Comment by doomlaser | Hacker News Reader

Comment by doomlaser | Hacker News Reader

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I'm no rocket scientist, so bear that in mind, but something has been bugging me about the flight paths for this vehicle [0]. It has to make some very large, specific orientation changes several times during descent, using the heat shield on one side of the vehicle, and then only rotating to upright orientation in the lower atmosphere, after quite a lot of falling sideways. The orientation maneuvers seem much more complicated than their current booster re-entry flight paths [1]. and even more complex than the space shuttle [2].

[0] https://www.youtube.com/watch?v=LQTnWEHl5qU

[1] https://www.youtube.com/watch?v=6YyV-otP3pI

[2] https://www.youtube.com/watch?&v=TOieURpnbm0

This appears to me to make each flight much more complicated to pull off, introducing more points for it to fail without contingencies, and it makes me nervous. But, again, I am an just an unenlightened observer from an unrelated field. I don't even play Kerbal Space Program! I'm sure they've weighed a million different options and their risks, and have arrived at this one after a lot of thinking, but it has been bugging me. The stainless steel construction certainly looks beautiful out there in Texas.

It’s actually similar to the shuttle, In a couple of ways simpler, and a couple of ways more complex. The shuttles body was also designed to both generate lift and be a blunt body - with a transition of mode between them. The blunt body allowed it to survive reentry while the lifting body allowed it to glide. However that glide was way more complex then NASA generally talked about. The shuttle pulled some aggressive s-turns to bleed off energy during the re-entry.

This approach is more or less the same as what the starship is doing here, except instead of using a glider trajectory for the controlled landing they instead use retropropulsive landing. This is needed because what SpaceX is landing here is a second stage, not a first stage. Way more heat, way more power.

Also they plan to land where there isn't any atmosphere, or an atmosphere too thin to brake significantly.

So they need propulsive landing.

A lot of the complexity is hidden in the second video. It's the exact same deal, it just doesn't look like it. In particular those innocuous 'waffle irons' on the side play an incredibly important role. You miss the scale in the video thanks to how big everything is. Those fins are each 4' x 5' large. And each of them can independently yaw/pitch/roll. And of course the exact angle that the rocket is at when it fires its boostback (as well as when it's coming down), exactly how long the boostback is fired, and a large number of other variables all need to be lined up exactly perfectly. Otherwise you end up literally thousands of miles from your destination, run out of fuel, or otherwise condemn your mission in a surprisingly large number of innovative ways.

The big difference you're seeing compensated for is that Mars atmosphere is less than 1% dense as Earth. So to slow yourself down you need a much larger exposed surface area. Do that exact maneuver on Earth and you'd get an 'unplanned rapid disassembly' during reentry thanks to the thicker atmosphere. That danger and complexity is also something that's hidden.

I cannot recommend Kerbal enough for anybody even vaguely interested in space. Just absurdly fun and does really help you get a feel for all of these issues.

> Do that exact maneuver on Earth and you'd get an 'unplanned rapid disassembly' during reentry thanks to the thicker atmosphere

That is the planned Earth re-entry maneuver. Belly first almost all the way down, using the heat shield on one side, then flipping and completely re-orienting near the ground so that the vehicle is pointed upwards, and then the final burn to land upright.

But it does make sense that this path has arisen from a secondary capability: to land on planets or moons with less atmosphere. It still doesn't make me less nervous about their ability to pulli it off reliably. Here's hoping.

Learning how to reliably control the damn thing on descent is certainly their greatest hurdle. Refueling in orbit being the second biggest. I expect things to go boom at least a few times before they learn to land properly. Saying people Wil go to orbit within a year is most likely not to happen.

Elon said they learned a lot of what they need for orbital refueling just from docking with the ISS.

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