- Aug 28, 2014
- 16,541
- 26,096
So Rocket Lab is a busy little bee. In fact I think they have 3 more launches scheduled before the next SpaceX launch. They did some interesting things (for me anyway) today that we can discuss.
Mission Name: Look Ma, No Hands
Launch time: Midnight NZT
Launch Pad: New Zealand (they are building a second launch site in Virginia)
Payload: A cubesat and 3 other small satellites to LEO
Launch starts at around 15 minute mark
Interesting tidbits:
· The Rocket labs first stage is black carbon fiber but you can see that the upper part is all white. That is because the liquid oxygen (LOX) has to be kept mighty cold (-183C/-297F) and the water vapor in the air freezes as it comes in contact with that part of the fuselage. The remainder is RP1 (AKA Kerosene Rocket fuel) which is liquid at room temp.
· Battery Hot Swapping – If you were paying attention in a previous post I mentioned that the unique thing about Rocket lab engines is that they are small enough that the fuel preburn turbo pumps can use electric motors. The motors run on batteries that are fairly heavy so as those batteries are depleted they are jettisoned to make the rocket lighter.
· First stage recovery – no they’re not recovering the first stage today however that first stage will likely have a few instruments or even tests on board to verify their plans of recovering the first stage by parachute. It will be interesting if they share that data.
The other interesting thing was their launch profile. So normally this is how it works:
First stage – The first stage rocket exists to get everything to orbital height (apogee). The first stage rocket engines are optimized for atmospheric pressure (as opposed to the vacuum of space). They also are designed for a relatively large thrust to weight ratio (TWR) as they must lift everything against the pull of earth’s gravity. Thrust = Mass x Acceleration, where Mass of the rocket, Acceleration of gravity. Of course if they equal you would only hover so in reality you want thrust to be much greater – the F1 Engine (Saturn V) had a TWR of 94.
Second Stage – Here’s where it gets interesting. The first stage will just get everything to altitude (apogee). If left alone everything will come crashing back to earth. So now you are sitting at apogee with rocket engines not designed for space and a whole lot of useless mass. But wisely you have done a bit of a gravity turn with the first stage to get you some Delta V going in the correct direction and you’ve dropped the weight of the first stage (to land harmlessly in the ocean*) or be recovered. But you aren’t orbiting yet. To do that you need to burn in the direction you are traveling (prograde). The arc that you are traveling in is actually an ellipse with a perigee somewhere IN the earth. By burning prograde you increase your speed, of course, but also increase the perigee (apogee stays the same). When perigee = apogee then congratulations you have a circular orbit. Generally that is when you can release your payload and, if you are a good space citizen, deorbit your 2nd stage so that it comes back to earth, hopefully burning up or landing in the pacific rocket graveyard.
So that’s a typical launch. Of course nothing says that you have to have a circular orbit. If you want an elliptical orbit you could keep burning so that the perigee gets greater than the apogee. Now the perigee becomes the apogee and the apogee becomes the perigee. And this is what Rocket Labs does. The 1st stage takes them to a lower orbit than the final desired orbit. When they burn the 2nd stage it is creating an apogee at their desired orbital altitude. Then they drop the 2nd stage (called a kick stage in this scenario). Their third stage has even smaller engine (Curie) which they can use to circularize the orbit. You can see an example at the end of the video.
Why do they do it that way? I’m not sure. It seems like it would be more expensive however rocketry is all about weighing costs and benefits. I’m sure they’ve done the math.
*Unless you are China, they just let the first stage land wherever, sometimes on villages. Seriously, google it.
Mission Name: Look Ma, No Hands
Launch time: Midnight NZT
Launch Pad: New Zealand (they are building a second launch site in Virginia)
Payload: A cubesat and 3 other small satellites to LEO
Launch starts at around 15 minute mark
Interesting tidbits:
· The Rocket labs first stage is black carbon fiber but you can see that the upper part is all white. That is because the liquid oxygen (LOX) has to be kept mighty cold (-183C/-297F) and the water vapor in the air freezes as it comes in contact with that part of the fuselage. The remainder is RP1 (AKA Kerosene Rocket fuel) which is liquid at room temp.
· Battery Hot Swapping – If you were paying attention in a previous post I mentioned that the unique thing about Rocket lab engines is that they are small enough that the fuel preburn turbo pumps can use electric motors. The motors run on batteries that are fairly heavy so as those batteries are depleted they are jettisoned to make the rocket lighter.
· First stage recovery – no they’re not recovering the first stage today however that first stage will likely have a few instruments or even tests on board to verify their plans of recovering the first stage by parachute. It will be interesting if they share that data.
The other interesting thing was their launch profile. So normally this is how it works:
First stage – The first stage rocket exists to get everything to orbital height (apogee). The first stage rocket engines are optimized for atmospheric pressure (as opposed to the vacuum of space). They also are designed for a relatively large thrust to weight ratio (TWR) as they must lift everything against the pull of earth’s gravity. Thrust = Mass x Acceleration, where Mass of the rocket, Acceleration of gravity. Of course if they equal you would only hover so in reality you want thrust to be much greater – the F1 Engine (Saturn V) had a TWR of 94.
Second Stage – Here’s where it gets interesting. The first stage will just get everything to altitude (apogee). If left alone everything will come crashing back to earth. So now you are sitting at apogee with rocket engines not designed for space and a whole lot of useless mass. But wisely you have done a bit of a gravity turn with the first stage to get you some Delta V going in the correct direction and you’ve dropped the weight of the first stage (to land harmlessly in the ocean*) or be recovered. But you aren’t orbiting yet. To do that you need to burn in the direction you are traveling (prograde). The arc that you are traveling in is actually an ellipse with a perigee somewhere IN the earth. By burning prograde you increase your speed, of course, but also increase the perigee (apogee stays the same). When perigee = apogee then congratulations you have a circular orbit. Generally that is when you can release your payload and, if you are a good space citizen, deorbit your 2nd stage so that it comes back to earth, hopefully burning up or landing in the pacific rocket graveyard.
So that’s a typical launch. Of course nothing says that you have to have a circular orbit. If you want an elliptical orbit you could keep burning so that the perigee gets greater than the apogee. Now the perigee becomes the apogee and the apogee becomes the perigee. And this is what Rocket Labs does. The 1st stage takes them to a lower orbit than the final desired orbit. When they burn the 2nd stage it is creating an apogee at their desired orbital altitude. Then they drop the 2nd stage (called a kick stage in this scenario). Their third stage has even smaller engine (Curie) which they can use to circularize the orbit. You can see an example at the end of the video.
Why do they do it that way? I’m not sure. It seems like it would be more expensive however rocketry is all about weighing costs and benefits. I’m sure they’ve done the math.
*Unless you are China, they just let the first stage land wherever, sometimes on villages. Seriously, google it.
