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Rockets launched to reach orbit usually start from a stationary position, engines are started and the rocket slowly gain acceleration upwards, eventually reaching orbit.

What I'm thinking is that the rocket "wastes" a few seconds reaching a speed/acceleration that could be gained by other means. That would save a little bit of fuel, but most importantly, it would mean the rocket leaves altitude zero at already some speed, making it easier/faster/cheaper to reach the final orbital speed it needs.

I'm thinking for example of a hole in the ground, a few hundred meters deep. You start the engines, then accelerate the rocket up until it reaches a speed of say 40km/h when at ground level, from where the rocket just continues with acceleration but already at an advantage. 40km/h doesn't sound like much, but by looking at rocket launch videos, they do take their time to reach that.

I understand it would be expensive to build such a mechanism, but it's a one time thing. I also understand that the calculations would be different, but again it would be a "somewhat" one time thing: once you figure it out, it's way easier to redo.

Am I subestimating these costs and complexities? If not, what would be the reason not to do something like that?

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    $\begingroup$ Why would it be hard to accelerate the rocket from zero velocity when standing on the ground, but easy to accelerate the rocket from zero velocity when standing in a hole below the ground? You have to accelerate the rocket from zero velocity no matter where you start. $\endgroup$ – Marius Ladegård Meyer May 18 at 6:14
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    $\begingroup$ There have been efforts made in this subject. $\endgroup$ – Sandejo May 18 at 6:26
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    $\begingroup$ @Marius part of my question is to clarify if it would be easier or not. I'm sorry that apparently I wasn't too clear, but what I had in mind is that the acceleration of the rocket when standing in the hole could come from another source other than the engines, like a lifting platform or something like a vertical "sled launch" I just found out thanks to Sandejo en.wikipedia.org/wiki/Rocket_sled_launch (but simplified to just help with the initial acceleration, instead of trying to replace a significant chunk of the rocket's work) $\endgroup$ – msb May 18 at 7:08
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    $\begingroup$ Re *Am I subestimating these costs and complexities?*That is putting it mildly. Moreover, this is an engineering rather than a physics question. You'll might get better traction for this question at the Space Exploration stackexchange. $\endgroup$ – David Hammen May 18 at 19:28
  • $\begingroup$ Good idea, thanks David. $\endgroup$ – msb May 18 at 19:31
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You don't actually want the rocket's speed to be too high when it first takes off. The rocket is primarily fighting against gravity, but it's also fighting air resistance (air drag), and flying faster increases the air drag. That not only consumes fuel, it causes a stress on the rocket known as dynamic pressure. From Wikipedia:

The max q condition is the point when an aerospace vehicle's atmospheric flight reaches maximum dynamic pressure. This is a significant factor in the design of such vehicles because the aerodynamic structural load on them is proportional to dynamic pressure. This may impose limits on the vehicle's flight envelope.

Dynamic pressure, $q$, is defined mathematically as $$ q = \frac12 \rho v^2$$ where $\rho$ is the local air density, and $v$ is the vehicle's velocity; the dynamic pressure can be thought of as the kinetic energy density of the air with respect to the vehicle. 

This stress can be so high at its maximum that it's necessary to throttle back the rocket engines.

During a normal Space Shuttle launch, for example, max q occurred at an altitude of approximately 11 km (35,000 ft). The three Space Shuttle Main Engines were throttled back to about 60-70% of their rated thrust (depending on payload) as the dynamic pressure approached max q; combined with the propellant grain design of the solid rocket boosters, which reduced the thrust at max q by one third after 50 seconds of burn, the total stresses on the vehicle were kept to a safe level.

To improve things, it's better to launch the rocket from a higher altitude, so that the air pressure is lower.

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  • $\begingroup$ Nice answer, thanks. :) But then wouldn't that mean that you can throttle the engines down earlier, saving energy and making the whole launch safer? Take off at an initial speed, engines at max until it approaches max q, throttle down to maintain speed while gaining altitude, resume engines to gain speed after you reach the point where atmosphere is thin enough? I mean, assuming H is the point where the air starts becoming thin ("point" to simplify, I know it's gradual), it doesn't matter how the rocket reached H, it cannot be above a certain velocity at H. $\endgroup$ – msb May 18 at 19:43
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    $\begingroup$ @msb You have to balance the conflicting requirements. Go too fast and the air drag & stress is too high, but if you're too slow you spend too much time in the thickest part of the atmosphere. (BTW, air density drops roughly exponentially). You don't want to carry fuel / propellant any longer than necessary, but you get more speed from a given amount of fuel when the rocket is lightest &/or fastest. How to achieve the optimum balance of all these factors is an engineering question. $\endgroup$ – PM 2Ring May 20 at 6:00
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The reason this does not happen is because it would take a nonzero amount of time to accelerate out of the hole. If we start in the hole, we have a longer ways to go with no benefit. In other words, we still take the same amount of time to accelerate from within the hole to 40 km/h as from ground level to 40 km/h, but all that the hole does is make travel time longer for the rocket.

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  • $\begingroup$ I'm sorry, I believe I wasn't too clear. I'm thinking of having the acceleration in the hole coming from something else other than the rocket engine. A lifting platform for example. So it could take a smaller amount of time to accelerate, and possibly use less energy. $\endgroup$ – msb May 18 at 7:02
  • $\begingroup$ @msb, it could be productive if the hole contained a launching device such as a railgun. The practical issues then come into play - would it be better to build this above ground, possibly obliquely? Could enough energy be added to be useful, given air resistance at the bottom of the atmosphere? Is there a suitable site, given that it takes less energy to launch to the East, from near the Equator. What about stresses on the rocket? Compare Navy aeroplanes which use a launcher to commercial planes, which don't. And cost/benefit of course. $\endgroup$ – Peter May 19 at 1:40
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Rockets work better when they are in vacuum than they do when they are in air. The less distance they have to travel in air, the better. Launching from a mountain-top makes sense, but launching from a hole in the ground doesn't, as it just adds to the distance the rocket must travel through the air.

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  • $\begingroup$ Before posting my question I searched online and found out that launching from a mountain top isn't good for several reasons, logistics being a big one, but also because the biggest time/energy is spent achieving the speed needed, not gaining altitude, and therefore you would be too close to the altitude you need without having reached the speed you need. But idk, that argument doesn't sound great. lol If you want to check it out, it's here: forbes.com/sites/quora/2017/01/04/… $\endgroup$ – msb May 18 at 20:03
  • $\begingroup$ Yes, this is correct. The main benefit of launching from a height would not be reducing the height it must gain, but minimising the amount of air it must pass through. $\endgroup$ – Peter May 19 at 1:23

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