Ask HN: How much delta-v is saved with Stratolaunch?

https://news.ycombinator.com/item?id=19656332

by walrus01 • 7 years ago

9 6 7 years ago

news.ycombinator.com

potiuper3• 7 years ago

Compared to what?

smallduck• 7 years ago

A launch from a balloon of course.

walrus01• 7 years ago

How everything except the Pegasus is currently launched, stationary, zero velocity, zero height AGL, with a first stage beginning at ground level.

potiuper3• 7 years ago

For the electron rocket https://www.reddit.com/r/RocketLab/wiki/vehicles, which is smaller than the Pegasus (smaller than XL) with a mass of 18,500 kilograms

first stage delta-v is: 303 s * 9.80665 m/s^2 * ln (10200 kg / 950 kg) = 7053.19 m/s

second stage delta-v is: 333 s * 9.80665 m/s^2 * ln (2300 kg / 250 kg) = 7247.06 m/s

LEO requires near 8600 m/s. So, the second stage is necessary for LEO starting from 0 m/s. The big unknowns are how much delta-v is lost to air resistance and due to the gravity of the earth (launch trajectory optimization). To calculate those would require numerical simulation, see https://space.stackexchange.com/questions/20848/how-to-deter.... Optimistically an air launch would occur at 40k' (12k metres) and 600mph (270 metres per second). The effect on the first stage only due to the imparted velocity reduces the delta-v to 6783.19 m/s or a reduction to a wet mass of 9314.03 kg (885.97 kg of fuel). 270 m/s / 7053 m/s = 0.0383.

tropo• 7 years ago

Directly, it isn't much.

Indirectly, it could be much more. You can fire the rocket from a different location on Earth. Switching to a different orbital plane is expensive.

There are other benefits.

Launch windows are usually limited due to the physical location of the launch pad.

Launches are very often cancelled due to weather.

There may also be something to be gained regarding ice hazards. Cryogenic rockets that sit along the Florida coast collect ice from the air. This doomed a Space Shuttle. Getting up to dry air before the launch could be a way to avoid the ice formation.

Ask HN: How much delta-v is saved with Stratolaunch?

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