There is a different perspective on that from large acquisitions around firm fixed price contracts. If you're doing some low risk building, say a 10 mile road

There is a different perspective on that from large acquisitions around firm fixed price contracts. If you're doing some low risk building, say a 10 mile road on flat ground with known soil, replacing a previous road, that is well-known to current specs with set-in-stone requirements (no pun intended) it is easy to pitch that firm fixed price because the contractor will know exactly what to do, and there are 20 road building companies that could do the work. Now, building an advanced spacecraft with a host of uncertainties in a vastly more complex environment that may require novel technologies development and a great deal of engineering is very different if you want to pitch that firm fixed price because the contractor perspective is that is very risky, and there are likely going to be changing requirements. The contractors will want to make the Govt pay for that risk, since they are on the hook for it. All risk is on the contractor. The contractor will also fight for $ for every changing requirement. By comparison, in a time & materials contract, the risk is on the government, and changing requirements don't matter much, if you change them, risk is still on the government. This explains why firm-fixed price is not always the best approach, and sometimes time & materials contracts may make for a more economical acquisition approach - because risk and changing reqs can translate into dollars.