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Comment by ucaetano | Hacker News Reader

Lifting a rocket that high before firing it carries another benefit besides the (small) altitude gain and lower fuel requirements due to drag: Max Q, the maximum dynamic pressure that has to be sustained by the rocket usually happens at a lower altitude than that (depending on the mission).

This means your rocket has to sustain a far lower structural load, making it safer and lighter.

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Max Q for the Peagus and Saturn V are actually at the exact same altitude. ~14-16km. And nearly identical pressures.

The thing is because all rockets follow the same trajectory to orbit, The gravity curve. So you hit similar speeds at similar altitudes. If you don't, you either aren't making it to orbit, or you're wasting fuel.

This is because the gravity curve is a straight line if you ask Einstein, just earth's gravity makes space-time look wibbly-wobbly.

Newton calls this the derivative of the balastic arc.

There isn't a magical trajectory to orbit that is cheaper.

I was disappointed to realize that "balastic arc" was just a typo. I thought I'd learned a new word. :)

Lighter? I'm not so sure. Most of the reduced mass has to be added back in again for Pegasus to have aerodynamic control during stage 1 flight.

The overall benefits of the platform are questionable, at best, especially considering its absurd launch cost.

Damn, $56M+, that is expensive...

The altitude gain is significantly less important than the horizontal velocity. An aircraft can easily launch from the equator and add 650+MPH.

Note that this launch wasn't from the equator, because this constellation needs to overfly hurricanes.

It's fairly uncommon for LEO satellites to want to be at the equator. GEO, extremely common.

Launching from the equator is useful for a wide range of orbits as a component of the final vector 1600km/h is really not that fast and your final orbit crosses the equator anyway. The only issue is if the ideal launch point is not over the ocean. Also, there is a wide band near the equator with close to optimal velocity.

Yeah, I suppose my comment was not the most clever. In this case they were only launching 1/2 the max payload so performance wasn't really an issue anyway.

It's about 5% of the final velocity, but that's way more than 5% fuel savings because you don't have to suspend million of pounds against gravity for a minute and also don't have to accelerate that fuel.

For example Saturn V used 40% of its total fuel to get up to this speed. That's a huge savings.

Unless I am reading the data incorrectly:

http://www.braeunig.us/apollo/SaturnV.pdf

Vector addition means you save energy, but not one to one. EX: if you add 1 going north and 1 going east then your final vector is 1.414 going north east. So, it might only be worth 1% or 5% of the final velocity.

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