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Comment by rland | Hacker News Reader

I have a question:

It seems to me like the rocket lifted off quite quickly, like the acceleration at first (right at t=0) was very rapid compared to other launches I have seen, especially launches with big rockets.

Is this because it's a satellite that can take higher G's? Is it because they're launching a payload with low mass so it has low TWR? Do these engines spool faster than the engines used in other rockets (like the ones used for Apollo or the space shuttle)?

Or, am I just incorrect that this one is faster...

Jealous of everyone who could witness this up close, as usual.

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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.

> 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?

I 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

Yup, that cleared it up. Thanks!

How 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.

A 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.

Yeah, I forgot that staying in place in the air requires burning fuel :/

a 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.

This one has about 10% more thrust than the previous Falcon Heavy launch and approximately the same weight, so it's going to be accelerating faster than last time. And as others have said, you have 3 cores but only one upper stage, so you'll have higher Thrust to Weight ratio than a Falcon 9.

The Falcon Heavy has three times the initial thrust, three times the booster weight, but only the normal amount of second stage weight. Going by numbers from Wikepdia that don't count the payload the normal Falcon 9 has a TWR of 1.41 and the Falcon Heavy has one of 1.64. So it should be lifting into the air about half again as fast.

The payload weight will make a small difference but it'll be pretty small to the overall weight in any event and not make a large difference. Arabsat-6A is just 6 tons compared to a total weight of 1426 tons.

For reference, Falcon Heavy's max payloads are 63.8T to LEO and 26.7T to GTO, so today's load was quite light. ArabSat would actually have (just) fit in a regular Falcon 9 as well.

Those maximum payloads are in expendable mode, though. SpaceX wants to reduce expendable launches as much as possible. For a reusable FH the numbers are substantially lower and the satellite might not have that much more mass to still fit.

So, just going off of what's on SpaceX's website[0], Falcon Heavy generates ~5 million pounds of thrust at liftoff, and weighs 3.12 million pounds. Max payload to LEO is ~140,000 lb That implies that acceleration at liftoff is somewhere between 1.53 and 1.6g, which would be fine for humans.

Further into the launch, I assume it's accelerating way faster than what humans can handle, but human limits wouldn't explain what it looks like anywhere where you can use the ground or apparent distance as a point of reference for speed and acceleration.

According to the Source of All Truth[1], Saturn V generated 7.6 million pounds of thrust, and had a launch weight of 5.1 million pounds. That means that, with no payload, it would accelerate at 1.49G. Meaning that the maximum possible acceleration is less than what you'd get with a fully loaded Falcon Heavy.

But that's not enough slower to be clearly visible. So I got no idea. Maybe it's some sort of perspective thing, or camera angles?

[0]: https://www.spacex.com/falcon-heavy [1]: You can figure out the URL

The maximum axial acceleration of a Falcon launch (in either configuration) is +6g[1]. That's certainly uncomfortable (and a manned flight wouldn't accelerate that quickly), but it's not beyond the limits of humans to tolerate (particularly for a short duration near the burn out of a stage).

[1]: https://www.spacex.com/sites/spacex/files/falcon_users_guide...

I think FH acceleration off the pad is slightly higher than F9. Makes sense since there's 3x as much liftoff thrust but generally the liftoff mass is not 3x that of a F9.

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