At all other times one pretends even to himself that he doesn't know it.
That is the only possible way to avoid leaks.

The top secret information was simply that it was still impossible to
supply the Platform. Ships could be made faster than had ever been
dreamed of before, but so long as any ship that went up could be
destroyed on the way down, the supply of the Platform was impractical.
But the ships were being built regardless, against the time when a way
to get them down again was thought of. As of the moment it hadn't been
thought of yet.

But building the ships anyhow was unconscious genius, because nobody but
Americans could imagine anything so foolish. The enemies of the Platform
and of the United States knew that full-scale production of ships by
some fantastic new method was in progress. The fact couldn't be hidden.
But nobody in a country where material shortages were chronic could
imagine building ships before a way to use them was known. So the
Platform's enemies were convinced that the United States had something
wholly new and very remarkable, and threatened their spies with
unspeakable fates if they didn't find out what it was.

They didn't find out. The rulers of the enemy nations knew, of course,
that if a new--say--space-drive had been invented, they would very soon
have to change their tune. So there were no more attacks on the
Platform. It floated serenely overhead, sending down astronomical
observations and solar-constant measurements and weather maps, while
about it floated a screen of garbage and discarded tin cans.

But Joe and Sally looked down where the ships were being built while the
problem of how to use them was debated.