Why Do Rockets Need Stages? The Quest to Build a Single Stage to Orbit (SSTO)
Science fiction has promised us a rocket that blasts off into space without stages. How close is reality to the science fiction dream of a single stage to orbit?
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Now, don’t get me wrong, Science Fiction is awesome. Like almost everyone working in the field of space and astronomy, I was deeply influenced by science fiction. For me, it was Star Trek and Star Wars. I had a toy phaser that made this awesome really loud phaser sound, and I played with it non-stop until it disappeared one day. And I was sure I’d left it in the middle of my floor, like I did with all my toys, but I found it a few years later, hidden up in a closet that I couldn’t reach. And I always wondered how it got there.
Anyway, back to science fiction. For all of its inspiration, science fiction has put a few ideas into our brains which aren’t entirely helpful. You know, warp drives, artificial gravity, teleportation, and rockets that take off, fly to space, visit other planets orbiting stars, land again.
The Millennium Falcon, Firefly, and Enterprise Shuttles are all examples of single stage to orbit to orbit spacecraft, or SSTOs.
Consider the rockets that exist in reality, you know, the Atlases, Falcons and Deltas. They take off from a launch pad, fly for a bit until the fuel is used up in a stage of the rocket, then they jettison that stage and thrust with the next stage. The mighty Saturn V was so powerful that it had three stages, as it made it’s way to orbit.
As we discussed in a previous video, SpaceX is working to make the first stage, and maybe even the second stage reusable, which is a vast improvement over just letting everything burn up, but there are no rockets that actually fly to orbit and back in a single stage.
In fact, using the technology we have today, it’s probably not a good idea.
Has anyone ever worked on a single stage to orbit? What technological advances will need to happen to make this work?
As I said earlier, a single stage to orbit rocket would be something like the Millennium Falcon. It carries fuel, and then uses that fuel to fly into orbit, and from world to world. Once it runs out of fuel, it gets filled up again, and then it’s off again, making the Kessel Run and avoiding Imperial Blockades.
This concept of a rocket matches our personal experience with every other vehicle we’ve ever been in. You drive your car around and refuel it, same with boats, airplanes and every other form of Earth-based transportation.
But flying into space requires the expenditure of energy that defies comprehension. Let me give you an example. A Falcon 9 rocket can lift about 22,800 kilograms into low-Earth orbit. That’s about the same as a fully loaded cement truck – which is a lot.
The entire fueled Falcon 9 weighs just over 540,000 kg, of which more than 510,000 kgs of it are fuel, with a little extra mass for the engines, fuel tanks, etc. Imagine if you drove a car that was essentially 95% fuel.
The problem is specific impulse; the maximum amount of thrust that a specific kind of engine and fuel type can achieve. I’m not going to go into all the details, but the most efficient chemical rockets we have, fueled by liquid hydrogen and oxygen, can just barely deliver enough thrust to get you to orbit. They have a maximum specific impulse of about 450 seconds.
Because the amount of fuel it takes to launch a rocket is so high, modern rockets use a staging system. Once a stage has emptied out all its fuel, it detaches and returns to Earth so that the second stage can keep going without having to drag along the extra weight of the empty fuel tanks.
You might be surprised to know that many modern rockets are actually capable of reaching orbit with a single stage. The problem is that they wouldn’t be able to carry any significant payload.
At the end of the day, considering the chemical rockets we have today, the multi-staged profile is the most efficient and cost-effective strategy for carrying the most payload to space for the lowest cost possible.
Has anyone tried developing SSTOs in the past? Definitely. Probably the most widely publicized was NASA’s X-33/VentureStar program, developed by Lockheed Martin in the 1990s.
The purpose of the X-33 was to test out a range of new technologies for NASA, including composite fuel tanks, autonomous flight, and a new lifting body design.