Cryo tanks are so cold, they tend to condense moisture out of the air, creating ice covering a lot of the rocket.
I've seen many videos where this ice starts shedding at launch, but I still have to ask, isn't this a (temporary) weight penalty? Is it a considerable penalty for a large rocket with large tanks?
EDIT: hopefully someone can post a numerical answer in the form of additional weight to nominal weight, or thickness and density of ice plus surface area for a given rocket (stage).
For the Apollo 11 launch, at S-IC ignition the frost weights were:
S-IC: 1400 lb/635 kg
S-II: 450 lb/204 kg
S-IVB: 300 lb/136 kg
But by holddown arm release:
S-IC: 650 lb/295 kg
S-II: 450 lb/204 kg
S-IVB: 200 lb/91 kg
So about half the ice was shed (due to vibrations) before the stack left the ground.
The Saturn V Skylab flight evaluation report has more data (see the tables on page 187). At first stage ignition 1400 lbs of ice was present on the first stage. By S-IC cutoff, 750 lbs remained. By S-II ignition, all ice on the S-II stage was gone.
On the Space Shuttle external tank, foam insulation was used to reduce icing. That didn't eliminate the problem completely. The insulation foam was porous, so ice would build up inside the foam. According to this test report, on an average launch about 1100 kg of water ice had accumulated in the ET insulation (depending on the age of the insulation, older tanks would accumulate more).
Most rockets don't use insulation AFAIK, so this is a bit of an outlier.
This study suggests that on LH tanks, ice is a problem because of thermal transfer from the water/ice to the LH, warming up the LH and increasing the boiloff. The study doesn't mention weight of the ice as a problem.
From Stages to Saturn:
Development of the J-2 engine turned up the inevitable gaggle of problems to perplex project designers, engineers, and workers. In using cryogenic propellants, it was obvious that great care was needed to ensure installation of very efficient insulation at critical points to control thermal losses. In the case of most early rocket technology using LOX as the oxidizer, the problem was not immediate. Designers simply took advan- tage of the fact that LOX components had a tendency to frost over. The frosty coating worked surprisingly well as natural insulation-so well that many components were designed without insulation from the start. The super-cold liquid hydrogen permitted no such easy design shortcuts. When air touched the extremely cold LH2 surfaces, it did not frost, but actually liquified. As a result, streaming liquid air not only became an annoyance, but also created a serious heat leak.
Icing on rocket tanks is not a significant problem. Only cryogenic fuels will cause icing so the vast majority of military rockets are unaffected. However some space launch vehicles use cryogenic oxygen or hydrogen which will cause ice build up.
Rockets are filled up with cryogenic fuels several hours before launch. This combined with the humid air found at some launch locations(Cape Canaveral/French Guiyana) will lead to a non-negligible amount of ice building up on the side of tank. Cryogenic fuels are pumped constantly into the tanks until right before lift off to minimize boil off of the propellants. The Space Shuttle External Tank and Delta IV CBC had foam insulation since the large surface area of hydrogen tanks leads to large amounts of thermal transfer. Rockets like Falcon 9 and Atlas V that only have cryogenic oxygen tend not to have heavy insulation.
One reason for the lack of insulation on these rockets is that the ice forms a layer of natural insulation preventing more ice build up.
Now the weight of the ice is not negligible however most of it falls off during launch due to extreme vibrations and air resistance.
