Reusability – legs and fins or wings and things?

The choice was made, so the outcome was determined, if not known. In engineering as in life. Not everyone accepts this notion quite the same way, or as gospel. Making a choice and then having to live with a determined if unknown future sounds fine in theory. In practice though determined leads to deterministic. As in destined. Fate. Meaning no longer in the realm of what can be affected or controlled. This is when the word “mitigate” comes in right on cue, because giving up control is not easy. In operations we were always on the tail end, facing the consequence of design decisions made long ago. The common refrain “sure we can” was part of the denial that the latitude in operations was thin, a reaction not a choice.

Design choices are like this. Something in the human mind wants it all. We want our choice (perhaps the easy one), but only with the very best outcome. Naturally. It’s easier to believe a design choice made up-front and in the here and now will echo through the ages, when the outcome is grand. Not so much when the assessment comes back and says the future is not so rosy, because of decisions made months, even years ago.

Usually, this design rule is pictured where Y has to do with freedom, like the number of decisions already set (or the money already spent) and X is time. Your first steps decide so much. Destiny awaits, worse, the die has been cast and the actual form and shape of that destiny is still to be discovered. It’s no wonder the concept was often touted about in any systems engineering slides and just as quickly set aside. A simple set of changes anytime ahead would still set the boat aright in the far-flung future-right? We could mitigate that. Actually, our design fate is what we make, but only if we make difficult decisions in the here and now.

A post-it note showing the design notion that the total consequences of decisions decline over time (a curve dropping) as the number of those total decisions made increases (a curve rising). Credit: Edgar Zapata,
You can see the NASA version of the same idea at (one more post-it note off my desk).

Choices, design and launch reusability

There are a particular set of launch system design choices around reusability. If we can think of examples where mass was missing, when it should have been there, being considered an enemy of the payload mass that is sacred, what happens when we think of adding mass so we can reuse a rocket? Better yet, to reuse a rocket cheaply? A first fork in the road is legs and fins or wings and things? Both designs can return a rocket stage to port. How to decide?

On first glance, each approach to returning a rocket booster would list pros and cons. For each, there are hidden items as well, things not as glaringly obvious as the fins or the flaps. Recently the DARPA XSP tried the wings and things on the booster, in contrast to the operational Falcon 9 booster returning thanks to legs and fins.

The Space Shuttle orbiter returning to the KSC runway (via wings and things like parachutes), the Falcon 9 boosters returning to the landing zone after launch, and the envisioned DARPA XSP booster returning to the launch site after releasing it’s second stage. Credits: NASA (left and center), and DARPA (right).

The assumption choices also have clear outcomes to expect, results written in stone alongside the choices, must also assume more information. To the degree that information is absent, any destiny is less clear. It may still be just as inevitable and determined a result, just that it’s not known. This we call risk.

 Reusing a rocket via:ProsCons
Horizontal Landing

Dissipate energy/velocity over the body, to slow down
Passive thermal protection does most of the work on return

Smooth return

Cross-range, can reach many landing runways very far apart
Complex thermal protection system to account for cryogens plus aerodynamic heating

Runways may not be where they are needed

Active systems, actuated aero-control surfaces, landing gear
Vertical Landing

Dissipate energy/velocity by re-igniting the engine(s), thrust to match descent to slow down
Landing sites can be mobile (a drone out at sea) or a smaller footprint on land

Extensibility from expendable systems

Extend learning to off-world systems (landers)
Must make the vehicle larger to hold extra propellant; propellant management

Engine re-ignition close to ground taxes the engine and areas aft; aft wear and tear

Active systems, actuated fins, landing legs

Difficulty exiting landed vehicle in an emergency
Reusable rocket landing, horizontal or vertical, pros and cons

There is a way to look at these two design choices that is not as stark as this or that – rather as decisions along a spectrum, that spectrum being the shape of the returning rocket. As measured by the ratio of length to diameter, go longer, skinnier and you have the Falcon 9. Add wings, like the XSP, or the SpaceX Starship, or the Space Shuttle Orbiters, and the ratio drops for the wider shape.

Seen this way, the choices for launcher reusability are on a spectrum. Curious, yet the conundrum persists – is there enough information that the measurable consequences of any of these choices are known? Probably not if we consider that reusable launch systems, weather the booster or the part that goes to orbit, are few and far between (TWO so far). Like the choices we make in our own lives, when there are few data points the consequences are not always clear (as much as we may have confidence otherwise).

Lacking information then, legs and fins or wings and things? Why not try each.

As much as seeing a Falcon 9 landing never gets old, as much as it may seem wings and things lead to extremely expensive Space Shuttle orbiters, we see in Starship mishaps that there remains much to learn about these design choices. Inside the extended barrel of a Falcon 9 (or Starship) comes things we don’t see – for liquid acquisition. Pumps prefer liquid, not gas. Very predictable liquid. Similarly, behind the scenes for a winged shape are many surface actuators, gear doors and separation mechanisms.

We can’t be sure yet of the real shape of things to come, shapes that may persist for generations, the way all airliners have looked alike since the 1960s. We can be sure there’s a need for more public and private investment to figure out these choices and what they mean. Only then can we all make better choices ahead, with results still determined by choices, but also known ahead of time from practice in the real world.

Also see:

  • From SpaceNews , the DARPA XSP program terminated in January 2020.
  • The Space Shuttle Solid Rocket Boosters also had an element of reusability (more akin to salvage), enabled by parachutes and the robust aluminum and steel cases that go with solid rockets.

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