This is a cutaway model of uknown scale of the upper stage of the Mars Electric Launch Vehicle. It was investigated by NASA in the 1960s and 1970s as one of several possible lauch vehicles that could be used in a crewed mission to Mars. However, it was never built.
NASA's Marshall Space Flight Center built the model and transferred it to the Museum in 1975.
This object is not on display at the National Air and Space Museum. It is either on loan or in storage.