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Comment by CodeGenie | Hacker News Reader

I ask you this question because you seem to know what you're talking about, whereas I simply don't.

Watching the launch today, I couldn't help wonder why they don't use standard jets to get the vehicle to a height where a rocket could take over.

The highest a jet has flown is about 37km and LEO is considered to start at about 160km. That 37km though in my mind would be the toughest and most fuel costly to ascend.

Would it not save both weight and money to use jets and fixed wings up through the low altitudes before turning to rockets for the higher altitude ascent?

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"The reason it's hard to get to orbit isn't that space is high up. It's hard to get to orbit because you have to go so fast."

https://what-if.xkcd.com/58/

I don't entirely buy this. You don't get to go any slower if you are going to Mars, which is not orbiting the Earth in any way. Or if I just want to escape Earth orbit and go drift in space, why do I need to be moving fast relative to the Earth? Why can't I be moving slowly but steadily away?

Everyone's responses are spot-on. But I can understand that it still may not be intuitive to you.

The difficulty is that we see pictures and videos of our satellites and shuttles in orbit, and they look peaceful and serene. What's missing from those images and videos is a visceral feel that they are going over 17,000 mph. If they went less, they would leave orbit, and hit the Earth.

Objects in orbit are not "outside" of Earth's gravity; they are not just up there, just floating in space. The reason they don't "fall to Earth" is that they are falling to Earth. Constantly. It's just that their horizontal speed relative to the Earth is so large that they are - literally - falling around the Earth.

> Or if I just want to escape Earth orbit and go drift in space, why do I need to be moving fast relative to the Earth?

Because gravity. If you aren't going fast relative to Earth, you aren't going to escape Earth orbit [0]. Heck, if you aren't going fast relative to Earth, you aren't even going to be in Earth orbit [1], you are going to be falling back to the surface.

[0] http://en.wikipedia.org/wiki/Escape_velocity

[1] http://en.wikipedia.org/wiki/Orbital_speed

The premise of your question is wrong. If you are in Earth orbit, you are moving fast. It's actually being in orbit that allows you to move slowly away, because you can take as long as you want to apply the force. If you're not in orbit you have to quickly apply enough force to escape Earth's influence in one go or you will fall back to Earth.

I can highly recommend having a play with Kerbal Space Program to get an intuitive grasp of orbital mechanics.

Basically, if you don't go fast enough, you're going to come back down to earth. Once you get to a speed that's fast enough that you aren't going to come back to earth, you'll find you are now in orbit around the sun, in a path remarkably similar to the one the earth is on. To get to Mars you need to get up into a higher orbit around the sun, so you need to go even faster.

It's not the height, it's the speed. (Though, Virgin is trying something like this for their rocket).

LEO isn't that far away, but it goes by _really_ quickly. And getting going that fast requires a lot of propellant mass, and that's judged by the tyranny of the rocket equation.

That's pretty much the approach of Virgin Galactic. Reasons why it works for them, but not for NASA would have to be explained by someone more qualified. I do know that VG barely gets into LEO, whereas NASA's requirements are much higher. In any case, I presume the scientists at NASA (and SpaceX & co) have considered just about every alternative, and the current approach still remains the most efficient for their needs.

Orbital Sciences is doing something similar, IIRC.

They are the two big approaches to cheap space being done now. 1) Be cost-effective at making rockets, 2) launch from jets. My money is on #1, which SpaceX is doing, but #2 has some things going for it that could prove me wrong. #2 is also strictly limited in just how big you can make a payload, while for #1 you can get up to around 200 ton payloads before things start working against you.

The issue is that people confuse getting into space with getting into LEO. Virgin Galactic are not even close to getting into LEO – their plan is to fly a ballistic trajectory which takes them just above 100km before falling back to Earth.

Since they don't need to achieve anywhere near the required velocity to enter LEO, they can use a much smaller solid-fuel rocket engine and launch from a jet.

I think there are strength/scaling issues with winged aircraft that, given the fuel requirements for any sizable payload to orbit even from the speed/altitude a jet can achieve, makes this extremely challenging, but its something people keep working on.

I'm sure this has been studied to death, but my guess is that the extra risk during flight/separation, and the extra weight of designing the rocket to handle the different forces encountered when attached to the mothership are not compelling enough to make it worth the extra few % in delta-V. (SpaceShipOne went to Mach 3, not 25, so the boost was greater by percentage)

Not even LEO; so far, all of Virgin's flights have been strictly suborbital.

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