# Why JAXA's tilted launch from a tilted rooftop?

According to the just posted NASA Spaceflight news item Japanese sounding rocket shoots for record-breaking orbital launch:

Japan conducted another attempt to launch a miniature satellite via a modified SS-520 sounding rocket Saturday, a little over a year after its first attempt failed to achieve orbit. Liftoff, from the Uchinoura Space Centre, occurred at the opening of a ten-minute window at 14:03 local time (05:03 UTC).

1. Why is the rocket launched at such a steep angle?
2. Is it an optical illusion, or was it launched from the top of that titled structure (and if so, why did that need to be tilted as well?)

fyi The original launch is discussed at length in Will JAXA try again to launch TRICOM-1 with the “world's smallest orbital rocket” SS-520-4 again?

• Both the linked NSF article and the linked question make it quite clear it's an optical illusion. Why bother leading with something that's already been answered? – Hobbes Feb 3 '18 at 11:51
• @Hobbes once in a while I simply miss something. I saw the other three images but somehow the one you linked to escaped me. It is possible that it's because it is displayed as a smaller inset i.stack.imgur.com/Zcvmn.jpg compared to the other three images, and when scrolling I mistook it for one of those embedded click-bait advertisements. It certainly wasn't intentional! – uhoh Feb 3 '18 at 15:44
• Reminds me of some of my launches in Kerbal Space Program... – Sean Jul 21 '18 at 22:30

The SS-520 lacks the guidance systems that more complex rockets have. It can detect where it is, and what it's attitude is, but can't make as many adjustments as more complex rockets. In order to make it to orbit, it has to start at a slight angle, to allow a gravity turn. Essentially a launched rocket at a slight angle will tend to point more at the horizon as the launch continues.

For an example of an unguided rocket, see this video of KSP play trying to do an unguided rocket, beyond launch. Note that it launches at a similar angle.

• Because a tilted launch rail is far cheaper than a guidance system and a gimbaled engine and incurs no weight on the rocket itself. A vertically launched, guided version of the SS-520 would carry less payload and cost more. – Russell Borogove Feb 3 '18 at 15:41
• @uhoh If you have particular reason to think so, you should probably write an answer. Rail launch is very common for small rockets, whether hobby model rockets or relatively large sounding rockets; it's an inherently cheap and effective, though somewhat imprecise, way of controlling trajectory. – Russell Borogove Feb 3 '18 at 15:52
• @uhoh In the description of the closely related NL-520: "During the boost stage and first stage burn, NL-520 flies through the atmosphere on an unguided trajectory, like L-4S Japanese first satellite launcher. Stability is provided passively through aerodynamics and aerodynamically induced roll rate (~2Hz)." – Russell Borogove Feb 3 '18 at 17:05
• @RussellBorogove right, the L-4S (Lambda 4S shown in Manley's video). It's seems unusual to put a satellite into Earth orbit without a guidance system on the first one or two stages. Since the lowest stage's fins are already articulated to induce the spin I'm not sure the weight of gimbaling the engine is the reason why there is no guidance. Perhaps this should be made a separate question. I'm having difficulty with the naming of stages. Are "boost", "first" and "second" the names of the first three stages? – uhoh Feb 4 '18 at 2:01
• @uhoh Yes, the NL-520 uses the first and second stages of the SS-520 configuration, but puts an additional stage under those, hence the odd naming. I'm pretty sure the fins aren't articulated; they're fixed with a slight cant to induce the spin. – Russell Borogove Feb 4 '18 at 2:25

From the same article, a better photo of the launch site, showing the rocket on its launch rail next to the building:

The JAXA site doesn't indicate why the rocket wasn't launched vertically. My guess: most rockets start a pitch maneuver pretty much as soon as they've cleared the tower. This rocket is small enough to be launched from a rail, which means it can be launched at a non-vertical attitude, and you can skip the pitch maneuver.

Scott Manley explains a little bit about this in his rocket in his video The Smallest Rocket - The SS-520-5 after about 04:00 where he explains that while the tilt does help guarantee that the spin-stabilized but otherwise not guided first stage starts off going East over the ocean, this does not really result in a gravity turn due to the extremely fast acceleration. There's simply too little time for the turn to happen.

Based on the altitude and down-range numbers and the diagram at about 02:50 in the video, almost all of the turn does seems to happen after 1st stage separation and before 2nd stage ignition, between about 50 and 80 seconds roughly.

Another thing you might have noticed is that this vehicle is placed on it’s launch system at an angle, so it already takes off eastwards, and the natural gravity turn pulls it over a bit, but this thing accelerates so fast it basically pops up above the atmosphere, makes a 90 degree turn, and then burns down-range to get up to orbital velocity.

In that respect it’s basically very much like a lot of Kerbal Space Program launches, where the orbital velocity is so much lower that you tend to pop yourself out of the atmosphere, then circularize, whereas real rockets with lower thrust-to-mass ratios don’t tend to do that.

GIF:

plotting script: https://pastebin.com/CWXGn0ty