Sounds like your ride is just too small. Make it bigger and you'll probably solve your problem.
Just to be clear on terminology, "prestige" is the actual ride stat, and "reputation" is the optional gameplay mechanic that causes prestige to vary over time. If you turn off reputation in the game settings, prestige will remain constant. In this post, I'll only be talking about prestige itself, which is what you have direct control over when you build the ride. This gives the ride its base prestige, which will then vary if you have reputation turned on.
With that out of the way, here's how prestige works...
Prestige is a meta stat that is a combination of the ride's:
* excitement rating
* duration
* track scenery rating (NOT queue scenery)
So, to improve your prestige, you have to improve these 3 factors. Here's how to do that:
Your track scenery rating is shown near the bottom of the ride's Testing/Results tab. It ranges from "low" to "very high". Values from "low" to "high" are determined by the total dollar value of all the scenery and building parts within about 12m of the track. "Very high", however, can only be obtained if you have at least 2 (two) triggers on the ride, regardless of the amount of parts. Going from "high" to "very high" adds about 60 prestige so if you don't have at least 2 triggers on the track already, add them.
For coasters, the excitement rating is mostly a function of:
* Speed. The faster you go, the more exciting it is, and vice versa. This is averaged along the whole track so the goal is to maintain as much speed for as much of the track as possible. A rule of thumb is that every 10mph of speed gives about 1.0 excitement for that part of the track.
* Vertical G forces. The most exciting vertical G forces are +4 and -1. Vertical Gs higher than +4 and less than -1 are scary (jack up the fear rating) and decrease excitement. Gs between +4 and -1 follow sort of a saddle-shaped curve that bottoms out at +1.
* Lateral and Axial G forces: These need to be kept between about +1 and -1. They work like vertical Gs in that getting right up to the limits produces the highest excitement but going beyond them increases fear a lot (and also nausea for lateral Gs) while killing excitement.
* Height above ground level: Every meter of ride height above the ground under that part of the track increases excitement by about 0.1 for that piece of track.
* NOTE: headchoppers, tunnels, and even inversions don't seem to affect excitement.
Duration is simply how long the ride lasts. For coasters, this conflicts with excitement because if the track length is constant, increasing speed to get excitement lowers duration, and vice versa. However, coaster prestige seems to weight excitement more than duration so in general it's better to go faster.
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Now, to your specific coaster. To maximize the prestige for In-Descent, you need to make it big. The longer the launcher, the faster it can go. The faster it goes, the higher the tower it needs at the end. The higher the tower, the more excitement for altitude, the longer the free fall coming back down (0 vertical Gs for good excitement) and the faster it's moving at the bottom so the higher the vertical Gs pulling out (just don't exceed about 3.5 - 4). And the longer the track and the higher the tower, the longer the duration, although this is never going to be the ride's strong point. And it has no turns so no lateral Gs at all.
So, make the launcher long enough to reach at least 60mph, and set the launcher's acceleration and braking to 9 or 10 m/s^2 (about 1 axial G for max excitement). Pimp the track as desired but be sure to have at least 2 triggers to get "very high" track scenery.