Do Spacecraft Really Have To Endure The Hazards of Reentry

An extended answer to a question I keep seeing - can spacecraft avoid the furious heat of reentry by slowing down before reaching the atmosphere, or, but flying above it until slow enough to avoid it.

The footage is from the Artemis 1 mission reentry.

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[baxtermullins]

There was one professor in the early days of Apollo that explained you could use rockets to overcome gravity to slowly deorbit but the amount of fuel was more than what it took to get to orbit. So, a heat shield was more efficient, less cost, less massive. His sketches showed multiple rockets pointing towards earth center and rockets slowing the spacecraft down to match earth’s rotation, then allowing the ship to slowly descend! Fuel requirement was enormous.

[Zeecontainers]

That reentry video always gives me a strong sense of relief and appreciation for the safe embrace of earth. Even compared to ending up alone in the middle of the ocean, which is normally considered an exceedingly horrible and deadly situation, it's a warm, protective and comfortable bosom compared to space's sheer hostility to life.

[benjaminhanke]

You make watching YouTube more efficient by presenting two videos at the same time.

[scottwatrous]

I feel like I'm in a capsule returning from the Moon and Scott Manley is on the intercom just rambling on and on about re-entry physics while I'm trying to enjoy this moment.

[schmodedo]

Although I had seen it before, I appreciate you leaving the re-entry video up as you narrated. The vortex of superheated gases behind the capsule is mesmerizing.

[JarrodFLifr]

I am amazed by the 'skipping' of Orion. The calculations to figure that out are truly incredible.

[homeopathicfossil-fuels]

I want more of these refutations of common "Why dont they just do X"
I love your content Scott, followed you since the early days of KSP, I remember being hyped every single time you released a "100% reusable space program" video because your solutions to things were so creative.

[theevermind]

Is reentry really necessary? If I went to space, I would insist on coming back, so yes, it's necessary.

[dallasangler]

Being utterly mesmerized by the parachutes interplay at the moment of splashdown "sparked joy" in this heart. Thanks Scott.

[StreuB]

A skipping rock on a pond is one of the best visual analogies to reentry that I ever heard. Really helped me understand and visualize it after that.

[dgkcpa]

Talked to a person who worked on the US X-15 rocket plane program (1959-1968). They considered putting the X-15 into orbit, but none of their pilots could fly the re-entry profile on the simulator without burning up. A non pilot member of the program asked if he could try re entry on the simulator, and they let him. He succeeded, and was able to repeat the manuever again and again.
Everyone wanted to know how he was able to succeed where others could not. Simple, he said, he watched the temperature guage. If the X-15 got too hot, he pulled up; when it cooled down he let the plane descend. He did this over and over, and showed that winged reentry from orbit was possible. The X-15's glide ratio was about 4 to 1. Constructed of inconel X alloy. An ablative coating was tried on the X-15, but was found to be unsatisfactory, and actually interferred with the plane's natural ability to disapate heat.

[brucewatt]

I never saw that re-entry footage before - my goodness, how amazing is that?!?!

Thanks for going through the details of re-entry Scott, you answered all my questions on that topic in one short, concise and easy-to-understand video.

[rafi]

Congrats Scott for explaining complex thermoaerodynamics without having to display a single equation on the screen 👏🏻

[Rick-the-Swift]

0:01 before I even watch this video or read a comment, I'm going to predict that the answer is no, it's not necessary for spacecraft to endure the hazards of re-entry- it's just a hell of a lot cheaper for them to do it that way. Can't wait to watch the video now and see what others think 😄

[jeromethiel]

I remember watching a documentary about the Apollo program, and they went to an aerodynamic expert and asked, "how do we stop our capsule from melting on re-entry." And he told them "make it blunt." The reason being, as Scott said, the bow shock of air formed by a blunt object pushes the super hot air away from the capsule, and actually insulates it from the hot air. Allowing a relatively small ablative heat shield to protect the capsule from the small amount of heat that gets through.

The space shuttle used the same concept, that's why it was all blunt shaped curves on the leading edges. And even then, the heat tiles were essential to insulate the interior of the craft from the extreme heating of re-entry.

[erdngtn]

Dude, thanks for being you. This is one of the dopest videos during your commentary I’ve ever seen. I’d have never sought this out but we’ve got you to show us something special while learning the best of human exploration

[thomassutrina]

Great talk! As a 1972 Aeronautical Engineer BS degree I knew the conclusions for both including the Blimp or Zeppelin solution. And even looked into it. So the lighter then air embodied rocket would float to a high atmosphere elevation with zero velocity effectively. Heating say hydrogen as the lift gas would help get higher but not significant.
Then collapse the blimp as the hydrogen lift gas and LOX/LH2 is burned would save almost nothing in fuel considering you have to lift the blimp also. So going into orbit is at best a wash but floating to altitude and being in the correct spot and surviving turbulence etc. in the air is a higher risk then flying or rocketing to that starting height.
Blimp would be so large that you would expend energy just to keep it from skipping or it would have to be a lift body that may actually be pushing towards the earth. Lift body means more weight and control surfaces, more weight. Trade off. And the gas inside the blimp couldn't be hydrogen or oxygen. Needs to be something that will not burn or be the oxidizer.
Skylon fly to space with inflatable heat shield for reentry that is discarded or retracted to finish by flying for a landing. Now that is maybe the best combination since the large wings already need to be there to fly at a low mach number early in the climb into space.

[AerialWaviator]

One thing missing from this video is an overlay of velocity and altitude. It would be cool to see the initial entry, climb out of the atmosphere, and reentry to final decent as plots in parallel with the video. (there's likely some national security restrictions to include, but just wishing)

Regarding the L/D (Lift to Drag) ratio of space capsules. Apollo had a L/D ratio of 0.52:1 (or ~1:1.92), and a Dragon Capsule has L/D of 0.18:1 (1:5.6). Apollo reentered at 11 km/s, while a Dragon reenters at a 7.5 km/s. Huge differences in amount of kinetic energy, as notes is a factor of mass*velocity^2.
Starship will be the largest (reusable) spacecraft to undergo reentry. It too will reenter at ~7.5 km/s.

[RobertDeloyd]

Nice to watch the re-entry all the way to the ocean!

[tlrmatthew]

This just blows my mind.
I already understand that re-entry is a rough experience to go through but the mathematical knowledge that is understood about it is just beyond me.
The way Scott talks about it, although i don't understand the most of it leaves me in awe of how much understanding there is about the subject.
How much Scott must put in as regards research & actually understanding then making it into such high quality videos is amazing.