Worth noting orbital refueling may not actually be that difficult, it was strongly rumored that Senate Launch System backers put the kibosh on refueling because

Worth noting orbital refueling may not actually be that difficult, it was strongly rumored that Senate Launch System backers put the kibosh on refueling because it would jeopardize the SLS jobs program if it worked: "We had released a series of papers showing how a depot/refueling architecture would enable a human exploration program using existing (at the time) commercial rockets," Sowers tweeted on Wednesday. "Boeing became furious and tried to get me fired. Kudos to my CEO for protecting me. But we were banned from even saying the 'd' word out loud. Sad part is that ULA did a lot of pathfinding work in that area and could have owned the refueling/depot market, enriching Boeing (and Lockheed) in the process. But it was shut down because it threatened SLS." https://arstechnica.com/science/2019/08/rocket-scientist-says-that-boeing-squelched-work-on-propellant-depots/ https://arstechnica.com/science/2019/08/rocket-scientist-say...