The Background

Well... for the first time, sort of, I have a question that doesn't tie directly into anything else. To define what I'm referring to, because I don't actually know the correct terminology, what I meant by a rocket is SpaceX's Starship, or the skiffs and shuttles of The Expanse. They look like blunt capsules, using armor to withstand reentry or engines to slow down, and then use engines to fly basically strait up.

When I said space-planes, I mean the Valkyrie's from Avatar, or the famous Space Shuttle or more recent Starchaser. For the uninformed, the Valk's are spaceplanes that use four VTOL jet engine nacelles to get up to speed, then use scramjets to get to hypersonic cruising velocity, and then use twin fusion engines to approach the ISVs.

The Question

This is a rather simple one, apart from the obvious fact that one takes off sideways, what differences do they have in regards to fuel efficieny, technology needed, advantages (like how you can fire one with giant railgun or catch skyhooks), or general reasons why one would be better than the other in circumstances?

How do they differ in use?

Why use one, or why use the other.

  • 3
    $\begingroup$ We answer questions about your worldbuilding problem. You seem to be asking about some unrelated third parties worlds: "what is the difference between a Starship trooper and a Balrog?" $\endgroup$
    – L.Dutch
    Jan 30 at 5:12
  • $\begingroup$ @L.Dutch I referred to the solutions for the how to get from ground to orbit problems, not the individual designs, which is why I included the real life examples. I need to figure out which would be the dominant piece of tech that humanity would use to get things from the surface into orbit. $\endgroup$ Jan 30 at 5:19
  • $\begingroup$ And because they seem to be the two most common solutions (by the fact that I can name about three examples off the top of my head), if someone is building a world with these technologies, they might also be interested in an answer to which is better, and for what. $\endgroup$ Jan 30 at 5:21
  • 1
    $\begingroup$ If the technologies are "current technologies", then the answer is a simple googling of their respective technologies. Otherwise, it depends highly on how they're fuelled, materials science... $\endgroup$
    – jdunlop
    Jan 30 at 6:42
  • 1
    $\begingroup$ VTC:NAW. You're asking for an open-ended, incredibly broad discussion that can't have an objective right answer. The differences touch on engineering, physics, economics, targeted use, level of technology, geospatial statistics, and a lot of other stuff. As asked, you're literally asking for an off-topic infinite list of things vs. an on-topic finite list of things. $\endgroup$
    – JBH
    Jan 30 at 17:00

2 Answers 2


The reason for the differentiation in terms is a question of how air-breathing the engines are. A rocket carries all of its own oxidizer. It has to, because there isn't any in the vacuum of space. A turbo jet (like in an airliner) compresses air to provide adequate oxidizer. The problem we've found is that a jet only works up to around 90,000 feet, and can only get us to speeds around mach 3.

This leaves a chunk of atmosphere between 17 miles and 60 miles, and a speed differential between mach 3 and mach 25 where you could be using a plane, but we have no technologies that operate there.

Why would we want to? Because oxidizer is 4 times as heavy as fuel, and being able to use atmospheric oxidizer for the worst part of the acceleration is a significant cost savings.

This is where a space plane becomes useful, and where ram jets and scram jets come in. These are technologies similar to the turbo jet, but they use the air compression from the vehicles' actual speed to compress the air up to the proper density.

These technologies just plain don't work in the lower atmosphere, so you'd wind up with:

  • all three technologies on the same craft (this is your space plane here)
  • engines that can shift how they work as they rise and fall
  • a three-stage system where the turbo jets drop off and land while the scram jets get the rocket-propelled part out of the atmosphere, then glide back down on their own.

It takes more than wings to be a space plane. They called the Shuttle a space plane, but it was just a rocket with wings. It didn't use the "plane" effect for lift on the way out.


Though this seems like a real-world terminology issue, I'm going to interpret this as one about the terminology people might use in a future setting. We do have a terminology tag, after all.

A spaceplane is simply a rocket that takes off and lands like a conventional aircraft and is capable of orbital flight, as opposed to suborbital.

In reality, a true spaceplane that is single-stage-to-orbit (and preferably reusable) is difficult to imagine, but likely possible. Many advances in material science, rocket science, and pretty much science across the board are needed to make SSTO spaceplanes a technological reality.

With enough fuel, you can get a single-stage anything into orbit, even things vaguely resembling a plane.

As to whether you'd use them over other vehicles, that's more for you to decide. A staged rocket can get more cargo into orbit because of multiple separations and shedding unwanted mass. It therefore uses propellent more efficiently. A spaceplane has to haul everything all the way up, including the empty fuel tanks. Elon Musk claims Starship's upper stage can make a SSTO trip, but cannot carry any cargo and would be devoid of many features that make it reusable or even a functional vehicle.


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