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[Robsoie]

The Earth to Orbit is what is burning the most fuel, i often consume way too much myself when playing with realistic fuel.

The SCRAM are engines you can use inside the atmosphere, though nowhere as powerfull as your main engine, but it can be usefull to spare main engine fuel to have both SCRAM and main engine turned on when ascending orbit, this way you should get there faster, or not using full engine, either way sparring a bit of fuel possibly.

Re-entry burns fuel only at the retrograde burn, to quit orbit and fall on your target, during the whole re-entry you're supposed to have no engine turned on, just gliding using the atmosphere to slow you down and landing (that's why it's important to be able to make precision re-entry, or you'll have to use lots of fuel and scram to be able to get back from your rentry miscalculation to the most nearby airfield.

[Carrion]

I see.  For some reason I was waiting till 20km or so before I even thought about SCRAM and then I was confused why the thrust was so weak. 

I went for another Orbital insertion, this time with the XR5 Vanguard.  I loaded some cargo and took off, made it about halfway then ran out of fuel on a ballistic trajectory.  Next time will be different.

Thanks for the info, Robsoie.

[sluissa]

I just completed my first orbital insertion from Earth, into LEO (Low Earth Orbit).  This was with the XR2, which I wish had more gadgets like the DGIV, but it's still cool.  The problem now is that I used nearly all of my main thrusters fuel.  I still have some reserve, but not much.  Really, though, I don't have a true goal... well, I just wanted to get into orbit.  What is a more fuel effective way?  Or, what are the SCRAM jets and how should I use them?  I could just look this up, but you all are much more insightful.

That's not bad, and really, most craft will use a large portion of their fuel getting into orbit, at least with their default settings.

The XR series is really designed to be used with the scram jets though, and take a slightly different strategy to get into orbit. With a traditional rocket you want to point your nose almost straight up and get into vacuum as soon as you can. The scram jets make it different though.

You'll still want to get high fast, but once you get to around 30,000 meters, you can level out a bit, maybe 10 degrees nose up or so. Accelerate beyond mach 4, keeping an eye on the craft skin temperature. Once you're moving that fast, the scram jets are usable. Less, and they become less efficient and if you're going too slow they won't even produce enough thrust to keep your acceleration up.

scram jets work on the principle that air is heated by compression from the air being forced into a narrow funnel at the front of the craft. This hot air has a relatively small amount of fuel injected into it and thrust comes out the back. It can potentially be a very efficient engine but it requires some other form of propulsion to get it up to speed and to a proper altitude. If you're too low, the needed speed will cause too much heat from friction on the craft. Too high and you no longer have enough oxygen. (Although the too high limit is VERY high.)

I know this isn't likely the most efficient method, but with the scram jets I'll usually get myself up to about 30,000 meters, level off, accellerate to mach 4-4.5 with the main engines, open up the scram jet doors, slowly open up the throttle a little, then lower the main engine a little, going back and forth, watching the guages of the heat, wing loading, scram temperature and the other normal instruments to watch until I can shut the main off completely and still get accelleration. You can rarely have the scram throttle open completely, it's very touchy as well and requires constant adjustments. Try to keep your dynamic pressure between 8 and 20, that's your safe range, with 8 being the minimum pressure for good engine functionality, 20 being a good pressure maximum efficiency without going into the danger zone as far as temperature. It'll take a few flights to get used to it. Once I'm on the scram engines alone, I'll set myself to a very shallow climb angle, letting my speed build as my height builds. The scram jets are good to at least 60km. keeping an eye on it you can get a bit higher. Once you're at the point where you can no longer use the scram jets transition back to the main engines for orbital insertion.

 I've never been able to use more than about 2/3rds of my scram fuel, but it has saved me a decent amount of my main fuel by letting me build up good speed over the altitude I can use it. I'm sure there are people who can do better than that, but It works for me. My last flight with the xr5 I managed to get into orbit with about 40% scram fuel left and almost 50% main fuel left. My first flight was closer to 75% scram 30% main. First flight with no payload, second flight with about 40% payload.

[Robsoie]

The best case is to have low angle when ascending, while using mostly only SCRAM .
It will increase only slowly altitude, but because SCRAM is far from as powerfull as the main engine, going high angle will get your speed falling.

If you can manage to keep using SCRAM as much as possible, you'll be able to spare lots of main engine fuel.
I can't count myself how much time i was out of fuel when trying to get myself ready for a re-entry.

But that's a good scenario for a rescue mission too, as you can then spawn a new ship on KCS and try a rendez vous with your previous one

(edit : posted while sluissa was posting )

[sluissa]

The Earth to Orbit is what is burning the most fuel, i often consume way too much myself when playing with realistic fuel.

The SCRAM are engines you can use inside the atmosphere, though nowhere as powerfull as your main engine, but it can be usefull to spare main engine fuel to have both SCRAM and main engine turned on when ascending orbit, this way you should get there faster, or not using full engine, either way sparring a bit of fuel possibly.

Re-entry burns fuel only at the retrograde burn, to quit orbit and fall on your target, during the whole re-entry you're supposed to have no engine turned on, just gliding using the atmosphere to slow you down and landing (that's why it's important to be able to make precision re-entry, or you'll have to use lots of fuel and scram to be able to get back from your rentry miscalculation to the most nearby airfield.

I've actually been able to get more thrust out of the scram jet than I can from the main, under certain conditions. with about 20 kpa dynamic pressure at mach 8 or so, I can get between 20 to 30% more thrust from the scram jet than I can from the main. But it's dangerous to let the accelleration go out of control like that.

[Robsoie]

Using sluissa method with the XR-2 and the default XR2RavenstarPrefs.cfg values ( that's the file in which you can change all the Ravenstar settings in the .\Config folder, so back it up in case you want to go back to the default values) for the 30km and mach 4 altitude, i experimented to see how much i could spare to build a low orbit.

Here is what i did :
Note : do not use accelerated time during this if you are using the attitude hold autopilot ! it will really screw-up your trajectory, the whole process should take you only +/- 10 minutes of real time, so there's no need of accelerated time.

-Full main engine
-Pull up and take off when reaching 150 kmh and pull the gear up immediately (at full engine it accelerate so fast you're going to break them if you don't pull them up as soon as they're not on the ground anymore)
-Pitch up at 20 (the attitude hold autopilot of the XR help)
-Wait until you're at 30 km altitude
-lower angle at 10
-Wait until at least mach 4 / 1300 kmh airspeed
-Open SCRAM doors
-progressively replace main engine by SCRAM, so you don't see the airspeed begin to go down.
-at some point you'll notice on your HUD the projected trajectory is going down slowly, and if it goes under level, altitude will begin to fall, but speed is always increasing so it will be fixed soon enough don't worry altitude will increase again .
-when you're sure the projected trajectory is not going down anymore and both speed and altitude are going up well enough go to 12.5 angle
-once you're above 60km , airspeed should not increase as quick as before (as you're above the atmosphere and O2 necessary for the SCRAM jets is very low. You should be nearly at between 3800/4000 kmh (mach 11-mach 12) speed now.
-at 75km go full engine and remove SCRAM - Close SCRAM doors (important to close it or it will burn!)
-at 80km  altitude go Prograde autopilot
-wait until orbit is fully built.


Not a perfect low orbit, but there should be enough fuel to come back.

I had a previous attempt in which i had 48% main and 18% scram left but the orbit was rather ugly.

[mainiac]

There's a firefly mod!!!!!111!!!1!!1!!eleventyone!!!!!!

[Il Palazzo]

There's a firefly mod!!!!!111!!!1!!1!!eleventyone!!!!!!

Huh?! Linky link please pretty please!

[sluissa]

the firefly mod is good for a bit of fun. Very soft sciencey, but fun nonetheless.

Robesoie: The only thing I can suggest to change there is to increase your angle between takeoff and 30,000 m. I'd go somewhere around 60 degrees up. Possibly even more. Your main engines can lift you straight up(in most craft, XR5 might be an exception, as I've never had the hover engines get me up on earth) and the longer you stay at low altitudes the more fuel you waste, both because rocket engines are simply less efficient at higher pressures but also because it's more time spent not using the scram engines on the xr series.

[Robsoie]

I have tried with your suggested 60, using a different procedure to avoid not having enough horizontal speed in the end and losing more main engine fuel in trying to regain enough :

-full main engine
-taking off as soon as possible (usually 150/160 kmh) and gear up as soon as it can be pulled up.
-ascent angle 60 degree (attitude hold help)
-reaching 20 km altitude, drop the ascent angle to 5 degree
-when your projected trajectory will reach those 5 degree too, you'll be at +/- 35 km altitude
-wait until you're at +/- 1300 kmh (mach 4)
-open scram door and full scram engine
-lower main engine until being shutdown, do it progressively so you don't lose totally the "Acc Z" indicator (it should stay above the middle) and do not lose speed.
-at 60km altitude, increase your ascent angle to 12.5 , you should be by then at 4900 kmh (between mach 14 and 15)
-at 75km altitude, main engine max, then drop the scram and close scram door
-altitude 85 km , stop the main engine, go Prograde and wait until your at the Apr Node
-When you're at the Apr node, (always in Prograde) , full main engine again until you have built your orbit.
-Once you're satisfied with your orbit, stop main engine.


I have burned more scram than before, but the great benefit is that i am in low Earth orbit with more than 50% main engine fuel left !
Enough for doing lots of things and a re-entry.

I'm sure i can drop the scram earlier without losing anything, will have to experiment to see the optimal altitude in which losing the scram does not affect the trajectory in any ways.

[mainiac]

There's a firefly mod!!!!!111!!!1!!1!!eleventyone!!!!!!

Huh?! Linky link please pretty please!

http://www.orbithangar.com/searchid.php?ID=3198

Damned if I know how to work the ship though.

If I can figure this out, we should do a trade the save let's play of building a lunar colony or space station with a firefly.

Well, I can make it into space.  This ship is about as fun and forgiving as you'd expect.

[Carrion]

I have burned more scram than before, but the great benefit is that i am in low Earth orbit with more than 50% main engine fuel left !
Enough for doing lots of things and a re-entry.

I have yet to attempt a re-entry.  How much fuel do you both estimate is necessary for the retro burn at a reasonable altitude?  Also, I have been playing with the SCRAMs on the XR5 and after a nice hull breach at Mach 6 I have successfully utilized them for orbital insertion, but still, after the Ap prograde burn, with less then 45% main engine fuel.

[sluissa]

Depending on the altitude of your orbit, you shouldn't need much fuel to deorbit. I'd say 20% is a really safe margin, but it should be able to be done at 10% pretty easily. Probably even lower, but that would be pushing it. Don't expect much fuel to be left over either, if you over or undershoot your reentry. I've still yet to get that down. I'm not any good at eyeballing that kind of stuff yet. I can at least get within a couple hundred kilometers though.

It's surprising what you can actually do with only a tiny bit of fuel once you're in orbit if you know how to deal with atmospheric braking and slingshots. In fact, one of the most fuel efficient ways I've seen of making large changes in your orbit is to actually do a free return around the moon to change your orbit inclination. (If you're interested in the method, take a look at this. http://flytandem.com/orbiter/tutorials/Challenge_2006/index.htm

Lots of interesting TransX tutorials available on that website as well, a good number of them video. http://flytandem.com/orbiter/tutorials/

[Carrion]

Just had a quick look at the tutorial.  Looks like something I will have to revisit when I am a little more confident and ambitious.

I just had a go with one of the DGIV deorbiting missions and I had no clue what to do.  In an effort to get back into the atmosphere, I just popped down, but waaay undershot KCS and ended up flying from the Gulf of Mexico all the way over to Cape Canaveral and with nary 10% fuel left, I landed too hard and blew out my gear.  Ugh.  It was good fun though and will give it another go soon.  Nextly, though, I need to learn how to align orbits so that I can do something with all those orbiting gliders I have quicksaved--read, dock with the ISS.

For de-orbiting, do you just estimate based on the map and your orbit position?  Can you burn at any point for re-entry?  It's good to know that the burn doesn't need much fuel, that will give me some leeway.

[Omegastick]

Woo, just completed my first landing (I missed the landing strip entirely, but I landed...).