Complex sentence: The Gulfstream X- 54 is a proposed research and demonstration aircraft, under development in the United States by Gulfstream Aerospace for NASA, that is planned for use in sonic boom and supersonic transport research.
Split into simpler sentences: The Gulfstream X- 54 was a proposed research and demonstration aircraft that was under development in the United States by Gulfstream Aerospace for NASA. It was planned for use in sonic boom and supersonic transport research.