stage 2 possible hollow cylinder design

Question

I was watching this video from Everyday Astronaut:

In the video, it highlights why Stage 2 landing is considerably more difficult than stage 1 landing. There is a 1:1 payload penalty in stage 2 as compared with 5:1 in stage 1. Also, stage 2 travels at 8km/sec vs 2.5km/sec in the first stage.

Another big problem (as per the video) is to keep heat shield pointing down during stage 2's return. Since the heavier engines tend to gravitate down towards lower CG while the empty fuel containers tend to point up.

The current designs attempt at using boosters to keep the shield facing down. But since the object is in an unstable equilibrium, any deviations are amplified, making the design an excruciating challenge.

Why don't we use a hollow cylinder design? By keeping the engine pointing down and a ventilator shaft at the top of the rocket to bypass the hot air from the atmospheric friction. Engine is already build using extremely heat resistant alloys. It should be able to withstand the friction heat, which is now lowered due to bypass shaft. The sensitive areas inside the engine can be shielded.

Hollow cylinder design will not help much to reduce the craft's speed during atmospheric re-entry. But it can reduce the heat problem, which is about 64 times stage 1. We will have to reduce the rocket's speed considerably after a successful re-entry though.

PS:

All the numbers and assumption are taken from the above video. Please point out if these numbers are wrong.

Answer 1

You stated a bunch of particular parameters; these will vary with what rocket.

It's true that because engines are heavy and empty tankage is light, that tankage tends to trail and engines tend to lead when an empty booster free-falls in the atmosphere. Still, this isn't universal; it depends on the exact shape and mass distribution. Hydrolox tankage is much larger (but also heavier relative to the engine) than kerolox or hypergolic tankage, for example. The SpaceX Falcon 9 deploys fins at the top of the lower stage to stabilize it in descent, for example, rather than trusting to blunt-body aerodynamics.

However, it does not follow from that that the highest-heat section of the heat shield will be in unstable equilibrium and will be driven away from the direction of flight. For example, some designs will have the heat shield on the same side as the engine, and therefore it will be on the leading edge. In other designs the vehicle reenters sideways with heat shield on one side (this also seems popular for Mars landers for some reason).

A hollow cylinder rocket, with the engine at the top of the tube, is problematic. In vacuum (i.e. where upper stages operate), pretty much any rocket plume expands a considerable amount. You're going to be cooking the inside of the fuel tanks.

Moreover, "bypass shafts" of any kind do not do much for reentry heat. Reentry heat is not due to friction but due to compression, and builds up at the leading edge of any object. Having a hole through an object simply means you have heat buildup in a donut shape rather than a spot -- and adds more surface area so heat will soak in faster.

Rocket engine materials should not be assumed to be able to survive reentry. Rocket engines are made of a variety of materials, but what lets most large, high-power rocket engines survive operating is not so much their high-temperature materials but the use of regenerative cooling. While some reentry vehicle designs do use liquid or phase change coolant, or perfusion cooling (which "blows" the superheated air away from the vehicle by bleeding coolant vapor through a perforated heat shield), this may be problematic for a minimal-mass rocket. Most successful reentry vehicles have used either ablative heat shields or high-peformance ceramics.