How To Do A Hoverslam - Things Kerbal Space Program Doesn't Teach.

hallo its scott manley here and today i would like to talk to you about the suicide burn now the suicide burn is the most Kerbal of maneuvers it's basically waiting until the very last minute to fire your engines and slow down to a stop it's kind of like driving towards a parking spot at 100 miles an hour slamming on the brakes and ending up inside it except in this case the brakes are leaking fluid and will only run for a few seconds and if you end up outside that parking spot there's an overzealous cop that all come over and tease you or something look the suicide part of this is because if you are late you are dead if you have an engine problem there isn't no time to correct it but even if you're early that can leave you running out of fuel just above the landing pad and then falling to the ground with great you know problems but basically not much room for error with those suicide burn now the Falcon 9 booster rockets when they land they use this type of approach but for PR reasons SpaceX prefer to call it the hover slam which is a bit of a lie since a near-empty Falcon 9 actually has too much thrust to hover on even a single engine at minimum thrust without the ability to hover the rocket can only slow down and if it starts to burn too early then the rocket could come to a stop while still in the air and at that point it either starts getting higher or shuts down the engine and falls onto the pad of course this maneuver is popular in Kerbal space program because well rocket engines and Kerbal space program are miraculous they can throttle smoothly from zero to 100% power that is something a real rocket engine cannot do but in theory it's the most efficient way of landing a rocket because it's the fastest way of slowing down and every second spent using engines against the force of gravity is wasting delta-v equal to the force of gravity if you've got the right mods they can actually do the math for you until you went to fire your engines to stop in time but I'm actually going to show you them because I'm a sick individual that likes mirth so say we have a rocket falling straight down towards the surface was a known thrust to mass ratio then we can figure out the acceleration in the absence of gravity but since the rocket is falling under the force of gravity we simply subtract the acceleration due to gravity and get the net acceleration due to the braking so taking that vessel speed and then dividing it by the net acceleration gives you the time that it takes to reach zero velocity so what we can do is draw a graph of a velocity versus time and here's one showing three different Landers with different accelerations slowing down from 200 meters per second now the distance each rocket is needing to stop is calculated from the area under the line and that's simply a triangle with the height equal to the starting velocity and the baseline equal to the deceleration time or 1/2 V squared over a of course you can do this with calculus but it's easier to illustrate using a graph so for different base accelerations you can now figure out the stopping distance from 200 meters per second and unsurprisingly this grows rapidly as a thrust drops lore but more importantly the lower the acceleration the longer those engines have to burn against the force of gravity and the more fuel is needed and one way if the Falcon 9 has to increase the thrust is to just use more engines so some booster landing attempts have been using three engines so somewhat unintuitive ly using more engines for a landing burn actually means less fuel used to decelerate the rocket to zero indeed the first time I heard about a landing attempt using three engines was the SES 9 launch which was right at the upper limit of the Rockets capabilities and that failed so hard that it knocked a hole in the deck of the landing barge the blast also destroyed the camera so the only video we have is from what is presumably considered a safe distance the downside of those high thrust landings is it the margin for error gets smaller and while we know computers can turn on a rocket at exactly the right moment it turns out that rocket engines or mechanical devices and they can take a moment to get started and ramp up to thrust and stabilize and it isn't always exactly as predicted nevertheless SpaceX has continued testing and about a week before the Falcon Heavy launch they launched a spacecraft called gov sat on an old block three pre used booster and according to Elon Musk it was testing a high retro thrusting it wasn't expected to survive and they weren't gonna risk the drone ship for a booster they plan to throw away anyway and yet the booster stuck the landing like a boss making a soft landing in the Atlantic Ocean and a few hours later pictures of the booster floating were also shared Elon had originally suggested that this booster could be towed home for research purposes but a few days later news leaked out that the booster had been scuttled instead since there was no safe way for recovery crews to depressurize the propellant systems to make it safe the initial rumors suggested that the US Air Force had tasked a combat aircraft to Sciuto which would have been pretty cool since there's not many aircraft that can claim to have killed a spaceship the later confirmation was far less romantic involving a marine demolitions contractor which still sounds like a pretty cool if dangerous job however during the Falcon Heavy booster landings we got to see a balanced approach with the initial deceleration burn lighting a single engine and then bringing three engines on lane for a few seconds of super high braking and then the final landing burn using a lower thrust of a single engine again a landing on a single engine is easier because the thrust is lower so there's some benefits from the increased thrust early on in improving the fuel margins and there's also the benefits of a more controllable landing however the third booster out at sea didn't manage to land since it ran out of the pyrophoric ignition fluid used to light the engines it had enough fuel but with only one engine lit it didn't have enough thrust to slow down in time which is kind of a perfect example of why Kerbal players call this maneuver a suicide burn because if anything goes wrong there is no time to fix it that's why the boosters that are heading for a barge landing actually aim for the sea next to the barge until the landing burn starts and everything is going according to plan only then does the flight computer skew the trajectory sideways onto the target ship of course all this is oversimplifying many things I've used a fixed landing speed whereas in reality when you're falling through a vacuum the velocity will be rising whereas if you're falling through the atmosphere you might be slowing down due to drag and that atmospheric drag force needs to be incorporated into the deceleration calculation early on the drag will be higher and later on the drag will disappear so you need to account for that also real rocket engines will simply show variations in their thrust and specific impulse as they air pressure changes and that air pressure will actually include the dynamic effects of reversing and hundreds of kilometers per hour finally the acceleration of the rocket will just increase over time as fuel mass is burned and the rocket gets lighter and that's of course one of the reasons why things like flight engineer and Meg jab will get their suicide burn calculations wrong so even although you're given a number in using these mods and Kerbal space program you still need to act like a pilot and actually adjust the controls as you're descending to make sure you land and exactly the right time so yeah despite their scary name suicide burns are actually a legitimate thing in rocket science that makes complete sense I'm Scott Manley fly safe [Music] [Music]

Channel: Scott Manley

Views: 1,014,419

Rating: 4.9442229 out of 5

Keywords: kerbal space program, rocket, spacex

Id: T3_Voh7NgDE

Length: 8min 50sec (530 seconds)

Published: Mon Feb 19 2018

Reddit Comments

[u/Dahbakon]

I've attempted the suicide burn lots of times in ksp so it was nice to know that I'm emulating a real life space contractor.

[u/selfish_meme]

I watched this expecting to learn how to do a Hoverslam. I was entertained but did not learn how to do a hoverslam.

Edit: Obviously my slightly tongue in cheek tiny critisicm of the title of the post has missed the mark. Be aware I have already been OCD'd and am suitably chastised.

[u/BreezyWrigley]

i feel like ksp totally demonstrates the complex nature of this maneuver. im not sure I agree with the title.