A single stage vehicle all the way to orbit

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http://m.space.com/19872-skylon-space-plane-human-spaceflight.html

I don't have a background in aerospace engineering but I am interested to know the main reasons as to why it is so difficult for single stage vehicle to go all the way to orbit. Is it about the fuel constraints and the weight? the weight of a multistage vehicle keeps going down as the initial stages are detached away, right ? Is that the reason for multistage rockets ? I would like to know all the reasons.
 
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Dr. Courtney

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https://en.wikipedia.org/wiki/Multistage_rocket

The bottom line is that it is harder to carry the entire mass of a single stage rocket into space after most of the fuel is burned.

It is easier to jettison the weight needed to hold and burn all the fuel at lower altitudes.
 
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https://en.wikipedia.org/wiki/Multistage_rocket

The bottom line is that it is harder to carry the entire mass of a single stage rocket into space after most of the fuel is burned.

It is easier to jettison the weight needed to hold and burn all the fuel at lower altitudes.
Harder in terms of amount of fuel required to lift them? So the only solution for a single stage vehicle is to use a very light weight material ,so that we don't have to throw away initial stages ? Or find a fuel source that is powerful enough to carry the whole vehicle ?
 
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boneh3ad

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A single-stage-to-orbit (SSTO) space vehicle has been an active area of research for years. One of the chief challenges is propulsion. Rockets use a lot of fuel mass and would require much for fuel to reach orbit in a single stage. Finding an alternative is a major challenge because you need a propulsion system that will work at sea level all the way up to space and from zero all the way up to escape velocity and needs to remain reasonably efficient throughout it all. This is one of the motivations behind trying to develop scramjets.
 
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Does building a single stage vehicle that takes off horizontally offer any advantages over vertical take offs ?
In terms of reusability, is it easier to land the vehicle horizontally on runway than what space X is trying to do ?
 
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boneh3ad

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Sure. You could launch it and recover it from a traditional runway instead of requiring specialized launch facilities.

You might be interested in the National Aerospace Plane program (NASP).
 

Dr. Courtney

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In terms of ease in reaching low earth orbit is there any difference between horizontal and vertical take-offs ?
 
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Dr. Courtney

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In terms of ease in reaching low earth orbit is there any difference between horizontal and vertical take-offs ?
Not in principle, but in practice, horizontal take-offs have the option of using jet engines and air lift up to a certain altitude. Jet engines use the oxygen in air rather than needing to carry their own oxidizers, so the weight burden can be significantly reduced.
 

berkeman

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In addition to the reasons given so far for using multi-stage rockets to get to orbit, a great point about rocket nozzle design was made in a different thread just now by @betadave

In a convergent, divergent nozzle, I think the limit might be imposed by the back pressure of the atmosphere causing the shock to form inside the nozzle. I am not an expert on the thermo involved in the calculations and so I don't feel qualified to take the argument much further than that. Any rocket nozzle is designed to a specific ambient pressure. When launched, the pressure is above ideal and it passes through ideal to where the pressure is below. If you have watched a launch at night, it is beautiful to watch the flame change as the rocket climbs and then the stage change occurs. It gives you a clear indication that the nozzle has limits.that depend on ambient pressure. It is a mistake to think you can change the throat diameter to get velocity changes, that changes thrust but not exit velocity for a given propellant.
:smile:
 

boneh3ad

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In addition to the reasons given so far for using multi-stage rockets to get to orbit, a great point about rocket nozzle design was made in a different thread just now by @betadave



:smile:
If you are referring to the second half of that post, then yes it is relevant. Otherwise, the first half wasn't really correct in the context of the thread in which it was posted. Or at least it isn't the whole story.
 

cjl

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Not in principle, but in practice, horizontal take-offs have the option of using jet engines and air lift up to a certain altitude. Jet engines use the oxygen in air rather than needing to carry their own oxidizers, so the weight burden can be significantly reduced.
At the same time though, a vertical take off rocket is, from a structures perspective, effectively an axially loaded hollow cylinder, which tends to be very strong and allows for a very light structure weight. Horizontal take off introduces perpendicular loading - you need to both withstand the thrust of the engines, and the bending moment from the wing, and the landing gear, and any other areas where the craft is supported horizontally. You'll never get a horizontal take off craft to have the same fuel fraction as a vertical take off.

In addition, horizontal takeoff at best (currently) allows you to use jet engines (and airbreathing propulsion) up to mach 4 or 5. While this sounds fast, it's only about 1.3-1.5 km/s, while orbital speed is upwards of 7km/s. Once you exit the atmosphere, you still need to accelerate that final 6 km/s on rockets alone, and you have to drag some (now useless) large and heavy jet engines along with you.

When you combine these two factors, horizontal take off and landing, though attractive at first glance, has never ended up being worth it on any practical launch vehicle. It's a great idea, but there are a lot of fundamental problems with it. I'm excited to see if we manage to fix them in the future, but for the time being, there's a lot to be said for the multi-stage, vertical take off/vertical landing reusability approach that SpaceX is trying, since it doesn't need to deal with these technical issues (though of course it brings its own set of problems to solve as well).
 

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