Background

I was recently asked by an aerospace journalist for comments on the reported aerobraking manoeuvres designed to bring the OTV-7/X-37B spaceplane down from its near-40,000 km apogee, described by Boeing as 'groundbreaking'.


My Response

I wouldn't exactly call it ground breaking. Aerobraking occurs every time something enters the atmosphere from orbit. If the object has a heatshield, it survives the trauma, if not - it breaks up.

As far as using aerobraking for trajectory control goes, you can look back as far as the 1960s to Apollo and Zond.

Zond used aerobraking to modify the incoming extremely elliptical orbit to a sub-orbital trajectory. Original perigee was within the atmosphere, above the Indian Ocean. Encounter with the atmosphere shaved off sufficient velocity to result in a ballistic arc with a second atmosphere entry point above Russia.

I believe Apollo had a plan to do something similar. There was a fear that a small error could strand the Command Module in low orbit so it was modified to keep the aerobraked trajectory within the atmosphere. I remember listening to the Voice Of America coverage of the re-entries, and the danger of 'skipping off into space' was often mentioned.

The most recent reported orbit of the X-37B was November 2 when an amateur observer, Toni Simola, observed it in an orbit of about 100 x 30,000 km, nearly 9,000 km down from its original height, see
https://www.orbitalfocus.uk/2023#210

There was never any way for the craft to return to Earth using a retro-rocket as it cannot carry sufficient propellant for the job.

It probably continued with the low perigee, with a plan to raise it once apogee reached 400-500 km so it can continue its mission in LEO. I notice that the Boeing press release mentions disposal of the Service Module so maybe it was planned to be released before the X-37B raised perigee, to re-enter above the southern hemisphere.

We'll not know whether the manoeuvre is complete or successful until either Boeing/Space Force announces it or an amateur observer detects it again.

Seeing how Boeing talks about the way it uses aerobraking reminds me of some stories that appeared from US politicians around five years ago about the X-37B using the atmosphere to change its orbit in order to 'confuse the enemy'. I wonder if they had a briefing from Boeing that talked about HEO mission profiles and came away with a slightly hazy, and optimistic, understanding of what they were told.