The biggest problem with rockets is the fact that they have to carry all their fuel into space with them. One of the reasons that rockets like the Pegasus and SpaceShipOne are so interesting is because they are carried up about 50,000 feet (~15 km) into the air with an airplane, are dropped, and then they fire and go into space. This is mainly to get above the thickest part of the atmosphere, but it also save a bunch of fuel to get up to the 15 km. Well, “a bunch” could be an exaggeration, but they save some.
As explained in another post, rockets are really heavy because they have to carry with them all of the fuel that they are going to use in the future. They also carry the infrastructure for carrying and using the fuel. This is the primary reason staged rockets exist. For a staged rocket, the fuel in one stage is used up, then the rocket engines and the storage tanks and everything are discarded (literally let go). This makes the rocket much lighter, so the next stage becomes more efficient, since it is pushing/accelerating a lot less mass. An ideal rocket would burn up the rocket motors and fuel tanks as it goes up. But, rockets can’t really burn up their storage takes and motors, so there are a distinct number of stages. That number varies, but is often two to four. One would think that four would be best, because the rocket could go up, dump some stuff, go up, dump more stuff, go up, dump more stuff, go up, and deploy the satellite. But, that is a lot of stuff to throw away, and while it may be more energy efficient to do that, it may not be cost effective. In reality, the number of stages is a balance in efficiency in both energy usage and cost. (Let’s be honest, it is all about cost, but the cost is driven by fuel usage and number of motors used.)
Let’s say you want to get to Mars. This requires a huge amount of fuel, due to the very large amount of delta-V that is required to get you there. At this point, we can land something about the size of a minivan on Mars. Not very big.
The reason that we can’t land anything bigger on Mars is because the rocket has to take all of the fuel with it. Imagine if you wanted to take a trip from Maine to California and you had to take all of your fuel with you. Ok, this is geeky – from Acadia National Park to Yosemite National Park is about 3,200 miles. In a Prius, you would get about 40 MPG on the expressway, which would require 80 gallons of gas, which weighs about 640 pounds. The Prius can’t carry that much, so you just can’t use it. Just like rockets like the Falcon-9 and Pegasus and stuff can’t put stuff on Mars. So, switch to an F150 instead. Google says that the 2020 F-150 gets 30 MPG on the expressway (I find this hard to believe, but, when had Google ever been wrong?). That would be 107 gallons of gas, which would weigh about 854 pounds. Google says that an F150 can carry 1,125 lbs. Awesome! We could do that trip, but we need a relatively big vehicle to do it (like an F150), and the payload would be relatively limited (only about 271 lbs), since the vast majority of the carrying capacity needs to be used for fuel. This is similar to using a Delta-4 heavy to get to Mars – we can do it, but there isn’t a whole lot of room left for the cargo! If you want to get something bigger there, you have to build a bigger rocket (which is what we are doing), or you have to change the fundamentals of the game. I would argue that we should be changing the fundamentals. I will suggest two ways, one of which I will discuss below.
The first is the easiest to implement and it is relatively obvious – we need to refuel rockets in space. The way we should get to Mars (and back), is to launch off the Earth, and go into an orbit around the moon. Perhaps a very high orbit, so we are not really strongly gravitationally bound to the moon, but it is enough that we are mostly outside of Earth’s gravity well. There would then be an orbiting space station there. There are several things that could happen at this point. The first is that the rocket could be refueled and launch off towards Mars. This is somewhat silly, since the amount of fuel to get to Mars from this point is actually relatively small compared to the amount of fuel needed to get off Earth, so the rocket would actually be a bit too large. What makes the most sense is that the people get off of rocket number one and they get onto rocket number two. This rocket could look and feel completely different, since it will have ways of creating artificial gravity and will never actually have to feel the accelerations needed to get off of Earth. It could also use vastly different fuels, which could save a huge amount of money. This is like a luxury craft. The sole purpose of this rocket-ship is to go back and forth between Earth’s moon and Mars.
After an 8-month journey, the ship gets to Mars, and it will go into a highly elliptical orbit around Mars. This is so it can not have to use a huge amount of fuel to break away from Mars’s gravity. The ship will dock with a refueling station, and the people will once again transfer to another rocket that will take them to the surface. Getting to the surface of Mars is relatively easy, since Mars’s gravity is pretty weak compared to Earth’s and it has an atmosphere, so the ship can use aerobraking to get a large cost savings in fuel usage.
When people want to go back to Earth, the process is reversed: (1) people take a rocket from the surface of Mars to an orbiting space station, where they transfer to another ship; (2) the ship takes them from Mars to Earth’s moon, where it dock with another space station, and they transfer again; and (3) the final rocket takes them back to Earth. The final leg is actually very cost effective, since the rocket can use the atmosphere of the Earth to slow itself down and not have to use much fuel at all.
This method is very similar to the days in which huge ships would sail from one place to another, but to ports were not deep enough to allow them to dock. The people had to use small boats to ferry out to the big ships, make their journey, and then use small boats to ferry to the shore on the other side.
I am supposed to not write super-long posts, so I am going to break this one up into smaller chucks. I will go through the math of this and discuss cost savings in one post, and then discuss the other idea for how to get to Mars (still with multiple ships) in another post.