A lot of that public money somehow ends up in the private sector, usually as corporate profits.
Libertarians may want to ask themselves what happens to the private sector - aerospace, energy, R&D, infrastructure, education - when public investment stops.
I am guessing that in your mind that if a private corporation bids on a contract it should only break even? Is that it? Or perhaps - for you - even better would be - the whole thing would be done by the government. There is no private corporation?
Libertarians may want to ask themselves what happens to the private sector - aerospace, energy, R&D, infrastructure, education - when public investment stops.