3 ms·
Falcon Heavy does indeed lift off the pad faster than many rockets (it's even faster than the single-stick Falcon 9). This is because of its higher overall TWR
by JshWright 8y ago
Falcon Heavy does indeed lift off the pad faster than many rockets (it's even faster than the single-stick Falcon 9). This is because of its higher overall TWR (since the two side boosters are functionally Falcon 9's without the mass of a second stage and payload). Another rocket that really leaps off the pad is Ariane 5 (once the SRBs light at T+8sec).
Generally speaking you want to accelerate as quickly as possible in the early stages of the flight in order to reduce gravity losses (every second you spend going "up", the more energy you have to spend fighting gravity). There are other factors involved here though (if you get going too fast in the lower atmosphere the losses due to drag start outweighing the savings, etc).
The spooling time of the engine doesn't really matter here, because the launch clamps don't release the rocket until the engines are all up and running and healthy, so when T-0 hits, the engines are already at full thrust.
- saagarjha 8y ago> Generally speaking you want to accelerate as quickly as possible in the early stages of the flight in order to reduce gravity losses (every second you spend going "up", the more energy you have to spend fighting gravity). This doesn't really make sense to me from a simple dynamics perspective. Why does time matter, unless efficiency comes into play somewhere?
- JshWright 8y agoI typed up a long explanation... then looked up the Wikipedia page to confirm the usage of a term, and realized it has a much better explanation, so... here's that... https://en.wikipedia.org/wiki/Gravity_drag https://en.wikipedia.org/wiki/Gravity_drag
- saagarjha 8y agoYup, that cleared it up. Thanks!
- bunderbunder 8y agoHow I like to understand it instinctually is: When you're going up, gravity is working against you. The more time you spend going up, the more time you spend with gravity working directly against you. Once you're out of the atmosphere and into the "over" phase, you're not fighting gravity, you're just trying to increase your lateral speed enough that the earth's falling away from you faster enough than gravity's pulling you toward it. Or you can think of the limit case: as your acceleration away from earth goes to zero, your energy spent fighting gravity in order to avoid hitting the ground goes to infinity.
- deleted 8y ago[deleted]
- CydeWeys 8y agoA really stupid simple way to think of it is -- Imagine you're hovering a hundred feet off the ground. You still need to be burning your engine at a full g, but you're not going anywhere. You're spending LOTS of fuel for zero movement. Now imagine you can only burn at 1.01 g; you will VERY slowly start moving upwards, but you'll be spending a colossal amount of fuel for the same result. Now imagine instead you're burning at 3g; suddenly you're actually accelerating quite quickly, and you're overall burning much less fuel to go the same distance.
- saagarjha 8y agoYeah, I forgot that staying in place in the air requires burning fuel :/
- walrus01 8y agoa really good way to visualize this is to build some fat, awkward vessel in Kerbal Space Program that has a thrust-weight ratio of only 1.2 or thereabouts, and use mechjeb to watch the data during launch. It'll eventually get to orbit (enough struts and boosters will do that, in KSP) but you'll see how inefficient and slow it is, how much time it spends in the lower atmosphere, compared to a properly optimized rocket.