Thread by @DuitsyWasHere

1/ Was glad to help with this. Now that it’s out, I can give some explanations. I have the first nine tweets planned out. Then I’ll need to pause and write the rest

This thread might end up rather long. I apologize in advance.

https://twitter.com/joshjonsmith/status/1126722078961364992

2/ Missiles like Russia’s Iskander, China’s M20/DF-12, the US ATACMS, and the new North Korean Missile fly on depressed trajectories, never leaving the atmosphere and maneuvering throughout the whole duration of flight. They are sometimes called “quasi-ballistic” missiles.

3/ Depressed trajectories are used for several reasons:
-Shorter flight time
-Enables maneuvering to avoid defenses and increase precision
-Takes longer for enemy radar to spot it
(These figures do not represent the North Korean missile’s trajectory or range)

4/ How does it enable maneuvering? Staying in the atmosphere means dealing with atmospheric drag. This results in increased airframe heating and deceleration. However, it is also advantageous because it enables the use of aerodynamic control surfaces (steerable fins or wings).

5/ In the vacuum of space, wings and control surfaces don’t work. Spacecraft must expend propellant in reaction control system thrusters in order to maneuver. Propellant has weight, so increasing maneuverability means increasing propellant weight, and reducing payload weight.

6/ Staying in the atmosphere avoids this problem. Even at 50,000 meters, where the atmosphere is very thin, control surfaces are still rather effective, and only require actuation motors and batteries to operate. In these pictures of ATACMS, you can see the fins and motors.

7/ The fin connects to the motor via an actuator shaft. The shaft is the only point of contact between the missile and the fin. This is called an all-moving control surface, because the entire fin moves.

8/ In this picture from yesterday’s North Korean launch, you can see the space between the fin and the missile body, as well as the location of the actuator shaft. The shaft is located in approximately the same position on the fin as in the Russian Iskander (third picture).

9/ (If you think the sheet metal fin on that Iskander looks weird, you’re right; that’s not the real fin. I’ll get to that later.)

10/ Please hold while I write more tweets. In the meantime, here is a video about ATACMS with a narrator that lies and claims that it’s not a ballistic missile.

11/ A few more...
The astute amongst you may have noticed these boxes at the base of the missile. These are the housings for the thrust vane actuators.

12/ Thrust (jet) vanes are heat-resistant fins protruding into the rocket motor’s exhaust stream. Rotating the vanes changes the direction of the exhaust, steering the thrust. This is a form of thrust vector control. Some missiles (Scud, V-2, etc) rely on jet vanes exclusively.

13/ The Iskander and DF-12 also use thrust vanes. In these missiles, the vanes are used to help control the missile during boost, especially during the first seconds, while the missile accelerates away from the launcher. After main engine cut off, the vanes are useless.

14/ UPDATE: I might have made a mistake about the effectiveness of control surfaces at high altitudes. Working through it with @GeorgeWHerbert. Any advice is welcomed.

Apologies in advance if I was wrong, I wasn't trying to mislead anyone.