# Is the launch of a rocket slow because the change in velocity (delta_v) should be as small as possible because of energetic reasons?

Does it make sense to keep the launch of a rocket slow because the change in velocity (a.k.a. delta_v) should be as small as possible because of energetic reasons?

• The delta-v should be what is required for the mission. Feb 13 at 13:31
• Until recently, heavy launches (30-ton +) did not have many options in terms of initial launch speed. But Elon Musk did mention the idea of a rocket on rails where engines would ignite when the rocket has already achieved 20+ km/h delta-v. And propellant savings were expected to be significant. Feb 14 at 3:48

No. Nor the opposite.

Smallest possible launch useful speed is barely >0 m/s. The rocket will mostly hover, spending all of its fuel just fighting gravity. Inefficient.

The other extreme is almost an explosion. Tons of engines (weight!), huge acceleration and within seconds huge speed. And that means huge aerodynamic drag.

So we balance.

Launch as fast as reasonably possible, which means take in account

• Weigh of the additional engines, or bigger engines to accelerate faster
• Structural supports needed for high acceleration.
• The aerodynamic drag

Which most of the time results in:

• Launch fast. Engines at max.
• Reach high speed (at still low altitude, so still high drag)
• Throttle engines down to keep drag in check
• Once out of most of the atmosphere throttle back up.

And of course, all of this varies per mission with different rockets, different payload, different target orbit, etc. etc.

• Another reason why real rockets launch slowly: If the initial acceleration is high, you can load more fuel until it isn't. That extra fuel is burned inefficiently, but still better than no extra fuel. Feb 14 at 10:23

It makes no sense to deliberately keep launches slow. The slowest possible "launch" is a hovering rocket. It is 100% efficient at converting propellant into sound and fury, but 0% efficient in converting it to lift.

The faster the launch (up to the tolerance of airframe, payload and crew), the more efficient. In the case of airframe, this is "Max Q". For payload, electronics can withstand the acceleration in artillery shells. For crewed launches, maximum G-forces at launch are usually well below what is tolerated at re-entry.

Fundamentally, this comes down to fuel being cheap and rockets being a lot more expensive than fuel tanks.

If your rocket takes off fast adding a bit more fuel tank to it isn't going to cost very much, the additional fuel is even cheaper. You lowered the cost per kilogram of payload. There is an issue of diminishing returns but the optimum point is with the acceleration pretty low at first stage ignition.

There is also the issue that acceleration goes up as the fuel burns off. The higher the maximum acceleration the stronger everything above it must be, thus you have an incentive to keep the acceleration down. Few rockets have enough engines that you could keep the acceleration in check by shutting off some of them early.

Something that has been implied but not spelled out in the other answers. If you launched going 0 m/s upwards, you would literally be wasting all of your fuel fighting gravity, and go nowhere at all. Therefore, higher accelerations are better. Of course, there is a reason that rockets down accelerate at 1000G:

1. Your fuel supply will be burnt very quickly
2. You will lose a lot to drag
4. You will destroy any crew
5. The rocket will blow up from the aero-forces
6. You will need a lot of mass for the engines

See Hennes answer for what we actually do and why. I won't repeat that here.

The other answers have quite clearly explained that, for a launch vehicle, launching as slowly as possible is inefficient.

But you mention 'for energetic reasons' in your question title: if you are, in fact, looking for the most energy-efficient rocket (rather than the most practical), the answer is slightly different.

The answers to this question explain this far better than I have been able to. My attempt to explain it is included below.

In this case, while you may still lift-off quickly, the velocity of your rocket engine exhaust should not (initially) be high. Decreasing this at takeoff is actually beneficial for efficiency (given a constant power) - so, in some abstract, tangential sense, your intuition about this is correct.

Consider: your rocket exhaust is moving very fast, therefore it has a lot of energy. That energy is wasted - we don't care about the exhaust once it has exited the rocket. If you tune your exhaust velocity so that the motion of the exhaust after leaving your rocket is negligible - in other words, the exhaust velocity is essentially equal to the rocket's velocity - more of the power put into the exhaust goes to the rocket, because the exhaust has minimum energy. For an explanation using maths, see this answer.

A more mathematical approach to this argument can be found in this NASASpaceflight forum thread, regarding exactly what effect a VASIMR-like engine has on the performance of a rocket.

Do note, however, that modern variable-isp engines are not particularly useful for launching a rocket from Earth. This is because launch vehicles are very mass-constrained, wanting to get as much energy out of each Kg as possible - they are not especially worried about being 100% efficient with that energy.

• So at liftoff your exit velocity should be zero? I think there is a flaw in that reasoning. Feb 14 at 5:08
• This is incorrect. Momentum is conserved. If the propellant leaves the rocket at zero velocity, it has zero momentum, and thus no momentum is imparted to the rocket. Feb 14 at 14:31
• There are related concepts where this is correct--for example, a turboprop or turbofan are more efficient than a turbojet because they have lower exit velocities, but only because they can also move more mass. Rocket exhaust is already mass-limited, and so we go to great pains with nozzle design to keep exhaust velocity and specific impulse high. Feb 14 at 14:37
• The problem with this reasoning (apart from the rocket starting at zero velocity) is that there is a finite supply of reaction mass available to the rocket, and for chemical rockets this supply is also directly linked to the available energy for propulsion. It is not useful to eject it at a lower velocity to "save energy" that can't be stored or used in any other way. Energy efficiency just isn't a very useful concept for rockets, you use the energy you have available. Feb 21 at 23:21
• A zero-velocity exhaust has high energy efficiency (which is why airplane engines try to lower the exhaust velocity -- they have access to unlimited mass, but finite energy). Rockets, on the other hand, are concerned with mass efficiency, since they need to carry all their mass with them. In a mass-constrained situation, you want the exhaust to go as fast as possible to get as much momentum change out of it as you can.
– Mark
Feb 22 at 2:00