Please answer my two ques:-

1. how do we know that jack was 20 year old, there is no mention in the movie.

2. there were three explosions in the movie( i mean rockets sent up as rescue signals)

a. At 01:55:31,000
b. At 02:05:00,900
c. At 02:07:26,000

But in one of these explosions i think, the third one, there is no hissing sound when the fire falls in the water of atlantic. Please explain why?

is it a mistake by james cameron or say, james horner, or it is deliberately done.

one doubt in ur question: Who says jack is 20 ? I mean whr did u find this fact? (Then only it makes sense to search in the movie as to how do v know)

I saw it in the movie reviews and also on the website of the movie.

This may not be the answer to ur Question but some clarification asking for specifics.

Allright from what I cud observe, there are not 3 but 7 rockets fired in the film. Below is the detailed list. The time is as on the 2nd disc of titanic colectors edition.

0.07.02 clearly everybody sees and hears the first rocket.

0.08.51 we only hear the release then we see its flash on rose' face and we hear the explosion sound simultaneously. But we never see the rocket. Commentry the rocket's height was 300-400 feet and so there should have been some delay in sound but it was anyway done for the dramatic effect in the movie.

0.18.55 we see and hear the release of rocket but immediately the shot changes to looking at titanic like an insect from miles away. from this particular angle we see the same rocket exploding in the air but we dont hear anything. wait...after 3-4 seconds we do hear the explosion sound. Why the delay? because sound travels slower than light and hence, for the sound to reach the position of the camera which was also the pov of audience, it took its own time. ref:cast and crew commentry: a scientific study had gone to find how far we were from the ship to calculate the exact delay.

0.20.45 We see only the release of rocket but no explosion heard/seen. This was just before Jack's friend Fabritzio with a few more guys was looking outside the ship from an emergency doorway.

0.24.55 we hear the explosion sound and see the flash of an already released rocket.

0.27.00 rose's fiancee enters the steering deck and behind him we see and hear the release of rocket. Soon it explodes but we only hear it. historical commentry: 8 rockets were actually fired when the actual titanic sank.

0.30.25 rose going down in her boat as Jack stares at her in a static. Behind him, a rocket gets released and we do hear the explosion in a very low volume. This is mainly because it was an emotional scene between jack and Rose and the bg score had to take over the sound effect in that slo-mo.

Now dear friend, first tell me which one are u talking about. I may have an answer then...