The US Federal government has a pension plan for government employees in lieu of Social Security. None of this is any sort of Financial Wizardry. Mostly depends on demographics and demographics seldom surprises.
The US Federal government FERS plan is not in lieu of Social Security, it is a supplement to it. It's relatively small, about 1% of high 3 year salary average per year of service.
CSRS was a very different pension plan, and those under CSRS did not contribute to nor draw social security. CSRS was phased out in the 80's, I don't recall exactly the year. All new federal employees since then contribute to social security and are under the FERS system.