7
$\begingroup$

The Falcon 9's second stage is currently disposed of each flight. I know there have been some concepts by SpaceX on making it reusable, but evidently they have not yet done so. From my understanding, the difficulty in bringing this stage back comes from the much higher re-entry velocities and the higher payload penalty for adding weight (and saving extra fuel). That being said, I tried my hand at ideating a way in which this could be done:

Concept Sketch

Basically, the stage would do its normal de-orbit burn and point engine first as it re-enters. It would be protected by either a regeneratively cooled piece or a traditional heat shield. It could then conduct a series of burns and land similarly to the first stage, with grid fins and landing legs.

I wanted to know a couple of things.

  1. Is SpaceX still planning to recover their second stage?
  2. And if so, what would be the technical challenges in doing it this way?
Improve this question
$\endgroup$
10
  • $\begingroup$ This would add a lot of mass to the second stage, the heat shield, the landing legs and the propellants needed for the series of recovery burns. You also need something to start the additional burns. The added mass would reduce the payload mass. You may need a negative payload mass made from unobtanium. $\endgroup$
    – Uwe
    Jun 5, 2020 at 9:37
  • $\begingroup$ Not answering the question but they have stretched Falcon 9 to its performance limit and now their focus has shifted to Starship development which would be completely reusable. It will be their workhorse albeit not anytime soon. Second stage recovery is not worth the attention because that would mean sacrificing payload mass for extra hardware which is critical to the rideshare missions for smallsats. Other companies like RocketLab are there to deploy a dedicated launch for smaller payloads. $\endgroup$
    – OrangeDurito
    Jun 5, 2020 at 10:17
  • 1
    $\begingroup$ @Carl Witthoft by going around once it would be close to the launch site again. $\endgroup$
    – lijat
    Jun 5, 2020 at 13:24
  • 3
    $\begingroup$ What beautiful illustrations! $\endgroup$
    – Anton Hengst
    Jun 5, 2020 at 14:44
  • 1
    $\begingroup$ I just want to further comment, that sometimes, the Falcon 9 launches lunar payload (like TESS, and Beresheet). So that's orbital speed + deltaV to get into the appropriate trajectory (around 3 km/s). That's a lot of velocity to cancel out! Anyways, those are really nice pictures! Wish I could draw like that. $\endgroup$
    – Star Man
    Jun 6, 2020 at 3:00

2 Answers 2

Reset to default
6
$\begingroup$

It's likely way too heavy.

Unlike the first stage, the second stage needs to accelerate all the way to LEO/GTO orbital speed. This means that all the extra stuff for reentry and landing needs to be first put into orbit and then deorbited again. All of the following thus cuts directly (gram for gram) into the payload capacity:

  • Thermal protection capable of handing entry from orbit. This likely has to be an ablative heat shield, you'd have to waste lots of propellants for cooling. ("Regenerative" only works when the stuff is going to be burned by the engine anyway.)
  • Deorbit and landing propellants
  • Attitude control system for atmospheric flight (grid fins including hydraulics)
  • Landing legs

Payload to LEO is about 15 metric tons when not flying expendable. It's anyone's guess how heavy all the stuff above would be, but it's obvious it's going to be a significant fraction of the capacity. Reusing the second stage would only make sense when flying really light satellites to LEO. However, that problem can already be solved by ride-sharing or using a smaller launcher.

Finally, even if you managed to make this work for LEO launches, it'd be impossible to use for GTO payloads where the current capability is just over 5 metric tons (and GEO satellites in general tend to be on the heavy end). So you'd have to have two very different designs of the second stage depending on the destination orbit, which would likely make all kinds of processes more expensive.

Improve this answer
$\endgroup$
2
  • $\begingroup$ Yeah, this makes sense. I haven't done a full estimate on the mass of these systems, but I'd imagine they're at least a few metric tons (the booster's landing legs alone are two tons). I read somewhere that they'd only get about a 30% cost reduction from saving the second stage, so it would make no sense to infringe upon more than 30% of the payload with added weight. This means the system would need to be at most 4.5 metric tons to be cost effective. Even if you get it under that, the engineering hours to do that could probably be better used elsewhere. $\endgroup$
    – Ben Woodman
    Jun 5, 2020 at 20:23
  • 1
    $\begingroup$ @BenWoodman Right, also see the new last paragraph. $\endgroup$
    – TooTea
    Jun 5, 2020 at 21:17
3
$\begingroup$

My guess is that they are not interested in recovering Falcon 9 second stages, because they are mainly focused on Starship/Superheavy. There the second stage: Starship, has been designed for reentry landing and reuse from the start.

Improve this answer
$\endgroup$
2
  • $\begingroup$ Meant to make that comment not an answer. Happy to delete if somone comes up with a more researched answer. $\endgroup$
    – Steve Linton
    Jun 5, 2020 at 10:56
  • $\begingroup$ Musk has said as much, in fact. They haven't lost sight of recovering the second stage, but their resources at the moment are better spent on Starship devel. $\endgroup$
    – user36480
    Jun 11, 2020 at 7:09

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge that you have read and understand our privacy policy and code of conduct.

Not the answer you're looking for? Browse other questions tagged or ask your own question.