 Subunit 6.3, trade tree. So let's talk a little bit about trade trees. So trade studies are a critical part of doing analysis to come up with selections for different components, subsystems of a spacecraft. Sometimes there are many trades required to get to a system decision. And so we're going to use a very complex example here. I'm going to try to walk through one path and kind of simplify it a little bit. But the idea is that when you have a lot of trades to do, a lot of studies are different options at different levels of complexity. You might want to organize them into a trade tree that allows you to maybe work on the hardest ones first. The ones that maybe have the biggest implication on the mission and then work your way down through a series of other ones that have some level of implication. But maybe they're almost in a priority order as far as maybe one would take a lot more mass to accomplish, much bigger satellite. So you maybe want to make that decision about whether you're going to go with a smaller satellite or bigger satellite for the mission right up front before you even go down to the more subtle different trades that you want to do on some component. So let's look at this example on the chart of the human spaceflight to Mars. So NASA has been studying this forever. We've done the Mars studies since the 1960s. And so what you see here is it goes through and looks at for a lot of the studies we've done, a trade tree where the most difficult or let's say difficult, the decisions that have the biggest implication on the mission are made first. So in the previous subunit we talked about the Mars mission with humans going to Mars. And we talked about a trade study that said should they stay on the surface for a short time or a long time. That trade is going to be done first. And in this case you can see that the decision was made in this analysis to pick the long surface stay. So the crew on the surface for well over a year with the shorter trip time back and forth to Mars. So again that was the first level decision. Now another set of trades that had to be done was do I want to send the cargo to Mars with the crew or do I want the cargo to go by itself? Why would you want to have the crew waiting around for the cargo? Why would you want to build a vehicle big enough to hold both the crew and the cargo, big station wagon or camper? Why not go ahead and take the crew and separate them out? They can launch to Mars once they know that all their cargo is safely at the planet. And so in this idea here of pre-deploy or all up, the next level of decision making, the trade that has to be done is what's better? To take the crew and cargo together on one big mission and send them out to Mars or to pre-deploy all the cargo? Maybe you've landed the habitat on the surface. They know that their house is going to be there when they land and it has all their food and all their supplies ready for them. Maybe you've even pre-deployed the return capsule that they will come home in and put it into Mars orbit for them so they don't have to carry it with them. This will allow the crew to take a minimal small size vehicle and get to Mars quickly with a very small vehicle as opposed to having to haul all that cargo with them. But now you look at the other side of that trade. Okay, how about if something goes wrong and they can't dock with the vehicle in orbit or they don't get to the surface? All their food for a year is down on the surface. Now they have to carry with them maybe extra supplies just in case. So the trade is very complex. There's a lot of different aspects that have to be looked at. Again, crew safety, which one is better from a propulsion standpoint? There's a lot of studies to be done. You could take the cargo on a very slow ride to Mars using an electric propulsion system. It could take well over a year and a half, two years to get there. You don't care, but you'll wait until it's there and then send the crew maybe with a chemical propulsion system that can get them there much quicker. So that may allow you to do some other trades to look at propulsive systems and how maybe you can save on fuel for the cargo mission. So once you've made that decision and here the decision was to go with pre-deploying all the cargo at Mars before the crew sets off on their journey, you've got other decisions to make. When the cargo and crew arrive at the planet, will they use the Mars atmosphere to slow themselves down? Arrow capture or arrow breaking, where they use the friction of the upper atmosphere to slow themselves down and put themselves into orbit? Or will they turn around, fire a rocket the other direction and use propulsive energy to put themselves into orbit? And so again, there's pros and cons of the vehicle design. Having to build something that could take a very large capsule, many tons and be able to slow that down by the friction of the atmosphere, the loads, the stresses on the vehicle, the heating of the vehicle, that has to be all taken into consideration how big that vehicle will have to be versus if I have to carry all the fuel with me to slow down, to create the delta V or change in velocity required to stop myself from flying past Mars but put myself into orbit around Mars. So that trade has to look at the differences in design and the differences in the mass required because for going to Mars one of the key things you want to know is in each one of these decisions how much mass is required because these are going to be really big vehicles that have to be launched off the surface of the Earth. I want to try to minimize that mass in low Earth orbit and a lot of times when we do these trade studies we have certain design factors or certain critical things that we want to keep in our decision making all the time. And for a humans to Mars mission, something like the amount of mass that will be in low Earth orbit when you start the mission, that's one of those key design factors. So in each trade that's one of the things you're thinking about. What's the implication on this decision back on how much mass I'm going to have to carry to Earth orbit. And so there's a lot of different terms for that but the idea is just that you're keeping these key factors in mind as you go through the analysis and making sure that those trades are keeping in mind. Things like crew safety and things like that are always in the back of your mind as you're making each one of those decisions in the trade trade. So once you've made that decision here you can see the decision was to propulsively capture versus arrow capture and so you now have to have the fuel for doing that propulsive capture. So the next level of the trade tree looks at whether you would use institute resources. A fancy word for things that are available on the surface to turn them into propellant to take you home or whether you bring all your fuel with you. And so to get yourself up off the surface of Mars you're going to need a rocket of some sort. And so this decision this trade where it says ISRU or no ISRU is will you be manufacturing your own propellant on the surface using the available carbon dioxide breaking it down and forming the chemicals you need to get yourself up or will you bring all of your propellant with you which is a big load to have to carry to the surface and then carry back up. But if you're manufacturing propellant on the surface how do you know it's safe to use? How do you test it in that environment? You know on Earth we do a lot of checks on the propellant to make sure the purity and the quality before we would let it be used in a rocket. How are you going to do all those same kind of analyses on the surface? And so there's a trade off. Yes it's less mass but it might be more risky it might have other implications. You have to carry the equipment with you to manufacture the propellant. How about if the equipment breaks? How about if it doesn't work the way you expected? And so the decision of lowering the mass that I have to carry with me may add risk to the mission. So risk always has to be incorporated in these trades as well. And you can see here that the decision was made not to use in situ resources or available chemicals to provide propellant but to bring all the propellant required with the mission. And so then the last one here is once you've made all those decisions there was another decision to be made in this trade tree which is what kind of propulsion system am I going to use? And there are many different types. Chemical systems which like on the space shuttle hydrogen and oxygen and there's other kinds as well mainly rocket satellites use some kind of chemical propulsion those chemical propellants you burn a lot of them you see the size of the shuttle fuel tank when it was launching that big orange tank that's a lot of fuel for eight minutes of flight and so if you carry chemical propellant you need a lot of it. The performance of the rockets is not such that you can do with only a little you need a lot of fuel. So they don't get good gas mileage. So there are other types of propulsion as well and you see some of them noted on the bottom NTR is nuclear thermal rocket and in nuclear thermal you use in hydrogen as your fuel and you're running it over a nuclear reactor that you've got on board and heating it up and using that expansion as propulsion that is much more fuel efficient and so that might require a lot less propellant and the other choice you have there is solar electric and the solar electric SCP is another type of propulsion we're using sunlight to add energy into their propulsion system again to build yourself a more fuel efficient propulsion system they are nuclear thermal rockets can provide high performance and get pretty good fuel economy solar electric propulsion can provide extremely good fuel economy but their performance as far as the amount of thrust you can get from them is pretty low and then chemical rockets are very bad in performance the miles per gallon is very bad but they can provide a good force to get you off the Mars and so you have to trade how much mass is going to be required in propellant how complex are these systems how advanced is the technology you may find that you have to invest so much money to develop the technology some of these like nuclear thermal rockets have not been flown they were developed in the 60s but never flown you may not want to have the risk of having to develop the brand new propulsion capability that's never been used before for your mission it might become too big of a risk for your mission so as you walk through these trade trees the goal is as you develop them for your mission is that you put in place a set of trade studies where the highest level trades are the ones that are most big and driving how the system is going to operate and what it's going to look like and then there's a whole series underneath where you can build this path and making selections along the way so that's really the key to going through a trade tree and coming up with an optimal design to meet your high level objectives so now click on the icon and make sure to answer the discussion forum post and collaborate with your peers