[Q] Why do rockets start vertically?

[sanctus]

I always wondered why rockets start vertically? It seems to me that this needs more fuel (but probably not true overall) and creates much more and stronger vibrations (problem for many scientific devices) than a start like an airplane. The only advantage I can see is that you are in less time in space...

 

I'm sure there are many more advantages, otherwise it would not be done this way, but I can't see them...

[JMJones0424]

It's all about escape velocity, pure and simple. There are alternative orbital designs that include a "flying" (or more horizontal than vertical) first stage (one that uses atmospheric lift rather than thrust), but the problem is that it requires more energy to accelerate to escape velocity rather than just punching straight through. It is hard to make "non-vertical" launch platforms economically efficient, but this does not mean that the added benefit of reusing conventionally flying early stages will not over-ride conventional thinking. Launching anything into space is not cheap, and I would think that we are taking the cheapest, safest option available. This does not mean that our approach will not change in the future. Perhaps you could design a reusable "first stage" plane?

[modest]

I'm not sure if SpaceShipOne could really claim to achieve what the opening post is looking for. It's lifted off the ground and taken to high altitude on a carrier craft and it doesn't achieve orbit, so probably not.

 

There would seem to be a large design difference between spaceplanes which use their wings to provide for a gliding reentry and recovery versus one which would provide takeoff lift. The first scenario has been successfully designed and built (such as the space shuttle) while the second scenario has not.

 

The problem being: a spaceplane on the ground getting ready to go to orbit has a much greater mass than a spaceplane reentering the atmosphere because the former includes the mass of the fuel. As an example, the mass of the space shuttle on the launch pad is 2,040,000 kg while it's mass upon landing is 132,800 kg. It looses 94% of its mas on the way up. Designing wings to lift the first mass from a runway (or anything approaching that mass) would surely be an entirely different thing from the wings the shuttle has now.

 

An interesting proposed solutions to this problem was the 1993 "Black Horse". The design called for an air-launched vehicle to be fueled for orbit only after takeoff by a midair tanker.

Winged, single stage to orbit launch vehicle using aerial refueling and lower performance, non-cryogenic propellants. Takes off from runway at 22,000 kg gross weight; rendezvous with tanker to load 66,760 kg oxidizer; then flies to orbit.

 

Black Horse

An interesting idea, I'd say.

 

~modest

[Pyrotex]

The basic things about rockets are: mass and delta-v.

The mass of the rocket, including fuel.

The total change in velocity needed to achieve orbit.

Since fuel will account for at least 75% of the total rocket mass, you must minimize the amount of fuel required.

This can best be done by designing for the highest accelleration that is practical -- if you're fighting against the Earth's gravity (and you are), then the faster you achieve altitude and the faster you achieve orbital velocity, the less fuel it will require, overall.

However, traveling fast through the atmosphere just increases drag. And it's not linear. The drag at Mach 2 is far more than twice the drag at Mach 1.

So, it's critical to take the fastest shortcut that gets you out of the atmosphere before your speed gets too terribly high.

And that requires you to launch vertically.

 

Atmospheric drag decreases sharply with reduced air density. Typically, we have our rockets go vertical until they pass through about 90% of the atmosphere. Then they start their "roll maneuver" which gradually has them accellerating horizontally.

 

You also want to minimize the amount of time you accellerate vertically, because that builds up vertical velocity, and you only need just so much. If your orbit is to be at 200 miles, then by the time you have a vertical component of 2 or 3 thousand miles per hour, you're gonna coast to 200 miles, even if the rocket shuts off. So, as soon as you have enough vertical velocity to "coast" up to your desired altitude, it is critical to swing your accelleration horizontally, because you're gonna need 17,000 MPH to achieve orbit.

 

You can even think of the total delta-v as being divided into its horizontal and vertical components.

You need about 3000 MPH of vertical velocity to carry you up to 200 miles;

and about 17,000 MPH of horizontal velocity to achieve orbit.

The total delta-v that the engines must deliver -- which equates to a certain amount of fuel burned over a certain duration -- is about 20,000 total MPH.

The total delta-v needed to achieve orbit (which is always significantly more than just the orbital velocity) is the parameter that rocket scientists try to minimize.

[enorbet2]

There is an additional issue and that is structural coming from the need to reduce mass to the minimum. The walls of rockets just barely support themselves along the vertical wall oriented axis. The structural integrity of liquid fuel rockets is so weak that it is substantially enhanced by the increased stiffness caused by volatile fuel creating internal pressure and extreme cold. Laying them down in any way would collapse most.