Why do traditional spacecraft need booster rockets to break out of earth’s gravity while the Virgin space plane not?


A spacecraft that takes off on a runway, with rocket engines that do not require external oxygen, seems like a safer approach to get off the ground than being strapped to a controlled explosion.

In: Physics

To put it simply: *VSS Unity* does not break out of the Earth’s gravity. Today’s spaceflight only lasted a few minutes because the Earth’s gravity pulled the spaceplane right back down into the atmosphere.

When you think of a vehicle “escaping gravity,” what’s usually happening is that the vehicle has achieved *orbit.* This does not mean it escapes gravity, but instead means that the vehicle is moving sideways so fast that the Earth’s curved surface drops away faster than the vehicle can fall.

>A spacecraft […] with rocket engines that do not require external oxygen, seems like a safer approach to get off the ground than being strapped to a controlled explosion.

*VSS Unity* is propelled by a rocket engine that contains its own onboard oxidizer, creating a continuous controlled explosion. Sure, the mothership uses regular jet engines, but there’s just no way to get people into space that doesn’t involve rockets engines at some stage.

The virgin plane didn’t get into orbit or “break out” of earth’s gravity. It just flew really high and fast to the edge of the atmosphere.
So not really a space craft, just a high aeroplane.

As far as I’m aware, while they reach what is classified as space, the rocket is not big enough for a sustained burst to put it into an a orbit. It’s only in a microgravity for several minutes before coming back to earth. The amount of fuel needed is huge, something which you wouldn’t be able to put into a craft that takes off from a run way.

Going up to space is not hard, the problem is to move sideways so you do not hit the ground when gravity is pulling you towards the earth.

The international space station ISS is a low earth orbit at 400km above the earth. You need to go sideways at a speed of 7,4 km/s to orbit earth there so the rocket needs to accelerate you to that speed parallel to the ground.

You also need to go up there against gravity and through the drag of the atmosphere. The getting up part is like accelerating to 2km/s.

So a total acceleration of 9.4km/s is required to go to ISS. 2/9.4=0.21 so 20% if the acceleration the rocket does is to go up and 80% is used to move fast sideways.

So Virgin’s spaceplane can be smaller than the rocket used to reach orbit because it only needs to do 20% of the work. 20% of the work do not mean the rocker is 20% of the size it will be a lot smaller. Rockets are the primary fuel, were are talking of over 90% fuel by mass. So most fuel initially is used to lift fuel that you need later.

Neither a rocket to ISS or Virgin’s space plane escapes earth’s gravity only problems that are sent to other bodies in the solar system can be considered to have done that. The gravity at ISS altitude of 400 km is 90% of the gravity at ground level, ISS is in freefall it has not escaped earhts gravity The Viring space plane only to go to 100km

Virgin space plane accelerates to Mach 3, orbital speeds are closer to Mach 30. There’s a reason they only spent 5 or so minutes in space… they were on a short hop, not in orbit

It’s because the private space flights abuse the arbitrary altitude of “space” that was set to 100km.

Other spaceships go there to do something useful, not just for the sake of going. They need to get to orbital speed and altitude. ISS is at the altitude of 400+km, which is considered low Earth orbit that’s 4 time higher. It also moves at over 7km/s, much, much faster than the Virgin craft.

So, technically, they are in space. But it’s just a short hop to that arbitrary altitude, so it requires only fraction of energy compared to a useful rocket.

For comparison, even ICBMs, the rockets that can carry nuclear weapons, get to higher altitude before they fall on their target.

The Virgin craft basically acted like a ballistic missile. It fired up, coasted up as far as the rocket burn would take it, then fell down. It did not get out of gravity, for gravity clearly pulled it back down.

Other space craft need to get into orbit, so they need to get up to higher speeds to avoid falling back down. Higher speed demands more fuel and more power.