“But direct heat is so inelegant. And it becomes less efficient in winter or at high altitudes or extreme latitudes, where there’s less ambient heat.”

“We don’t care about elegance,” said Alf. “We just need it to never run out of fuel.”

Buck couldn’t believe what he was hearing. These people thought they could separate water vapor into hydrogen and oxygen on the fly. They thought they could extract ambient heat and put it in the engine.

And they could.

The tank went in circles again, faster than ever—and without a lick of fuel in it. It stopped only when the four-star asked if the crew could get back on in order to monitor the instruments.

“Just don’t try to change anything,” said Alf.

“Not that they can,” said Gerd.

“But they could break bones trying,” said Alf.

The tank went around and around. Alf and Gerd led Buck and Thor and the four-star back to Buck’s office. Naturally, the four-star took the chair behind the desk, while Buck stood at attention to one side and behind him.

“I don’t know what you did,” said the four-star, “but I’m satisfied. Can you give us a redesign to allow all our future M1A3s to run like this? And what will it cost us? Because you can ask for the moon and I’m betting you’ll get it.”

What a clever negotiator the four-star wasn’t, thought Buck.

“Oh, dear,” said Gerd.

“You can’t build it to do what we got it to do,” said Alf. “Besides, you want your tank crews to be able to control their own tanks.”

“Well, yes. But the fuel thing—do you have to be present for that to work?”

Alf nodded.

“No, dear,” said Gerd. “He means the whole time, and the answer to that is no. We have to start it up, but once Alf and I have spent a few minutes with each tank, they’ll all run like this … well, forever.”

“We need to be able to shut them off,” said Buck. “Do you need to be there each time the engine restarts?”

“Oh, that would be so tedious,” said Gerd.

“No, it’s a one-time treatment,” said Alf.

“No cost,” said Thor. “We don’t need money, anyway.”

“I think we do,” said Gerd softly.

“What we need is a certain percentage of the tanks,” said Thor.

Silence.

Finally, the four-star showed he had a little spine: “I’m sorry, but it’s illegal for private citizens to own tanks.”

“We don’t really want tanks per se,” said Thor. “We want the tanks, their crews, all the supporting crew and spare parts and all that.”

“It’s really illegal for anyone to use the U.S. Army as mercenaries.”