see that? No brute force here: no huge nuclear V-2s. Just proven technology, and elegance, and style. A little thought, gentlemen.

      ‘In conclusion, it has been shown that the Venus swingby mode is generally applicable to all of the Mars flyby and stopover round trip launch opportunities, with very favorable benefits.’

      Dana stepped back from the podium, retreating from the glare of the light. He was numbed, a little giddy, unable to feel his hands or face.

      Seger thanked him, then opened up for questions; with a glance at his watch he signaled that these should be brief. ‘… What about guidance and navigation? Don’t you realize that you’re now talking about devising a mission profile with possibly four planetary encounters? – Mars, Venus maybe twice, and Earth on return? And at each encounter the accuracy of positioning will have to be of the order of a few hundred miles, after traveling tens of millions. How can we navigate so accurately? Why, we haven’t yet proved we can manage a single swingby on such a scale.’

      ‘But we will,’ Dana insisted. ‘Remember, NASA committed to the lunar-orbit rendezvous mode for Apollo – which required a rendezvous a quarter of a million miles from home – before a single space rendezvous had been demonstrated.’

      There was some muttering at this. Hardly a valid comparison.

      ‘What about the design constraints? Near Venus the sunlight is four times hotter than at Mars, so you’ll be sacrificing payload space for a cooling system that will be dead weight at Mars. And there’ll be problems with the increased level of radiation coming from the sun …’

      Dana tried to answer – I’ve incorporated spacecraft design modifications into my weights analysis, and … But he was all but drowned out by the noise of an audience which had little interest in him.

      Now Hans Udet stood up, and a hush gathered. Udet said precisely, ‘On what basis have you arrived at your figures? I am aware of the preliminary analyses of the complicated mission classes you describe. I am aware of no detailed analyses which show the savings you claim.’

      Dana began to stammer out a reply. But our understanding of spacecraft systems has advanced since those early studies, and with the figures I have compiled, we can now show that –

      ‘These results are false.’ Udet glanced around at the audience, tall, aristocratic, in control, charming even now. ‘This is obvious. The figures we are shown are based on unstated suppositions. The speaker doesn’t know what he’s talking about. It may be incompetence, or malice, whatever. We should not expend further energy on this red herring.’ He sat down, his back ramrod-straight.

      There was an uncomfortable stirring in the audience, some nervous laughter.

      Bert Seger got to his feet, quickly thanked Dana, and turned away from him.

      Udet’s

Скачать книгу