How to get the Gemini spacecraft back to Earth - Specialist's Guide

Hello everyone and welcome to Reentry. In this video we are going to take a look at how the Gemini spacecraft could return back home with a splashdown in the ocean from its orbital missions. The spacecraft is currently in space traveling around the earth in an elliptical orbit. The orbit got its highest point that's Apogee at 120.5 nautical miles. When the spacecraft passes this point it will then start to fall down towards earth and will hit the lowest points in orbit that's Perigee at 84.2 nautical miles before it will climb back up to Apogee. And this is something that it does now basically for infinity if it haven't been for kind of some orbital occupations. So with this current orbit we will not be able to get back home. To be able to get back home we will need to somehow reduce our orbit so that it intersects the atmosphere or even the surface of earth to get a proper entry angle. And from an earth orbit we want the entry angle to be around two degrees. So if I now go into the spacecraft itself we will start to kind of configure our spacecraft for performing this deorbit burn which is also called the retrograde burn. If I take a look at the external view here you can see that the spacecraft contains a few sections. The topmost section is the spacecraft itself that's the capsule the Gemini capsule. And this is where the two astronauts are staying. It's a pressurized cabin and there's a couple of systems that are able to keep the astronauts alive for long duration missions. Most of that equipment is located in the adapter section that's the lower section here. This contains the fuel cells that's the primary power source as well as oxygen and some other things that are required. And this middle section contains the retrograde engines that's our retro sections. We have adapter retrograde and then we have the capsule itself that's the re-entry module that will re-enter the atmosphere. There's a heat shield where this red piece here is. And then you can see that there's a couple of thrusters on this white section. This is the thrusters that are used to maintain our attitude in orbit. Since we're going to jettison both of these two parts before entering back home we will lose these jets. But you can see that on the top here there's also some jets available for attitude control. So we will need to first of all switch the power source and internal system source into the internal systems as well as enabling the reaction control system. This is called the OEMS and this is the RCS. So basically the first thing that we need to do in order to perform a retrograde is to switch everything to internal systems. So I will first of all plan a retrograde burn at Apogee. That's one hour and 18 minutes away. And there's this orbital cursor that you can move around to just get timing information and then perform maneuvers and planning. So I know that this is the phase. So zero is always perigee. That's your lowest point in orbit. And 180 is our highest point in orbit. The retrograde engines will be able to take us down to a splashdown from this kind of lower earth orbits like this. So I'm going to plan a burn at Apogee. That's 180. So if I now go back into the cockpit there's a tool that allows me to do this. I have a retro Apex Apogee. I could use this or I could select a retrograde point using the orbital cursor. So I'm just going to hit that button and you will see that this doesn't really work. The reason for this is that mission control or ground will need to be able to uplink data to our computer. We'll need to make sure that the computer is running to perform this. Now if you take a look at this light over here I will request the burn and okay yeah so that didn't illuminate. I'll of course fix that. Then the thing that now happened is that ground uplinked a message to us in which is core 19. And I'll go ahead and read that and that's basically a time to retrograde. And using core 19 I can basically see the amount of seconds left before I have planned my retrograde engine ignition. So that basically leaves us with about 1 hour and 15 minutes until the burn. So let's prepare everything that's needed. So first I will go into checklists and I will find the entry section. You have pre-retro, 256 seconds to retro, one minute to retro and post-retro jettison. Those four checklists are the ones that we will go through in this video which will make sure that we are on an entry trajectory ready to hit the atmosphere to perform the entry and landing itself. So I'm going to go ahead and start this checklist and I'm gonna press run. The first thing that it asks me to do is to perform a sequential lights test. So I will make sure that this is set to bright and then there's this switch next to it saying sequential light test. So I'm going to use this to perform this test. The first test is the amber lights. You can see that if I now set this to the test switch to amber all of the amber bulbs are illuminated. And secondly I'll perform the red and green light bulb test and I want to make sure that all of those critical light bulbs that I need mostly during entries such as the sequential things are working. So that's the sequential light test. The next is to reconfigure our fuses. So I want to power the retrograde engines by setting this to arm and then we want to set our event timer to 20 minutes and I'll explain to you why pretty soon. First of all I'm going to hit to set this to stop and make it countdown and then I'm going to increase the minutes to 20. There we go and then I'm going to decrease first the six digits in the end to zero and then the one to zero. Okay we're now set the event time to 20 minutes but I'm going to start it yet and I'm going to explain to you very soon why this is important to do. Then we'll continue to depower stuff. You can configure lighting as you want. You can set the lights using this. I'll keep it in the middle. If you want it into more dark configuration you can also set some lights in the front. You can see that you can also unthread if you want. But I'll leave it to off and then let this light do its main job because I want it to be nice and bright for this video. All right let's set the fans on and then we'll change our radio because during entry you want to use the re-entry antenna instead of the adapter antenna and there's a couple of antennas here and these will be jettisoned as we remove the sections. So remember to change the antenna or you will lose comms. The next thing that we want to do is to perform our battery test. So then I'll set this to BT that's battery test. Just go down here. Let's test these batteries. So to test the battery set this to BT. The voltmeter will now read the voltage of a selected battery and then on the main batteries you can see that it has non-position off and then test. It's important to only keep one of these in test at the time. If not you will break them. So I will first test battery one. You can see that it's now 24 volts. Battery two also 24 volts. Battery three 24 volts and lastly battery four also 24 volts. So everything is fine with the batteries and this is important because currently the main fuel cells are powering the spacecraft but once we jettison the adapter the fuel cells will be gone and we will lose power if we don't switch to internal power. The internal power is mainly these four batteries here so we will then turn these on at a later time. I will continue to depower some more adapter specific things and then we will set up our event timer. Remember that I could use core 19 to see the amount of seconds left to the burn. I can use this to start the event timer and 1200 seconds that's equal to 20 minutes. So 20 minutes before the burn I'm going to start the event timer. So now I'm just using time scale to get to that point. So okay getting close three two one. Okay the event timer is running. The reason for using the event timer here is that this countdown method doesn't really work and it's a little bit hard to see the ignition time here. We want to be able to use the event timer to see the ignition time because it's close to the other instruments and secondly the computer will be off for about 10 minutes during this countdown. So what's important to do is to set the event timer to 20 minutes so that it matches this countdown and then we will turn off this module pretty soon. It will take 10 minutes to load the re-entry module and that's the program that the software or the computer will run during re-entry. This is located on a tape which is this system here. The tape will spend about 10 minutes on playing through this entire program to load it into the memory of the onboard computer. So during this time the computer will be unavailable and this is why it will be off for about 10 minutes. So we have the event timer running and we can now go ahead and change our computer mode to re-entry and notice that the comp light is off and then that the green light on the auxiliary tape system is green. This means that it's playing and while this is playing it means that the computer is loading data. So now I will not be able to use a computer at all so I will now go ahead and time skip for those 10 minutes it takes to load this. Once this extinguishes loading is complete. There we go. That leaves us about 8 minutes for preparing the rest of the things for our retro burn. Go ahead and start the computer. Everything should be now fine and that's the end of this first checklist. Now things are going to be a little bit more hectic so the 256 seconds to retrograde is our next checklist. The reason for this is that there's a variable inside of the computer which is named tr that's the time to retrograde and it will start counting down at 256 seconds. So now I'm going to hit run. I'll leave the checklist open for reference and then I'm going to timescale to about 4 minutes and 16 seconds. That's those 256 seconds. Once you hit tr the sequential lights here will illuminate indicating that the retro sequence is about to start. So now we are going to power our capsule using the internal systems instead of the fuel cells. We are going to enable the proper motor valves for our reaction control system. Remember that the OMS which we are currently using for attitude control will be removed. We are going to check our propellant temperature and pressure of the reaction control system A. We can also check B and we are going to reconfigure a couple of things. First the radio and then we want the needles here to show a specific attitude. Then we are going to send our control mode to rate command re-entry. That's basically using the RCS for rate command. Then we are going to depower the OMS and enable the controller powers for RCS and this is to be able to use our RCS once we enable it. This is what we're going to do next. So first we will press the indicate retro attitude. That will tilt our attitude indicator slightly to more match our retrograde attitude. Then we are going to enable RCS by pressing this button. Now we should be able to maneuver the spacecraft and if I take a look out the window you can see that I can fire the jets and they are working well. So I'm going to set the spacecraft to somewhere like this. I'll just follow the pointers for now. Set the pilots to also show attitude and then let's go through here just to verify everything. One minute to retro checklist clear. Okay the next thing when the countdown reads one minute we will perform a few more steps and this is basically executing the burn itself. So I'm going to hit run here and I'm going to go to that pitch down attitude. This will basically point the retrograde engine slightly up to help us with our current trajectory. 20 seconds. At the one minute mark we will continue to seal the lines going from the adapter section into the spacecraft itself. The re-entry section which we are sitting and then separate it. Okay one minute. Separate the lines that's oxygen and stuff and we are going to separate the power cables. Nothing happened so that means that we are now fully powered. Everything is well so we can go ahead and separate the stage and there she goes. Okay at TR 30 seconds we are going to arm our retrograde rockets. One, two, three, four. And as you can see the four retrograde rockets are now exposed and ready for ignition. At five seconds we are going to arm auto retro and one second after ignition we are going to fire them manually just in case and we are also going to set the event timer to up. Okay I hit the wrong one but up anyways that's fine. And now you can see the current retrograde burn counting up on the delta v indicator. This is basically showing me the current burn vector of the retrograde engine and you can see that I've now burned about 320 feet per second in the retrograde direction. The next step will be now and I will set this one to free and then I'm going to at 45 I'm going to arm this and turn on the squib and perform the separation. Okay start post retro checklist. The retrograde section is now also removed which means that now the heat shield is exposed. This will now let me travel through the atmosphere but this is it is very important to perform this right. So in the next video I'm going to do an entry from this condition but first we will complete our post retro jettison checklist. I can depower the retrogrades the rockets and I will set the platform to the computer. I'll set the sensitivity a little bit higher. I don't need controller B that's the pilot's controller for entry. The commander will perform the entry and I will power down a couple of things no longer needed. Fuel cells turn those off. In terms of the switching and then one important is to arm the landing system. So with the landing system armed we should be ready to start deploying chutes when we start landing. Also take a note of the delta v vector here. So you could do that in a couple of ways. You could use notes here or you can use for example data form blank paper and you can say that delta v of retrograde was 319 on the x-axis and then zero on the y-axis and that's a negative and 10 in the z-axis and that's a positive. Just to store this for later. Okay and then we are going to start the entry checklist. I will do that in the next video because there's quite a lot of ground to cover before that. But for now if I now hit F3 you can see that this is the planned splashdown point of the spacecraft. This is where I'm currently at and I will travel across the entire lower part of Africa before hitting the entry interface which is 400 000 above earth to perform the entry itself. So with that I thank you for watching. If you like these kinds of tutorials please let me know in the comments below. Give me feedback and hit that subscribe button. Thanks a lot!