 Subunit 6.2, trade study examples. Okay, let's go through a very complex trade study. So a lot of trade studies you might do would be very simple, right? You're trying to pick between two different things that you could use to perform a function, say, two different types of gyroscopes, and two different companies build them, and you want to make a selection, you do a trade study. Well, that's a fairly straightforward trade study. Let's look at one that has a lot more subtleties and layers to it and kind of bring out some of the things about how trade study is done. So what you see here is some folks walking around on Mars, and this has been a goal of NASA and many other space agencies for a lot of years, right? So robots have gone to Mars, some have gotten into orbit, some landed on the surface, some have even roved on the surface at this point. They've always gone on one-way trips. One's never come back and brought samples back or anything else. And the idea of taking humans to Mars, once you add humans, they're kind of a consumable nightmare, right? You have to bring air for them to breathe, water for them to drink, food for them to eat. You've got to keep them alive. It's very difficult compared to a robot, which doesn't require much of that at all. So getting humans to Mars and bringing them back is quite a challenge. You think about in the history of spaceflight, people have only traveled to the Moon. That's only about a quarter of a million miles away. So going off to somewhere that's 50 or more million miles away from Earth and bringing and crew with you and then bringing them back. That's going to be quite a challenge. So those trips to the Moon back in Apollo took on the order of a week to two weeks to complete. That's from, like, going from Earth off to the Moon and come back. Missions to Mars are going to take years for a crew to accomplish from the time they leave Earth until the time they return. So it's going to be a pretty big challenge and that's why it wasn't like it was the next step and then all of a sudden at the end of Apollo people were headed to Mars. It's a much bigger step. And so we're going to talk a little bit about how NASA has gone through to try to figure out how do you do this kind of mission where you send humans to Mars and bring them back? How do you do that as optimally as possible? Because it's going to be very expensive. It's going to take a lot of massive equipment. And so we want to try to optimize this mission as much as we can to try to make it affordable so that someday we can actually execute this mission. So we're going to talk about this trade in a little bit of detail. So the first thing you see on this chart here of the orbital mechanics or the two planets going around the Sun is that you notice they don't go around at the same rate and so there's a lot of orbital mechanics equations you could use to look at this Kepler's laws of motions and things like that. But the basic is that the one closer to the Sun, the Earth, is going around in one year. It takes a year to go around the Sun, 365 days. Mars, much further away from the Sun, is taking almost twice as long to go around the Sun. So you think about that. As you think about the two traveling around the Sun, there are very few times when Mars and Earth are anywhere near each other. Now you can see here that they could be only 55 million kilometers from each other if they were on the same side of the Sun near each other. There could be other cases where they're on opposite sides of the Sun and it's almost 400 million kilometers. And so there's a lot of distances to be traveled here to get humans to Mars and bring them back. And so the other thing is that because of this fact that one is going faster around the Sun than the other, the times when they line up close to each other on the same side of the Sun only come about every few years. So every few years there'll be an opportunity where Mars and Earth are on the same side of the Sun. And that's the primary time when you'd like to take the crew and get them there because it's going to be the shortest path. And so when you look at that orbital mechanics and you look at the alignment of these two planets as they go around the Sun and see, well, what's the easiest way to get them there and back? If you go to the next chart, what you see is that it breaks down into the two different kinds of missions that you could execute and they're very different. So let's take the one on the left first, the short stay option. So in this mission scenario, you wait for the two planets to be on the same side of the Sun. You launch to Mars from Earth, take the shortest path as you can. You spend only about a month or a little bit more than a month on the surface and then you blast off from Mars and you try to get back to Earth. Now Earth and Mars have moved since you got there. So now it's going to be a little bit of a difficult journey back. And as you can see here, you actually, if you follow the path back from Mars you actually swing in to that green planet there shown on the chart, Venus. You're actually going to fly all the way past Earth's orbit, all the way to Venus. And I'm not going to go into the orbital mechanics, but let's just say this is the cheapest way to get back from a fuel standpoint is to fly all the way into Venus, let Venus use its gravity to slingshot you around and get you back to Earth. And so that sounds like a complicated mission, right? So what happens here is the mission takes about 650 days. So not quite two years, but it's a quite a long mission. And on that mission, most of your time is being spent going to Mars and coming back from Mars. Very little time is spent on the surface. And so that's the short stay mission. So in our trade study, that's going to be one option. It's a short stay option. The other option is long stay. And so here, again, you look at the planets going around the sun, when they're on the same side of the sun, you launch and go to Mars. Then you sit on Mars for well over a year and you wait for the planets to get back on the same side of the sun and then you drive yourself back to Earth. And if you look at that plot there, that looks a lot more straightforward but you spent a long time on the surface of Mars, right? So you spend these shows here 496 days on the surface. So these two trade, when you do this trade study, these two options don't look a lot alike at all, right? I mean, one is you spend most of your time in space and zero gravity in a spaceship, a little bit of time on the surface a month, and then you scurry back home. The other option, you get to Mars and you just sit there, wait for the planets to come around in a line again, and then you drive yourself back. Is it safer to be on the surface for a long time or safer to be in transit? Is it more fuel economical to be on a short stay mission or fuel economical to be on a long stay option? What are my figures of merit, my measures of effectiveness, my measures of performance that I want to use to evaluate which one of these two is the way I would like to go? So that's what's on this next chart here. I'm not going to go through them all, but these are all the different figures of merit or again, as we talked about before, measures of effectiveness or measures of performance that you might use to be able to evaluate which one of these options is the way I'd like to make my decision. So you go through things like total mass in low-earth orbit. It's a really easy thing to look at, the amount of mass of a ship, and you start to think about things like how complicated will it be. If it's really massive, it's going to be complicated to get it up in the space. It may be complicated because you have to dock a lot of pieces together. It's also sometimes a corollary to say how much will it cost because a big spaceship is going to probably cost a lot more than a little one. So if I can say that one of these is going to be a much smaller spaceship, I might also be able to say it's probably going to be a less expensive mission to be able to develop the system for. So total mass in low-earth orbit is a key measure that I'm going to want to evaluate for both of these options to see which one of them will provide me a better answer for mass in low-earth orbit. So you can walk down the slide here. There's things like crew exposure to radiation. Which one of them will put the crew in a more risk of radiation exposure? Some of these are going to be discrete and defined things like mass. How much mass for option A? How much mass for option B? Some of them are going to be a lot more perception, like how safe is option A versus option B for the crew. There might not be a way to calculate that, but it might be more of a sense of complexity or exposure to radiation or other risks that you might say, well, this one sounds more risky than this one does. And so there could be some parameters in your figures of merit that are judgmental versus just discrete answers. So the other thing to think about is that all these figures of merit are in this case kind of being treated equally. In a lot of cases you might give them maybe values against each other, like a weighting, right? You might decide, hey, what's most important to me is how much mass I have in low-earth orbit or what's most important to me is the cost through the first mission. And so if you take some of those figures of merit and say, well, when I do my evaluation, if one of these options is much better in those areas, that's going to be the dominant part of my decision. And some of these other areas might be just a very small part of my decision-making process. So you could put a weight against each one of these, some kind of a measure of how important that criteria is in your final decision. But in this case, we're going to treat them all equally. We're going to say that all of these are important figures and we want to evaluate each one of these options against all these different figures. So we're only going to talk about a few, but this is just to give you an idea of how you would develop a table of figures of merit to be able to do that evaluation. One of the things we're going to look at here in this trade study is how much delta V, very technical term, right? Change in velocity is going to be required from the beginning when you're sitting on the ground ready to take off to go to Mars all the way until you return back to Earth. Think of what is delta V, change in velocity, what does all that mean? Well, we have to think about how much fuel we're going to have to carry. Mars is a long way away, it's going to take a lot of propellant to get there. Well, we want to know is a short-stay mission or a long-stay mission going to be better when it comes to propellant. It's going to take less fuel. So one of the easiest ways, we don't know what kind of rockets we're going to use yet when we do these trade studies sometimes. So we're at a very high level. So first if we know how much acceleration is required to go from Earth to Mars and how much acceleration to go from Mars to Earth, that's going to help us understand how much fuel. It's kind of like you're going to leave your house and go to the store, right? You're going to get in the car, you're going to accelerate, get out on the highway, you're going to decelerate when you get to the grocery store. On the way back, you're going to do the same thing. Now, in a car, you've got to keep your foot down on the accelerator burning gas because you have road resistance, you have air resistance, but think about this in space, right? You leave Earth, you give yourself a push to accelerate with some rocket engines. Once you've accelerated to the right speed, you just coast all the way to Mars. There's nothing to slow you down, but unlike on Earth, you can't just step on the brakes. When you get there, you have to actually turn around, fire the rocket the other way to slow down enough that you can go into orbit or you just go flying past Mars. And so, same thing is going to happen on the way back. And so, that change in velocity or acceleration that's required at the beginning to get going, to stop at Mars, the deceleration, and then the acceleration to get going and then the deceleration to get yourself back to Earth, those are all going to take fuel. And so, this is an easy way to look at the amount of fuel that might be required. Even before you know what kind of rockets you're going to use is which one of these options is going to require more change in velocity from the beginning of the mission to the end of the mission. So, what you see here is up on the top side, you see the long-stay missions. And so, you see the amount of fuel required to go to Mars on the bottom. You see the amount of fuel required to come back from Mars on the top. And then you see that one other bar that says mid-course corrections or whatever deep space corrections. It's just a time when you might need to steer a little bit along the way and so you need to fire some rockets to steer yourself to make sure you get to your destination. So, all of those fuel requirements add up. And so, you can see that the bar here in delta V or kilometers per second of acceleration on the y-axis just says that for all these different Mars missions that are long-stay in nature where you're going to have a long over a year on the surface, these different opportunities. And remember, we talked about that only every couple of years do the Earth and Mars line up where you can actually go to Mars. So, that's why you see a bar in 20, you know, 2031 or so, 2033, is that there's not an opportunity to leave for Mars whenever you want to. There are specific times. And this repeating cycle where they're close to each other, they're not always in the exact same position relative to each other. So, sometimes you're going to need a little bit more gas. Sometimes you're going to need a little less gas depending on which specific opportunity we're talking about. Then scientists have predicted for many, many years out into the future where the planets will be aligned on any one of these given opportunities. So, you can do this analysis of how much change in acceleration would be required, for any one of these specific opportunities. So, what you can see here is for all the long stay missions, the amount of acceleration is somewhere in the, what, 6 to 8 kilometers per second. That's all of the changes that will have to happen throughout the mission, all added up together, right? Well, and you can see that from opportunity to opportunity, there's not a big change. Although, when you get out here in the 2040s, there's a little bit of a hump. And so, you've got to think about that. If I'm going to design a system to go to Mars, to take crew to Mars, I'm probably not going to design it for one mission and one opportunity. I want to be able to use it again and again and again. So, that's why I'm doing this plot, not just for one opportunity, but I'm looking out across time at many opportunities to try to figure out if there's, you know, sometimes when the acceleration is going to be a lot more than in other times because my fuel tank is going to have to accommodate probably the worst case scenario. Unless I want to say, well, gee, I'm just not going to fly in that one opportunity, but then you're going to have to wait four years to go back to Mars and you might not be willing to do that. So, we've kind of laid it out here. And so, that's the long stay options. And then if you go down to the lower part of the graph, you see the same kind of chart for the short stay options. And again, you see, well, how much change in acceleration does it take to go to Mars for each one of these opportunities? How much does it take to come back? And then how much is required for some steering along the way to do any mid-course corrections? And you can see here now that in these cases, it's eight to, well, the highest one is what? About 15, almost 16 kilometers per second of acceleration required to complete the mission a lot more for each one of these cases. Now, the other thing you'll notice is that when you go from opportunity to opportunity, the amount of acceleration required to complete the mission is very different, right? So, there are some here that are all the way down below eight, a little below eight kilometers per second. But then there are these others that go up to 11, 12, 14, 15 kilometers per second. And so, again, if I want to build a system to be able to go back and forth to Mars and use it again and again or maybe build another copy of it, my fuel tank is going to have to be a lot bigger for some of these opportunities than for others, and I have to kind of think about that. Because if I say, well, I'm just not going to go when there's these parts where maybe the planets are at a line very well, so it's going to take a lot of acceleration to get home, well, gee, that's taking out a number of opportunities. And so I might not only be able to fly to Mars every five or 10 or 15 years if I take out the ones that have the higher requirements. So this was a good part to do. This analysis is really helping me understand what's a better mission to fly should I go with a long stay or a short stay. But this is only from one very limited perspective, and that perspective is change in velocity, which is kind of equivalent to how much fuel I'm going to have to carry. So there's a lot of other things I want to keep in mind, not just how much fuel am I going to have to carry to complete the mission. So the other thing is these different opportunities, again, because of their planetary alignment, it's going to make the mission maybe longer or shorter, depending on which one of those opportunities I fly on. So, again, look at this chart here, and you look in the upper right-hand side, and this time it's mission duration on the y-axis and all those different opportunities to fly to Mars for a long stay mission. And you see the time it's going to take in red to go to Mars, the time in blue that you'll be on the surface, and then the time in beige up at the top is how long it's going to take to get home. And so you can see that from opportunity to opportunity, there's very little variation again. They're all about 900 days in duration, right? So if I'm going to build the system to support a crew for 900 days or so, I could pretty much fly whenever I would want to. I could choose any one of these opportunities. And so that might be one of my big constraints is if some of these options I can only fly at certain times, I can make this a less attractive option for me in my trade study. So the other thing you see noted on this slide is solar activity. Now, the sun goes through a 12-year cycle where it gets kind of quiet, and then it ramps up. There are more solar flares. You hear about that in the news. Things called coronal mass ejections where particles come off the sun. So they can pose threats, both the radiation from the sun and the particles, can pose threats both to the crew on board a spacecraft if it's flying out to Mars or back. And to the systems on board, the computers and things like that. So you kind of like to know on this, where is the time when the sun is going to be more active? And then you could say, well, gee, if I have very long mission durations around that time, that might be really unhealthy for the crew or dangerous for the systems to be flying for long durations during that period of time. So it's just something you want to keep in mind as you're doing this part of the trade study analysis. So if you look at the lower part of the chart, see a very similar plot for the short-state mission. So here you're looking at for short-state missions, again, the time to go from Earth to Mars in red, the time on the surface above that, and then the time to return above that. And again, you're looking to see a couple things. One is, is there a big variation between the length of time from beginning to end of the mission? And you see here there is. And because these options for short-stay, most of your time is being spent in transit between planets, very little time on the surface. Now you do have to worry about that solar activity because the Mars atmosphere will protect you a little bit from radiation when you're on the surface. When you're flying between the two planets, you're exposed to that radiation. And remember, on some of these, all of these short-stay options, you're going to have to fly in towards Venus to get home to Earth. Well, gee, if the sun's very active, I surely don't want to be flying right in its face to be able to get home. So again, this could be a very dangerous kind of situation when I combine the length of the mission that I'm spending in space, the time I'm in space with the fact that the sun is going through active periods every 12 years, that might be a bad combination for short-stay missions. So again, this is only looking at it from one perspective. So to complete this trade study, I'm looking at a number of different things that I can evaluate that I could then apply my measures of effectiveness and measures of performance and be able to say, hmm, which one of these is going to be my better option. So just two things here. Here's some examples of what you look at. Now, another big criteria we talked about before was how much mass has to be in Earth orbit to start the mission. So when I go to leave to go to Mars, and I've got all my stuff in Earth orbit, how heavy is that vehicle? And that's going to give me an indication of how complex it's going to be to build this thing and how expensive this is probably going to be to build. So to do that, you have to have some sense of an architecture. So we've talked about architecture before. Now, early on in the mission planning phases here, we're starting to develop some minimal ops concepts and architectures. And so to do this trade study, you can start to look at what are the vehicles required in this architecture. And that's going to start to allow you to evaluate how much mass is going to be required. We talked about Delta V before. Once you take Delta V and add what kind of propulsion system you're going to use, you can figure out how much propellant you need. So the mass of the vehicle, the mass of the propellant, you could add that together and get kind of a sense of how much mass and orbit is going to be required to start this mission off. And so let's just look at two architectures and we'll look at the short stay first. And the goal in both architectures is to try to keep the architectures as similar as possible and only make changes that are related to the thing you're trading. Remember, we're trading short stay on the surface versus long stay. I'd like to have the rest of the architecture remain as constant as possible so I could do this head-to-head comparison of only one element of the architecture, which is the changing the requirement for a longer stay on the surface. And so what you see in the short stay architecture is I'm going to, working from the left, I have some equipment that I'll only need when I take the crew down to the surface of Mars. Well, I'm going to go ahead and launch that on a big rocket and I'm going to go ahead and send it to Mars orbit. The crew is still on the ground. They're still getting ready for launch. I'm going to go ahead and ship their supplies to Mars orbit. They don't need that stuff on their trip. Then separately, I'm going to launch the vehicle that's going to take them to Mars. And then last, when everything's ready, I'm going to launch the crew on that smaller rocket. They'll dock in space with the vehicle that's going to take them to Mars and then they'll shoot off. When they get to Mars orbit, they'll meet their supplies, which are already there in orbit. They'll dock with their supplies, their landing vehicle, which is kind of represented as the upside down here. So you land on Mars with your lander, you spend your month or so on the surface, and then you have the lander blast you off. You dock in orbit with your return vehicle that's going to take you home and then that takes you all the way home and you splash down in the ocean just like in the Apollo missions of the old days. So this gives you a sense of how much equipment is going to be needed from the beginning to the end of the mission to accomplish a short stay mission. To figure out, at least at a gross level, how much mass is going to be required for this mission in all these different components. So this basic architecture, this basic ops concept is going to really help me drive out my requirements for mass. So you flip to the next one and you look at the long stay mission architecture, it almost looks the same, right? And that's the goal. The goal is to try to keep all the other parameters the same except to say, how about if I stay on the surface for more than a year now instead of just for a month? Well, and the original one in the short stay option, when you land that you're in a small lander. You have a small crew, they're going to live in the lander for a month. Okay, it's not going to be real comfortable, it's going to be a little close quarters, but for a month you can put up with having a neighbor three elbows away and you can survive and get yourself back. But if you're going to be on the surface for more than a year, not only do you need a little bit more room to have more equipment, because they're going to be able to go off and explore greater distances, they've got a lot of time. Maybe they want to bring a buggy with them to drive further away and maybe a camping outfit to go off and be able to camp at different spots on Mars, do further exploration. And they're going to want to have some privacy, so maybe you have to develop some bedroom facilities, some wash facilities, they're going to be on the surface a long time. I don't want to take a shower once in a while, so it's going to have to have a lot more facilities developed on the surface to support that crew. There is the same architecture as we saw before, but what's added all the way over on the left-hand side is you now have another set of vehicles that's going to launch and go directly to the surface with a large habitat module. So that adds a complexity too, because not only do you have to launch more mass to launch this habitat that they can live in while they're on the surface, this bigger habitat, but you notice that it lands on the surface independently. So now when the crew gets there later and they go to the surface, they're going to have to land near that habitat if they don't and they can't get to their house, they're not going to be able to leverage all the supplies and probably get all their food in and everything else. So this second landing, when the crew comes down, they've already got their house sitting on the ground, they've got to make sure they pinpoint land right next to it so they can walk over and live there for a year before they have to come home. So not only does it add more mass to the mission, but it also adds a lot more complexity because on a short-stay mission you have your house with you, you land wherever you land. You kind of know where the scientifically interesting place would be, but if you miss it by a little bit, no big deal. Here if you miss your house by a long way, you don't have a way to get there. It's going to be a very challenging mission. You might have to come home early. So, since we saw that the real driver here is the surface components, right, that there's a big difference between these two architectures, let's look at the surface in a little bit more detail, right? So here you have the lander, which is for the short-stay mission, and the lander which looks very similar for the long-stay. So you're going to, again, try to keep the architecture the same as far as the vehicle that will actually have the propulsion units and everything else to land you and then take you back off up into orbit. But then you have this separate habitat that's landed on the surface separately, and then a habitat's going to have everything from, again, some personal space for the crew, some common areas where they can work together, and then it's also going to have things like medical facilities, because now that you've said, I want to be on the surface for more than a year, well, the chances are going up that somebody's going to get hurt or have some other medical condition while you're on the surface. And so now your surface facility better have some kind of medical care facility, too, to deal with all the kinds of things that might happen to a crew member while you're on the surface for this long period of time. And so, again, you've maybe done enough study of this, and you've kind of thought about what the requirements would be so that you could then make a guess at how big it has to be and how heavy it has to be, what materials it has to be made out of to withstand a year or more on the Mars surface. And so, so that that analysis is just helping you again. You're going into a little bit of detail on what the surface systems are going to look like so you can make your assessment of how you're going to do this. So now the next thing is you've got all this data now on propellant, because you did your delta-v analysis. You've done your surface architecture analysis. You've done your transportation architecture analysis. Now you're ready to compare the mass in orbit at the beginning of the mission for long-stay versus short-stay. And you might be surprised if you go, we'll go to the next chart here and we'll look at two different options. One is you use chemical rockets, which is very similar to how the space shuttle worked. It's very similar to how all rockets work, most of the rockets work today, where they use some chemicals that are mixed together to provide propulsion for the system. Now for missions out to Mars, we've also thought about doing something a little different, which is to use nuclear rockets. The nuclear rockets would have a small nuclear fission reactor inside and you would flow hydrogen over that reactor and you would heat up the hydrogen and expand it, and that would allow that would push you the other direction. So it's a hydrogen fission engine. So that nuclear rocket is another option that's a lot more fuel efficient than current chemical rockets, but it hasn't been developed yet. So it's still a technology that needs to be developed. So if you had that technology you could see over on the left-hand side that how much mass you would need in low-earth orbit to do long-stay missions and short-stay missions is pretty similar. Same thing if you use chemical rockets, pretty similar. So you're not seeing a big differentiation now. You might have thought when you looked at that architecture and you saw the all the ground equipment you needed for the long stay, for that year-long stay on the surface that, jeez, that's probably going to make that a much heavier architecture than the other one. Well, no, not true, because the trip, when you get on Mars and you stay on the surface until the planets come back into alignment and then you come home, that's the lowest delta V. If you went back to that delta V discussion, that's the lowest amount of energy you need to get to Mars and come back from Mars. When you go for only a short time, you're going to need a lot of propellant to get home because you've got to take that shortcut in through Venus to get home. And so that's going to take a lot of propulsion. So what's happening is the advantages of the short stay mission and not having to have more surface architecture is outweighed in a lot of cases by the fact that you need more propellant to execute those missions. So what you see here is in the end the mass in low Earth orbit is really not a good deciding factor between the two because it's pretty similar. And I don't know that that would have been intuitive when you first started doing the trade study. So we'll finish up this set of options in this trade study by looking at, not going to go through again all of them, but this is what the team at NASA came up with for a comparison of short stay versus long stay missions. And you can see they've highlighted in green areas where they thought there was a significant advantage to one or the other. And so when it comes to things like the total mass in low Earth orbit they said, well gee, it's pretty similar. So I don't know that I would make a decision on one or the other based on that. But there are things like the complexity and the size of some of the vehicles because of the changes in propulsion. They said, boy, you know, I think the long stay the actual vehicles taking you back and forth would be a lot simpler to build. There's a lot less propellant, there's a lot a smaller vehicle would be adequate. And so there's actually a big advantage to the transportation part of this by going with long stay missions. So things like expected useful crew from a sole being a Mars day it's about 24 and a half hours a little longer than an Earth day but so the time it takes to spin on its axis. And so, you know, so here you said, well, gee, of course, I'm going to be on the surface for way over a year. I have a lot more productive time where the crew could be out doing scientific research. So from a scientist standpoint I'd say, ooh, boy, you know, long stay sounds great to me because I can get a lot more research done. So you can kind of go through all of these different factors here and just see that in the end this set of trades and in this option analysis, it looks like the long surface stay missions might be the best ones. But now let's stand back away from that for a second. So these tools are always a way to help in decision making, right? So it gives you a lot of information to help make a decision. You saw that you do a lot of very detailed technical analyses to help drive this decision process. But in the end you have to use kind of human thought too is that maybe in the end it's better to go with a short stay mission because I'm a little worried about the crew being on a 900 day mission away from Earth and all of the risks that can come up with a long mission like that. And so maybe overriding all of this is the fact that I just don't want to have the crew be away from Earth for that long. So that's the thing about this is it really tries to drive out what are the differences between the two options and a trade study tries to then give you as a decision maker the information to help make a decision. But in the end it's really never a perfect analysis it's always something that you're going to have to add a little bit of your own analysis on top of that to say which one do I think is really the way to go in this trade. There are no additional resources for this subunit.