2534: Retractable Rocket
Title text: Hard to believe that for so many years once they were fully extended we just let them tip over.
This comic documents another of Beret Guy's absurdist ventures. He explains to Megan that "we" (possibly his company) are testing their new "retractable" rocket.
Reusable rockets are a growing industry, as they are more economically viable in the long run – though technically much more difficult to operate – than rocket boosters that are just discarded after use (which have been standard throughout the majority of space-faring history). Thus, Megan is understandably confused about Beret Guy's assertion that theirs is "retractable", asking if he misspoke. In typical fashion, he assures her that he did not misspeak, with a single "No" without further explanation.
They proceed to watch the rocket "launch", proving that it is indeed retractable. In fact the rocket does not launch, but merely extends – apparently all the way to the International Space Station (ISS), a height of over 400 km (over 250 miles) – before retracting, as promised, to its original position. The top part, with the astronauts in it, has been left in space. Presumably, it is docked to the ISS, as the crew onboard the ISS say hello to them in panel 4.
Of course, it would not be possible to extend anything this far. The top would need to be moving very fast compared to the bottom part, or it would bend westwards and break, and even with the strongest material a fully extended, very thin, presumably, hollow structure with a payload on top would buckle very soon after extension began. Also, the ISS moves at 27,600 km/h (17,100 mph) compared to the ground under it, making an orbit in about one and a half hours. So making the tip follow this long enough to dock would be even more impossible.
Beret Guy's retractable rocket has more than a few similarities to a space elevator which has been discussed in real life. The chief difference is, a space elevator is only extended once (and most likely this would be down from space, not extended upwards), and never retracted unless it needs to be dismantled. Randall has referenced space elevators in 697: Tensile vs. Shear Strength. A more similar theoretical means to attain orbit is that of the space fountain. He has also examined the problems of a solid metal object extending through the atmosphere in a what-if.
The current method of sending rockets into space requires huge amounts of fuel, and the more fuel you attempt to carry, the heavier the rocket, leading to more fuel being required, etc. (Tsiolkovsky rocket equation), which makes the current method inefficient. Alternate methods are being explored, such as using a slingshot (SpinLaunch had a successful test flight of a smaller scale launcher just days before this comic was published, probably the influence for this comic), theoretical space elevators, or this comic's impossible retractable rocket idea, all of which would leave the majority of the "fuel" requirements on Earth or elsewhere rather than having to carry heavy fuel with the rocket. The only fuel carried might be minimal amounts for course adjustments once in space rather than large amounts used to get there. However, many of these methods are less flexible than rockets; the space elevator, for instance, operates on the basis of constant angular velocity relative to the Earth's axis of rotation, meaning that it cannot launch payloads directly into low-earth orbit, polar orbits, or many other orbits frequently used by satellites for their desirable characteristics, and satellites intended for these orbits might still need to carry considerable amounts of fuel, even if less than that required to launch directly from the ground.
The title text parodies the 'old' single-use boosters. It appears that the predecessors to the 'retractable rockets' were capable of controlled extension only. Once they had lofted the payload to orbit, they were then allowed to fall over, destroying them in the process so they could not be used again just like booster rockets. However, if a 250 mile/400 km high construction just fell over, it would be much more difficult to avoid other damage, than to the rocket (booster), than for just a few small booster rockets falling out of the sky.
This comic was released four days before (and possibly refers to) SpaceX's Crew-3 mission to send astronauts to ISS with a reusable rocket on 31 October 2021.
- [Beret Guy and Megan is talking. Behind them near the horizon is a tall rocket on a launchpad.]
- Beret Guy: We're testing our new retractable rocket.
- Megan: You mean reusable?
- Beret Guy: No.
- [A zoom in on the launchpad and rocket. It has the appearance of having a long first stage, a second stage with slightly wider fairing and an Apollo-style capsule with escape-tower atop it all. There is a directionless speech-bubble at the top depicting a count down voice.]
- Count down: Three...Two...One...Liftoff!
- [Same view as before, but while the base of the rocket-stack remains stationary, the first stage is apparently elongated, with a hint of a bend to the right, to raise the total height to which the upper-stage and capsule assembly reaches almost to the top of the panel.]
- [In a wider panel, with the base to the left, the first stage is now elongated far enough to disappear off the top of the center of the frame, thus clearly bending to the right. Two peoples voices are indicated as coming from the space capsule far above, as it reaches it destination.]
- Voice 1: Hi, welcome to the ISS!
- Voice 2: Hello!
- [The final panel shows the same view as in the third panel. The first stage is now retracting, and has similar length as in the third panel, but the capsule is no longer atop the 'second stage' fairing. Four movement lines above the top of the retracting rocket indicates that it is returning back to the original position.]
- The original comic misspelled "retractable" as "retractible". Has been documented on the web archive.
- This was done both in the comic itself, and the title.
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However it is done, note that the base of the 'first stage' extends/retracts in advance of the bit further up, as can be seen by the panelling position in the standing/extending/extended/retracted frame-sequence. Which might be worth noting if you're wanting to copy the technology. 220.127.116.11 03:09, 28 October 2021 (UTC)
- I think this might be theoretically possible, ignoring the apparently topological outer skin of the "rocket", with some kind of steerable space fountain. Accelerating the astronauts at 10g for about two minutes might be survivable, and could limit the overall length to just over 800 miles. Though the shroud only has to extend to where the atmosphere is "sufficiently" thin. No idea whether this kind of design would tend to fall over when finished or not. -- Ken g6 (talk) 04:24, 28 October 2021 (UTC)
- I believe it is 100% impossible. --Kynde (talk) 07:45, 28 October 2021 (UTC)
- Only things which are 100% impossible are the ones mathematically proven so. All physical laws - including basic stuff like thermodynamic laws - are theories based on statistics and confirmed by statistics, but just because something didn't happened since the beginning of universe doesn't mean it can't happen. For example, even assuming no proton in whole universe ever decayed doesn't mean it's impossible: there are indeed theories that protons decay with half-life about 1031 to 1036 years. -- Hkmaly (talk) 22:37, 28 October 2021 (UTC)
- I believe it is 100% impossible. --Kynde (talk) 07:45, 28 October 2021 (UTC)
If (as it appears) the Bot created the page with various references to "Retractible" rarher than "Retractable", does this mean Randall made that error initially? (I say this as the guy who failed to notice he'd put "resuable" in his Transcript edit. ;) ) 18.104.22.168 03:54, 28 October 2021 (UTC)
- Looks like yep, Randall originally made that error persistently, as the image here still has "retractible" in the comic, but the image on xkcd.com has been updated. Not sure how this is usually handled, I think it's mentioned in the FAQ.
- Edit: okay, I've uploaded the new image, and added a trivia bit about the original. No idea how to update the page URL, that'll have to be someone else. Esogalt (talk) 04:02, 28 October 2021 (UTC)
In my own opinion I think this is actually a brilliant idea, if you think outside the box. Imagine those floppy noodle blow up baloons they use at car dealerships and the like. Now replace the continuous membrane with a membrane combined with multiple stages of internal baffles that can collapse and move around vertically, and scale the whole thing up. Now think about a straw. What if we engineered a straw that reaches all the way to the target (and then back into the atmosphere), and inflate it? we could have  the ThothX tower but that is a stacked kevlar cell system that only reaches 12 miles in height. The weight is not truly supported by the air, but rather by the tensile strength of the membranes. The question is how much air we'd need to move and how big would it need to be to function. 22.214.171.124 04:45, 28 October 2021 (UTC)
Is there a compelling reason to refer to the countdown as spoken by "Tannoy" rather than "PA" or "Loudspeaker" or something? I had to look up what that was. Esogalt (talk) 05:09, 28 October 2021 (UTC)
- I have correct to a count down voice. We have no idea how it emanates. --Kynde (talk) 07:44, 28 October 2021 (UTC)
I tried to update the page and image URLs to fix the misspelling, and I think it mostly worked. But if you click the "Next" button from the previous comic it doesn't work so smoothly. Anyone know how to fix that?? Orion205 (talk) 05:40, 28 October 2021 (UTC)
Erm, this seems to have broken the main page as well. I've found [], which says to use the "Move" option in the "More" menu at the top of the page. Did you use that?Never mind, I think I was just the victim of cache weirdness. Esogalt (talk) 05:51, 28 October 2021 (UTC)
As a rule, when you’re correcting someone else’s mistake, proofread your correction, or you might make a new mistake that will leave “it is” mark on explainxkcd. 126.96.36.199 09:13, 28 October 2021 (UTC)
When I first read this comic, I thought it was a KSP reference. I feel like half the rockets I build do that if I don’t add struts. I realized in a second it was extending and not just noodling but it made me laugh, and then I laughed again after understanding the real joke. I got two jokes out of one. --BlackBeret (talk) 11:36, 28 October 2021 (UTC)
Can't believe nobody connected this with SpinLaunch, who had a successful slingshot of a "rocket" days earlier. Added it to the explanation. -boB (talk) 22:32, 22 November 2021 (UTC)
0% Penis Joke?
It's hard for me to believe this is in no way a penis joke; particularly with the falling over rather than gracefully retracting, afterward. Is everyone truly certain there's not a penis joke intended, here?
ProphetZarquon (talk) 14:06, 28 October 2021 (UTC)
- I don\'t think so. --GcGYSF(asterisk)P(vertical line)e (talk) 18:29, 28 October 2021 (UTC)
- Me neither. randall hasn't done anything even remotely like that since over a thousand comics ago. kinda sad, really 188.8.131.52 19:58, 28 October 2021 (UTC)Bumpf
- How was the joke ... "Who can tell me which organ of the human body expands to 10 times its usual size when stimulated?" ... ... iris of the human eye. -- Hkmaly (talk) 22:18, 28 October 2021 (UTC)
Of course it's (also) a penis joke! https://www.theguardian.com/uk-news/2021/jun/11/digested-week-should-we-tell-jeff-bezos-his-rocket-looks-like-a-penis