“I just knew you’d bring that up,” said Brennan. “I’ll tell you in detail, but first let’s talk about the battle.”
“Screw the battle!”
“You did well,” said Brennan. “There isn’t much left of your weapons pod, but that’s okay if you don’t meet any more scouts. You don’t have reserve fuel to get into orbit around Wunderland; you used too much. But you can abandon Protector and land with the cargo ship.”
“That’s nice. That’s very reassuring. Now tell me how a Pak scout can burn my own exhaust and come tearing up my tailpipe!”
“It’s one possible configuration. In fact, it’s the one I’m about to start looking for, because it’d be easy to find. I can show you better with diagrams.”
Roy had calmed down a bit when they reached the Flying Dutchman’s control room. He had also started to shake. Three days in Protector’s control chair had left him exhausted.
Brennan looked at him thoughtfully. “Want to put this off?”
“No.”
“Okay, I’ll make it quick. Let’s look at what your ram field does. It picks up interstellar hydrogen in a path three thousand miles across. It sweeps it in via magnetic fields, pinches it together hard enough and long enough to produce some fusion. What comes out is helium and some leftover hydrogen and some higher-order fusion products.”
“Right.”
“It’s also a hot, fairly tight stream. Eventually it’ll spread out into nothing, like any rocket exhaust. But suppose a ship were following you, here.” Brennan made pictures on the screen: two tiny ships, the second following a hundred miles behind the first. He spread a wide cone before the lead ship, converging it almost to a point behind the ship. A needle shape with the ship in its point — the ship’s protective shield — brought the incoming hydrogen into a ring shaped constriction.
“You’re collecting the fuel for him. His ram field is only a hundred miles across—” Brennan drew a much narrower cone. ” — and it gives him finer control over his fuel flow. It’s already hot and dense. It burns better, in higher-order fusion. The exhaust would be rich in beryllium.
“It’s just one of the things those last remaining Pak might have tried. The lead ship would be nothing but a ram: no onboard fuel, no insystem motor, no cargo. It would have to be towed up to ramscoop speed. The following ship is heavier, but it gets more thrust.”
“You think that’s what’s coming at us?”
“Maybe. There are other ways to work it. Two ships, independent, held together by a gravity generator. In a pinch they could split up. Or the lead ship might be the ship proper, with the hind ship only an afterburner. Either way, I can find them. They’ll produce beryllium frequencies like a neon sign on the sky. All I’ve got to do is build the detector.”
“Need help?”
“Eventually. Go to sleep. We’ll try another dry run in a month or so.”
Roy stopped in the doorway. “That long?”
“Just to keep you on your toes. You’re as ready as you’ll ever be. Only, be more careful with that electromagnetic projector. When you wake up I’ll show you what the Pak scouts did to it.”
“What you did to it.”
“What they would have done. Go to sleep.”
Brennan was in the machine shop for three days. If he slept he slept there. He skipped meals there. Whatever he was doing filled the machine shop with constant racket and sent a humming vibration through the rock of the Flying Dutchman.
Roy read a couple of old novels stored in the computer. He floated through bare rock caverns and corridors, and was oppressed by the sensation of being underground. He worked himself to exhaustion in the exercise room. Free fall had cost him some muscle tone. Have to do something about that.
He researched Wunderland and found about what he expected. Gee: 61%. Population: 1,024,000. Colonized area: 3,000,000 square miles. Largest town: München, population: 800. Farewell, city life. Come to that, München would probably look like New York to him by the time he got there.
There was a time on the fourth day when he found the machine shop quiet and Brennan apparently asleep. He was about to leave when Brennan opened his eyes and started talking.
“You depend too much on those long, slow turns,” he said. “The way to dodge Pak weaponry is to vary your thrust. Keep opening and closing the constriction in the ram field. When they throw something like a laser pulse into the constriction, open it. Nothing’s going to fuse if you don’t squeeze the plasma tight enough.”
Roy wasn’t flustered. He was getting used to Brennan’s habit of resuming a subject that may have been broken off days ago. He said, “That last ship could have done that when I threw radon at him.”
“Sure, if he did it fast enough. At good ramscoop velocities the shit should be in the constriction before he knows it’s reached the ram field, especially as you didn’t put any rocket thrust on it. That was good thinking, Roy. Memo for you: don’t ever follow a ship that’s running. There are too many things he can throw into your ram field. Hopefully we’ll be doing the running in any battle.”
Roy remembered what he had come for. “You’re two days past dinnertime. I thought I’d—”
“Not hungry. My prism’s in the oven, and I’ve got to wait for it to cool.”
“I could bring—”
“No thanks.”
“Any significance?”
“Didn’t I tell you I was predictable? If there aren’t any Pak scouts in the vicinity, you could just as well go on to Wunderland alone. Most of what I know about the Pak is stored in the computer. When a protector feels not needed, he doesn’t eat.”
“So you’re kind of hoping we find Pak scouts.”
Brennan laughed: a credible chuckle, though his mouth didn’t move. His face wasn’t hard, exactly; it was like wrinkled leather. It was his mouth that was like hard shell. Too much of human expression is in the mouth.
On the evening of the same day he came out towing three hundred pounds of machinery, of which a big, solid crystal prism was a prominent part. He wouldn’t let Roy help tow it, but they set it up together at the focus of the Flying Dutchman’s telescope. Roy brought him a sandwich then, and made him eat it. The Jewish mother role irritated him, but so did the thought of going on to Wunderland alone.
Brennan was gone when Roy came looking for him, around mid-afternoon of the fifth day. Roy found him in the one room from which he was forbidden, the hydroponics garden. Brennan was moving down the side of an open tank, consuming sweet potatoes one after another.
The prism threw a rainbow spectrum across a white surface. Brennan pointed to a bright green line. “Beryllium light, blue-shifted,” he said. “And the helium lines are up in the violet. Ordinarily beryllium is in the infrared.”
“Blue-shifted.” Any school child knew what that meant. “He’s coming down our throats.”
“Maybe not. He’s coming toward us, but maybe not dead on. We’re only a couple of light-weeks out from Sol, and he’s a light-year away, and I think he’s decelerating. I’ll have to check to see if we’re getting his exhaust. But I think he’s headed for Sol.”
“Brennan, that’s worse.”
“It’s just as bad as it can get. We’ll know in a month. He’ll have moved by then. We’ll have some paralax on him.”
“A month! But—”
“Just a minute. Calm down. How far can be go in a month? He’s way below lightspeed; we’re probably going faster than he is. A month won’t cost us much — and I’ve got to know how many there are, and where they are, and where they’re going. And I’ve got to build something.”