Why don't we use sound suppression system when rocket lands propulsively?

Question

We know that the sound suppression system (water deluge system) is used to protect the launch vehicle from the acoustic energy produced by the engines. For most of the expendable launch vehicles, it is meaningful to use this only at liftoff, since they don't have any kind of soft landing.

But for launch vehicles such as Falcon-9, Falcon Heavy, Starship, Starship Booster, etc., why don't we (will not be) use sound suppression systems? Or when Starship lands/launches on mars we couldn't afford a sound suppression system using water. Then how do we actually protect the launch vehicle from the sound energy during propulsive landing, and launching (from other celestial bodies)?

Answer 1

The amount of energy involved in a take off and landing will be roughly equal to the mass being moved, and approximately the same proportion of energy will be turned into sound given the same engines are involved. Going up a falcon heavy is around 1400 tonnes, coming down the stages have split apart and are almost empty. Have not found authoritative numbers but seem to around 30 tonnes.

Even allowing for different acceleration/thrust profiles that is a major reduction in energy involved, allowing landing without a flame trench or sound suppression. The same probably holds true for an empty Starship but no real numbers yet to pin that down.

Landing a Starship on the moon or mars is a much more interesting question, since they will need to land with a reasonable fraction of fuel still on board even if in a lower gravity and with lower (or near zero) local atmosphere.

Unsure on the math for determining how much of the exhaust volume of a landing starship would hang around being an 'atmosphere' propagating noise energy for the times involved in a landing.

Answer 2

Another take on the question: you have to get your aiming perfectly right.

Others have already pointed out, that landing does not cause as much noise as launching.

However, if we consider using a sound suppression system, then this comes along with a lot of plumbing and other infrastructure.
For launch, this is not a problem, since the location of the launch is very well defined and fixed in space. So, you build your sound suppression system around the launch vehicle.

For landing, however, you can define a landing spot, but where the rocket actually lands is not in your hand. Depending on the quality of your location tracking, vehicle guidance, interference from wind, and many other factors you may miss your target landing spot by meters. Good luck designing a sound suppression system that covers a wide area, yet does not obstruct the landing of the rocket.

Aim for the wet spot, don't hit the plumbing.

Image credit: Wikipedia

Answer 3

As other answers have pointed out, the necessary noise is much lower for landing than take off, but this doesn't mean that the actual amount of noise will be so reduced. As control theory gets better, and landing on a larger fraction of the available thrust becomes an option, I think we will see it happen as it makes sense from a performance point of view. F9 has already started on this trend, having used 3 engines to land as opposed to 1 (See: Why were three engines used for the F9 1st stage landing burn (BulgariaSat-1)?).

I think the major reasons not to are:

• Aiming accuracy required
• Expensive stuff to hit if you miss

Major reasons to desire it less are:

• Less likely to hit resonant frequencies
  • The high power densities of rocket flume sound are quite low, which is more likely to excite a full/heavy rocket than an empty light one.
• Less time spent near the ground (landings are faster than takeoffs)
  • Less critical that the rocket is protected on the way down
  • It has done its job so the loss does not effect the outcome of the launch
  • only one stage to be damaged
  • no sensitive payload on the top

That said, none of the major reason not to are long term issues, 'just' engineering and control theory, and these are getting better quickly. Also no of the reasons the desire it less are reasons not to desire it at all. Maybe we will start to see this soon.

Answer 4

TLDR

A landing booster creates < 10% of the noise of a rocket launch. This is possibly reduced much further in near vacuum conditions (Moon/Mars). No sound suppression could be fine at these noise levels.

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To expand on the previous answer;

A rocket's engines must create more force than the force of gravity on the rocket in order for it to accelerate skyward (when taking off this would increase its velocity and when landing this would decrease its velocity).

Therefore, the thrust-to-weight ratio of a rocket is a key metric in its performance. At launch, this ratio was 1.5 for the Space Shuttle while the Saturn V was at less than 1.2, resulting in a very slow takeoff.

As the rocket exhaust gasses create both the thrust and the damaging sound waves, the sound created by a rocket is roughly proportional to its weight for any given acceleration. Assuming the acceleration of a landing booster or spaceship is equal to that of one taking off, we can say the sound created in either scenario is (very roughly) proportional to the rocket weight.

For the Falcon 9, a completely empty booster is given by Wikipedia as weighing 22 Metric Tonnes, while a fully loaded stack is over 549 Metric Tonnes. Therefore, only ~4% of the takeoff sound is generated during landing if the booster was using its last drop of fuel to land. This is backed up by the landing boosters using just 1 of 9 engines (presumably throttled down).

The problem would be increased slightly during a landing if there is no flame trench (landing on a flat surface will reflect more of the sound upward toward the rocket engines) but this is still too little to require active sound suppression for the Falcon 9.

As for sound suppression in vacuum or near vacuum conditions, the answer to Does a rocket sound louder to the astronauts as it leaves the atmosphere? indicates that sound would be much less of a problem on the Moon or Mars due to the exhaust gasses being able to flow away from the rocket without the interference of external air. The exhaust gases from a rocket engine are much less dense than air at sea level and rapid compression an expansion of them occurs after leaving the rocket (see https://en.wikipedia.org/wiki/Shock_diamond). The above answer indicates the shock-waves created by this are responsible for most of the sound, a problem not encountered in vacuum.