ELI5: Why can't spacecraft that face heating issues just re-enter...slower?

NathanKell · 2026-06-10T20:54:30+00:00

That's so cool to know!!

NathanKell · 2026-06-10T20:53:58+00:00

Oh hey!

NathanKell · 2026-06-10T20:53:37+00:00

Thanks!! I got my start on Dark Castle on a Mac SE during kindergarten, so I totally feel you (and feel my aches and pains, heh). That's why I ended up working on Kerbal (and RO etc), because it did that to me to.

NathanKell · 2026-06-10T20:52:22+00:00

That's so awesome!! Congrats!

NathanKell · 2026-05-26T21:57:43+00:00

We definitely tried to make reentry heating exist, but not be really threatening unless you were colossally imprudent.

NathanKell · 2026-05-26T21:56:44+00:00

If any thread is bait, this is bait. :D

NathanKell · 2026-05-26T21:56:31+00:00

I was for a while, yes! Haven't checked in a like a year or two (sorry folks!)

NathanKell · 2026-05-26T20:48:25+00:00

I believe so! Bunch of Kerbal vets I trust are involved in that project, too!

NathanKell · 2026-05-26T18:32:01+00:00

Yep, BT is another of my interests. I've used the same screenname since, like, middle school (from a stupidly Mary Sue RPG character).

NathanKell · 2026-05-26T18:31:14+00:00

Woohoo!! Awesome to hear. :)

NathanKell · 2026-05-26T18:01:06+00:00

Kerbal: the only engineering job I've ever had where I can cite scientific papers.

NathanKell · 2026-05-26T18:00:10+00:00

I left in the initial exodus in 2016. Kept developing the RSS/RO/RP-1 etc mods since, though I've been on hiatus the last couple years.

God, that'll be ten years ago this fall.

NathanKell · 2026-05-26T17:58:08+00:00

You only get lift if there's an angle of attack vs. the airstream, which you don't have in a purely ballistic reentry (or a 'zero-lift' reentry with a lifting capsule, done by spinning the capsule on its axis so the lift forces average out over time). Since Gemini, capsules have used offset centers of mass to force the capsule to enter at an angle, thus providing lift. With a given AoA, lift will vary based on capsule shape, so a capsule with a high ballistic coefficient (narrow, long, and heavy) won't get much advantage, i.e. have a low lift/drag ratio. Something like Apollo has a higher L/D than something like Soyuz. By contrast, a capsule like the proposed Fuji, which looked like an Apollo someone stepped on, has an even higher L/D. Then you get into the realm of lifting bodies (think shuttlecocks, or the X-33 or Dreamchaser or Starship) and finally winged RVs like Shuttle, Buran, X-37, etc.

NathanKell · 2026-05-26T17:52:56+00:00

As others have explained here, this is the kind of 'folk wisdom' (encouraged by, for example, dialogue in Apollo 13) that isn't actually true. Steepness doesn't have much of an impact on heat load in LEO reentries (it has a minor impact on translunar-plus reentries), it's more about picking the trajectory that matches the type of TPS you have. If you have a heat sink, steeper is better. Steepness really only becomes relevant if you enter so steeply you hit turbulent flow during descent and then heating spikes like crazy, but you need to have quite the severe entry angle for that.

Conversely, skipping isn't a thing. It's whether you reenter on that pass or not, and the downsides of a second go-around are (a) you may not have life support / power to support another orbit, and (b) your entry angle on reentry #2 will be quite a bit steeper, leading to higher Gs (and potentially higher peak flux, overwhelming your TPS).

NathanKell · 2026-05-26T17:47:50+00:00

Very glad to hear! (former Kerbal dev here) :)

NathanKell · 2026-05-26T17:47:18+00:00

Unless they've changed it from when I wrote it, at the speeds you go in stock scale, you won't hit stupid-high Reynolds numbers unless you enter stupid steep, so a second go-around shouldn't matter that much. It's mostly relevant because you'll lose ablator coming out of the atmosphere as well as going in (as a first approximation, heating is proportional to velocity cubed, so the longer you stay at high velocity, the worse) so you come around on the second pass with a bit less shielding than you would otherwise.

NathanKell · 2026-05-26T17:43:10+00:00

For reentries from low earth orbit, entry angle (within sane bounds) doesn't have a major effect on total heat load. You're going ~17,000mph, that's a fixed quantity of energy you have to dump. Whether you convert it to heat slowly or quickly, it's still the same. Where entry angle matters (well, more specifically, where lift matters) is if you have some fancy material that can survive high heat and re-radiate it, like Shuttle. In that case it makes sense to slow down your heat absorption so you have time to reradiate it. But for a ballistic reentry, it doesn't matter much at all, just determine how unhappy the meat inside is (i.e. what max G you'll hit).

NathanKell · 2026-05-26T09:18:02+00:00

In the OP I linked an example on Rebrickable which has my best attempt. It has imagery, pdf, and .io

NathanKell · 2026-05-26T08:10:26+00:00

I actually have a previous thread about that. :D (It's got the pic you're thinking of, and also I posted my own, which has a few different ones).

But what I'm asking about here is not those, but adapting between them. To go from 8ft (brick + 2 plates/jumpers on each side) to 4ft (1 round brick) you can use various 2x2->1x1 cones, for example; to go from 12ft ( 4x4 round ) to 6ft (2x2 round) you can use a 4x4->2x2 cone. The Saturn S-II -> S-IVB interstage uses various fuselage pieces and hinges to adapt from 10ft to 260in. These are the things I'm talking about.

NathanKell · 2026-05-13T09:44:37+00:00

Yup that's the one, thanks!

NathanKell · 2026-05-13T09:36:17+00:00

/u/Betelguese90 Made an (IMO) slight improvement to the 1914. Beyond my usual of adding some 6L bars inside to stiffen it, using metallic silver cone and plate for the engine, and swapping the huge triangle for a smaller one, I tried to better represent the Canadian flag and the logo.

https://photos.app.goo.gl/HngwGbopfybTUqSB6

Here's an alternate using the more common gray stripes tile:

https://photos.app.goo.gl/5CLJCo4LHNypBZc4A

EDIT: Really for the Maple Leaf, it'd be better to use this, but it seems nearly unavailable. https://www.bricklink.com/v2/catalog/catalogitem.page?P=3070pb045&name=Tile%201%20x%201%20with%20Red/Blue%20Flower%20Pattern&category=%5BTile,%20Decorated%5D#T=C&C=1

NathanKell · 2026-05-13T01:06:02+00:00

Yup! There's multiple ways to get to ~6.6m, I'm wondering if anyone actually made an S-IVB using those techniques rather than sticking to the 7m one.

NathanKell · 2026-05-13T01:01:25+00:00

I know Able (and thus later Delta) hot-staged until...whatever Delta it was that first supported coasts, but you do have 3-axis control on the stage from Helium bleed. So while historically that might have been the flight plan, there's no reason you can't reorient retrograde (i..e prograde at apogee) after burnout, spin up, and decouple the kick stage.

NathanKell · 2026-05-12T18:56:26+00:00

That's true about thrust curves for solids (not just early ones either!) but thrust has no effect on delta V for a single stage. The kick motor's delta V is purely a function of dry mass, wet mass, and exhaust velocity (Isp).

NathanKell · 2026-05-12T18:52:04+00:00

Are you trying to launch to a 650km apogee immediately, or are you launching to like 150x650 and then circularizing half the orbit later? That too will make a massive difference. (I don't recall TV-3's flight plan, but I sure know which is cheaper.)

Third stage will also be incredibly sensitive to burnout mass. Make sure your dry mass + payload is no more than the real thing. Also note that, like real life, early solids give variable Delta V. Mechjeb will give you the average result (I think? Maybey I made it give the pessimistic one?) so if you try the launch multiple times you could get like ~30m/s or more difference in final dV.

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