Most efficient way to reach LEO?

[SpaceShuttleFan]

Hello O-F!

I'm an Orbiter newbie. I've been getting a Dragon to LEO for a while now-with a Falcon Heavy. :rofl: I'd like to find a way to LEO that takes the least amount of fuel... any ideas? Thanks.

 

[Cras]

Well, the vehicle your are launching does have a say in the best track to orbit, but two good things to keep in mind

1.) Get up out of the thick air as fast as you can

2.) start to pitch over and pick up horizontal velocity as fast as you can.

You obviously cannot get close to orbital velocity while still in thick air, so you need to get up and out of that first, but applying some pitch early certainly doesnt hurt.

Keep an eye on the Surface MFD and more specific, the dynamic pressure. It will go up during the initial parts of launch, nothing you can do about that, you wanna get up and out of the thick air quickly so you dont add too much dynamic pressure. I am for something like 50kpa....but usually achieve lower. Launch up and when reaching like 20 km can realy think about pitching down and gaining speed.

 

[BruceJohnJennerLawso]

Well, the vehicle your are launching does have a say in the best track to orbit, but two good things to keep in mind

1.) Get up out of the thick air as fast as you can

2.) start to pitch over and pick up horizontal velocity as fast as you can.

You obviously cannot get close to orbital velocity while still in thick air, so you need to get up and out of that first, but applying some pitch early certainly doesnt hurt.

Keep an eye on the Surface MFD and more specific, the dynamic pressure. It will go up during the initial parts of launch, nothing you can do about that, you wanna get up and out of the thick air quickly so you dont add too much dynamic pressure. I am for something like 50kpa....but usually achieve lower. Launch up and when reaching like 20 km can realy think about pitching down and gaining speed.

I would suggest a pitchover at around 7-10 kilometers altitude on Earth. I read somewhere, but cant seem to find right now, that the Earths atmosphere is 99% under 90 k, 90% under 9 k or something like that. The point is that orbital velocity can only really be reliably held over 100 k altitude, but the total drag during a launch drops off very quickly after around 10k

 

[Kyle]

It all depends on when you pitchover if you're going manual, I usually do it around 8km. Anything beyond 12km is pushing it as you now have more upward momentum than forward, and that's hard to balance out.

 

[Cras]

I will look at how my trajectory is on the next flight. I haven't done a manual ascent in a long time, I have used programs to control the pitch for me so I can pay attention to other things during the ascent.

But the early pitch start, yes I agree with what has been aid 100%. The earlier the better, and pretty much start it as soon as you have established a good velocity vector upwards. One thing I remember aiming for was a pitch of 70 once starting the pitch over.

Once you see dynamic pressure dropping you can pitch down a great deal and really start to go fast. Once you get to that point it is just a matter of being the pilot and watching your apogee and your vertical speed to judge if your current pitch angle is too conservative or too agressive. Meaning, are you still soaring up hill at this point, or are you pitched too far down and are sinking back into the atmosphere?

I know for my XR-2 vertical ascents, I aim for a target 105 km for the turn. For the Space Shuttle, you can be further down around 90 or so and be alright. Anything lower and you will find yourself in more trouble than you need to be. Not unrecoverable, but that amount of drag will make working out the rest of the orbit just a bit more complicated.

Sorry I don't have more firm numbers to tell you such as "When reaching X Km do this", you just got to keep in mind the basic principles of what you want to do and how the pressure of the atmosphere changes as you go higher, then keeping an eye on the dynamic pressure, the apogee, and the vertical speed, you should soon find the sweet spot for what ever vehicle you are hand piloting up to orbit, and then of course try try try again and see if you can get better and better efficency.

 

[BruceJohnJennerLawso]

I will look at how my trajectory is on the next flight. I haven't done a manual ascent in a long time, I have used programs to control the pitch for me so I can pay attention to other things during the ascent.

But the early pitch start, yes I agree with what has been aid 100%. The earlier the better, and pretty much start it as soon as you have established a good velocity vector upwards. One thing I remember aiming for was a pitch of 70 once starting the pitch over.

Once you see dynamic pressure dropping you can pitch down a great deal and really start to go fast. Once you get to that point it is just a matter of being the pilot and watching your apogee and your vertical speed to judge if your current pitch angle is too conservative or too agressive. Meaning, are you still soaring up hill at this point, or are you pitched too far down and are sinking back into the atmosphere?

I know for my XR-2 vertical ascents, I aim for a target 105 km for the turn. For the Space Shuttle, you can be further down around 90 or so and be alright. Anything lower and you will find yourself in more trouble than you need to be. Not unrecoverable, but that amount of drag will make working out the rest of the orbit just a bit more complicated.

Sorry I don't have more firm numbers to tell you such as "When reaching X Km do this", you just got to keep in mind the basic principles of what you want to do and how the pressure of the atmosphere changes as you go higher, then keeping an eye on the dynamic pressure, the apogee, and the vertical speed, you should soon find the sweet spot for what ever vehicle you are hand piloting up to orbit, and then of course try try try again and see if you can get better and better efficency.

Yes its all about finding the sweet spot :lol:. I would say that number is quite low through much of an ascent using an XR vessel, since staying low allows for better scramjet operation & more speed, even at the cost of having to pitchup later in the ascent. For a variety of reasons, however, I think the "sweet trajectory" for any vehicle like Falcon Heavy which uses conventional rocket propulsion & generates little lift would go something like this:

-Lift off the ground (obvious I know)
-begin roll to desired azimuth/heading almost as soon as the tower is clear
-Pitch over to something like 60-70 degrees after 8 k altitude
-watch orbit MFD until PEA (periapsis altitude) hits about 140-160 k
-pitchover hard to point towards your horizon (in the direction you were travelling in already)
-burn (hopefully) to orbit

So far, thats the best flightplan that Ive found in launching my Shuttle-D on a Themis core stage, lower target altitudes just seem to take a bit too much fuel for some reason

 

[Cras]

Yes its all about finding the sweet spot :lol:. I would say that number is quite low through much of an ascent using an XR vessel, since staying low allows for better scramjet operation & more speed, even at the cost of having to pitchup later in the ascent.

Ahh,yes a SCRAM ascent would be much different, but I no longer use those for getting my XR-2s into orbit. I use velcro rockets instead to launch.

 

[garyw]

-Lift off the ground (obvious I know)
-begin roll to desired azimuth/heading almost as soon as the tower is clear
-Pitch over to something like 60-70 degrees after 8 k altitude
-watch orbit MFD until PEA (periapsis altitude) hits about 140-160 k
-pitchover hard to point towards your horizon (in the direction you were travelling in already)
-burn (hopefully) to orbit

For most of the awesome forum italia addons like the Jarvis and Neptune I tend to set the heading of the rocket to be the heading I would normally roll to after launch, this saves all the messing around with the post launch roll and is fine for those types of rockets. After launch and at about 1-2km I start to pitch over, this is normally a 1 or 2 second 'nudge' downwards and after that I just let the rocket continue this motion so that it's still pointing above the horizon, gaining altitude but also gaining speed.

have a look at the google earth addon and try a few launches then have a look at how it affects your trajectory. A pitch over 3,4 or 5 seconds later really does affect your final orbit.

 

[BruceJohnJennerLawso]

For most of the awesome forum italia addons like the Jarvis and Neptune I tend to set the heading of the rocket to be the heading I would normally roll to after launch, this saves all the messing around with the post launch roll and is fine for those types of rockets. After launch and at about 1-2km I start to pitch over, this is normally a 1 or 2 second 'nudge' downwards and after that I just let the rocket continue this motion so that it's still pointing above the horizon, gaining altitude but also gaining speed.

have a look at the google earth addon and try a few launches then have a look at how it affects your trajectory. A pitch over 3,4 or 5 seconds later really does affect your final orbit.

so youre saying 8k is still too low, needs a little more height (ie those 3-5 more seconds lifting the rocket about 1500-3000 meters higher, and clear of more of the atmosphere?

 

[garyw]

Depends on the rocket and payload. As with any, your mileage may vary!

Personally I make a note of the weight and performance and use that to work out the launch parameters for the next.

 

[BruceJohnJennerLawso]

Depends on the rocket and payload. As with any, your mileage may vary!

Personally I make a note of the weight and performance and use that to work out the launch parameters for the next.

Aha . I think the reason my launch combo worked better with a higher target altitude was that the whole stack has somewhat lower thrust than I would like & needs more time to burn to orbital velocity. Any time you need to pitch up just to maintain altitude in the later portions of a launch is really wastefull on fuel.

 

[Urwumpe]

The acceleration profile of the rocket is one factor there, not just how fast it accelerates on the average, but when it has which acceleration in flight.

One maneuver that is pretty helpful during launch is the "gravity turn". It simply means how much your velocity vector changes/flight path angle drops by gravity while you maintain zero AOA (which means minimal drag). It is a rather simple function and can be approximated with minimal effort by a human pilot. You do a short initial pitch over to give you an initial rotation and from that point on just let your rocket drop and keep the AOA zero until you reached the pitch angle at which you want to leave the gravity turn.

This also saves a lot of fuel, since you keep AOA low and don't need to spend much fuel for rotating your rocket, since physics do most of the work for you.