Getting to know the Ravenstar:)

[TMac3000]

I downloaded her about two weeks ago. She really is a dream to fly, but quite a bit different from her more automated city cousin, the DG-IV. It's a bit like driving a stick when you are used to an automatic. Okay, so here is the takeoff procedure I've been using, for my fellow orbinauts' evaluation:

1) Take off, accelerate to 200 m/s and turn to launch heading.

2) Pitch up 20 degrees and climb to 10 km (32,000 feet)

3) Pitch down to 10 degrees and accelerate to 925 m/s

4) At Mach 3, disengage airspeed autopilot, engage SCRAMs, and pitch up to 15 degrees. Reduce main engine throttle to 30%.

5) When SCRAM thrust drops below 50 kN, shut down SCRAMs, close the SCRAM doors, and advance main throttle to 100%. Proceed to orbit normally.

This way is hard on the hull (temp peaks at aroun 1300 C), but my record so far is about 32% main fuel remaining after orbital insertion. That's with default config: crew of 14 and about 3000 kg (I think) of cargo. Is there a way to do better without reducing crew/payload?

 

[Krikkit]

My record is near 56% main fuel at 200 km circular orbit. No payload, no passengers. I don't know how to translate that.

I only use main engines until the SCRAMS fully kick in, then I just skim the mesosphere until at mach 27 I get the SCRAM diffusion temp warning and have to close the scram doors. At which point it is just a quick main engine burn to get me on a parabolic trajectory. At AP another short burn to round out the orbit

 

[N_Molson]

Try to keep hull temperature and dynamic pressure low. If those rise too much, you are wasting fuel (that is converted into heat).

 

[Tommy]

I get to orbit with 50% to 60% main fuel, depending on cargo mass. The profile is still a work in progress - just haven't had the time to work on it lately. Here's the steps, as they exist now.

Step 1; Launch. You may want to use the airspeed AP set to 200km/s to avoid excessive dynamic pressure and damage to the gear. Maintain about 10 to fifteen degrees of pitch and turn to heading as quickly as possible. Max speed during this step is 200 - 225 km/s

Step 2; Initial Ascent. Our goal here is to get above 10k altitude. The lowest portion of the atmosphere is thick, and drag increases dramatically with velocity, so we don't want to gain velocity until we are above 10k altitude. This gives us a limited airspeed, but we can choose the ascent rate. Drag is an acceleration, and "accumulates" over time - and the same goes for gravity. This would dictate a very steep pitch so that getting above 10k as quickly as possible will result in less losses due to drag and gravity. However, with an XR-2 we could also use a slower ascent rated (at a low pitch) and throttle down to maintain airspeed. This would allow us to use the wings to counter gravity rather than the engines. However, lift does create some drag (and we will be climbing) so it will take quite a bit of fuel to maintain speed. Optimum is somewhere in between, but much closer to the steep side.

For my initial ascent, I pitch up to 60 - 70 degrees (more payload = less pitch), and throttle down a bit if I need to keep ACC slightly positive. I ensure that I am still on heading, and wings level, then I let go of the stick.

Step 3; Transition. Once I get to 10k altitude, I use full throttle and add some downward elevator trim - about halfway (more if you are "empty") with the goal of being fairly level by 24k altitude. Airspeed will increase - you should go supersonic around 14k.

Step 4; Scram Ascent. We will fly level (if needed) until airspeed reaches mach 3.5. Engage full Scram, and reduce mains - but keep enough on to keep ACC at 5m/s. As scrams began thrusting more reduce throttle until mains are off and ACC is >5 m/s. Trim up to bring VS up to about 150 m/s and keep an eye on hull temps. You may need a bit more than 150m/s of climb - watch the Dynamic Pressure. It should be holding steady or dropping - we want about 15 kP by the time we hit 35k alt. By then we should have reduced our VS to about 100 m/s, and we'll try to keep the DynP at 15kP, and slowly falling, as we continue to climb.

Step 4; Final Ascent. At somewhere around 65k - 70k, our scrams will either overheat or run out of fuel - and we should be at around Mach 18. Shut off scrams, close scram doors, and engage full Main engines. Slowly turn elevator trim until it's one or two clicks in the nose down direction. When your ApA hits the target altitude, shut off all engines. Once above about 125k you can engage prograde autopilot.

Step 5; Orbit Circularization. When close to my Ap, a prograde burn to elevate my PeA and reduce eccentricity.

If done properly, you should have very little (less than 5% at most) scram fuel remaining. If you had to shut down engines due to heat, and have much scram fuel left, you've done something wrong!

 

[Robsoie]

I think this could be a great theme for a challenge.

-XR2 Ravenstar with default unmodified settings.
-Goal is to get on a defined circular orbit (roughly).
-Then see how much of main engine fuel you always have once you achieve the circular orbit.
-Obviously provide replay file to validate your record (and help other to learn).

 

[n122vu]

I just follow the profile in the XR Flight Ops Manual:

Ready for takeoff! Push the main engines to full power. You will notice the Y
axis G meter fluctuating as you roll down the runway; this is normal.

6. The computer will call out 100 knots, then V1 (which is Takeoff Decision
Speed), and finally Rotate: pull up when Rotate is called out and press ALT-S
to engage Airspeed Hold, or throttle back to about 40% manually. As the
ship goes airborne, by default the computer will call out Wheels Up, which is a
pilot term that means "the wheels are off the ground." It does not mean that
the gear has been retracted. NOTE: you can change or disable the liftoff and
touchdown callouts by editing your vessel’s configuration file; refer to the
LiftoffCallout and TouchdownCallout properties in the file for details.

7. Once you are airborne raise the gear with G. Turn smoothly to 136 degrees
(or whatever your ascent heading is), level out, press SPACE BAR to
disengage Airspeed Hold, and push the main engines to full power. Pitch up
to about 70 degrees. Do not engage the SCRAM engines yet! As you
accelerate you will hear a sonic boom as you pass through Mach 1 unless you
have disabled the sonic boom sound in your vessel’s configuration file.

8. Once you approach 24-28 km altitude, level out smoothly and begin to
accelerate to Mach 3.5 (XR1/XR2) or 4.5 (XR5) and open the SCRAM doors if
they are not already open (CTRL-G, or use the switch on the upper panel)
Adjust the elevator trim controls to maintain level flight (CTRL-. and CTRL-, or
INS and DEL). Once you reach your target velocity, push the SCRAM engines
to full power and gradually throttle back the main engines until they are shut
down. Keep an eye on the thrust generated by the SCRAM engines; if you
throttle back the main engines too soon the SCRAM engines will not be able
to accelerate the ship by themselves. Adjust the elevator trim controls to
maintain about +100 m/s (meters-per-second) ascent. If you want to you
may engage the Attitude Hold autopilot at this point (CTRL-L) to maintain a
smooth ascent profile.

9. As you accelerate uphill keep an eye on the Dynamic Pressure gauge: it
should read about 35 kPa at 24 km at Mach 4. Adjust your climb rate via
elevator trim to gradually decrease dynamic pressure until it reaches ~10 kPa
at 40 km altitude. Then adjust elevator trim or Attitude Hold settings to
maintain 10kPa. Note: you may need to adjust your ascent profile somewhat
depending on the vessel you are flying and its payload mass.
10. When the sky becomes dark switch your secondary HUD mode to mode 2
(press the 2 Secondary HUD button or press CTRL-2 on the keyboard). This
will switch to a transparent HUD with different data fields useful during
ascent. Remember that you can customize each of the five secondary HUD
modes via your vessel’s configuration file.

By default the five secondary HUD modes are configured for (1) reentry, (2)
ascent, (3) atmospheric flight, (4) docking, and (5) on-orbit operations. You may customize all five secondary HUD modes by specifying which data fields to display for each mode as well as the color and transparency settings for each mode in your XR vessel’s configuration file (e.g., XR5VanguardPrefs.cfg).

11. You will notice the fuel flow and, correspondingly, the thrust for the SCRAM engines increase as velocity increases up until the fuel flow reaches the maximum flow rate. The flow rate determines the maximum amount of fuel
the engine can burn, and therefore the maximum thrust the engine can
produce.

12. You will also notice as speed increases the SCRAM TSFC (Thrust-Specific Fuel Consumption) will go down, which means the engines are being more
efficient. As speed increases beyond Mach 7-9, however, engine efficiency
will gradually decrease. This is normal and is due to a combination of two
factors: 1) the shape and design of the engines, and 2) the decreasing delta
between the compressed freestream temperature and the SCRAM burner
temperature as velocity increases.

13. Use elevator trim (CTRL-, and CTRL-. or INS and DEL) to maintain a smooth, gradual ascent as you accelerate. Adjust ascent rate to keep dynamic pressure around 10 kPa. Try to fly smoothly by making small adjustments to the elevator trim. As your velocity and altitude increase, adjust your ascent rate to keep lowering dynamic pressure gradually below 10 kPa down to about 4 kPa. For optimum efficiency you want to have enough oxygen for the SCRAM engines to operate at peak thrust but still have minimum air resistance.

14. If you fly your ascent correctly (and assuming no payload), at about 60 km you should reach mach 20+ before the SCRAM engines approach their
thermal limit or run out of fuel, with ~60% main fuel remaining (with the
default fuel ISP settings). You may want to keep a few percent of your
SCRAM fuel in reserve in case you need it later during reentry if you are short
of the base by a few hundred kilometers. If that occurs, remember to engage
the SCRAM engines at velocities greater than mach 4, and preferably about
mach 7-9; below mach 4 the TSFC is much higher because the engines make
much less thrust for a given amount of fuel. The engines operate at peak
efficiency around mach 7-9.

15. Once you shut down the SCRAM engines, close the scram doors! (CTRL-G,
or use the switch on the upper panel.) If you don't do this and you engage
the main engines, the SCRAM engines will begin to overheat because the
temperature of the diffuser (which compresses the incoming air) is directly
proportional on the velocity of the freestream. Overheating the SCRAM
engines may damage them or even destroy the ship if you overheat them too
much!

16. After you close the SCRAM doors, push the main engines to full power and
pitch up about 10 degrees initially. Then adjust pitch to enter your desired
orbit as you ascend smoothly. Once your apoapsis altitude (ApA on the
Orbiter MFD) reaches 200 km, shut down the main engines. Note that you
are not in orbit yet, but you have to wait until you reach apoapsis (the “high
point” of your orbit) before burning your main engines again to raise your
periapsis (the “low point” of your orbit).

I usually end up with between 50 and 60 percent main fuel remaining, with about two to five percent SCRAM fuel remaining.

 

[Robsoie]

I usually get roughly 55% main engine fuel left in a circular orbit by doing the following :

-On ground Full Main Engine
-Start the Attitude Hold autopilot
-At 100km/h set the Attitude Hold pitch to 70 degree
-Gear up when your XR-2 is taking off
-Wait until you reach 20 km altitude
-Set the Attitude Hold pitch to 5 degree
-Wait until Mach 4 (roughly 1250/1300 kmh)
-Open Scram door and full Scram engine
-Shutdown the Main Engine (progressively so you keep the Acc.z always above 0)
-Don't worry if you lose a bit of altitude, it's going to go up again very quickly
-Wait until you reach altitude 70 km
-Full Main engine, shutdown Scram engine and close Scram door
-Set the Attitude Hold pitch to 45 degree
-Wait until the Orbit MFD display your target ApR
-Shutdown Main Engine
-Go Prograde
-Deploy Radiator to avoid a nasty surprise and shutdown APU to spare it
-Wait until you're near your target ApR
-Full Main engine to build your circular orbit by matching your PeR to your ApR

You should have roughly 10/15% Scram engine fuel left with this and usually 55% Main engine fuel.

 

[Wishbone]

Any SCRAM fuel left is dead mass. If you don't run it to depletion, you're doing a sub-optimal ascent (provided you won't use more main engine fuel to get from SCRAM cutoff to LEO).

EDIT: this is true for handwavium-based craft, in real life there are performance reserves and some fuel that simply cannot be spent. Plus, safety would dictate not running the pumps dry.

 

[Grover]

i thought it was impossible to use all the scram fuel in an XR2 thats NOT fuly loaded, since you accelerate faster, so your efficiency (proportional to velocity compared to freestream) reduces faster, and your engines heat up real fast...

anyway, there are some easy ways to do it, and im sure youve heard them all already, just keep fiddling around with your ascent, and AIM TO GET RID OF ALL YOUR SCRAM FUEL, even if you need to dump it (try to predict how much longer you can use scrams for, then drop the fuel earlier, gives better efficiency (for what goes into the engines anyway). i usually do a check, if scram temperature is above 4000k when scram fuel is at 50%, i need to drop 10-15% fuel

 

[Robsoie]

I usually get roughly 55% main engine fuel left in a circular orbit by doing the following :

-On ground Full Main Engine
-Start the Attitude Hold autopilot
-At 100km/h set the Attitude Hold pitch to 70 degree 65 degree
-Gear up when your XR-2 is taking off
-Wait until you reach 20 km altitude
-Set the Attitude Hold pitch to 5 degree
-Wait until Mach 4 (roughly 1250/1300 kmh) 1000 kmh
-Open Scram door and full Scram engine
-Shutdown the Main Engine (progressively so you keep the Acc.z always above 0)
-Don't worry if you lose a bit of altitude, it's going to go up again very quickly
-Wait until you reach altitude 70 km roughly only 1% scram fuel left (should be at +/- 72 km altitude)
-Full Main engine, shutdown Scram engine and close Scram door
-Set the Attitude Hold pitch to 45 degree
-Wait until the Orbit MFD display your target ApR
-Shutdown Main Engine
-Go Prograde
-Deploy Radiator to avoid a nasty surprise and shutdown APU to spare it
-Wait until you're near your target ApR
-Full Main engine to build your circular orbit by matching your PeR to your ApR

With these corrections i went from 10 % Scram fuel / 55 % Main fuel left to 1% Scram fuel / 59 % Main fuel left, that's a bit better.

 

[Grover]

you should try to keep your altitude no higher than 60km, so your scrams still have air to breathe, even at high speed, they need a fair bit of air, that might help you get rid of all your scram fuel (it works for me at least)

 

[Robsoie]

I gave a try and on my procedure, i lowered my Attitude Hold pitch angle to 2.5 instead of 5 to get less altitude speed and so keep the Scram engine burning while staying under 60 km altitude, but something it wasn't doing when i kept the pitch to 5 is leading into some serious Scram temperature problems.

And in the end, as i have to shutdown the scram engine and use the main engine unlike previously in which i could just burn the whole scram fuel without problem before going back to the main engine.

And i come to the roughly the same result, 58% main engine fuel left (my previous method left me with 5% main engine fuel left), while having additional dangerous situation that wasn't existing in my previous method.

I'll play around some of the speed and angle in my procedure to see if i can keep burning the scram fuel under 60km without getting very bad temperature issues and get more main fuel left.

I'll see if getting rid of 15% of the scram engine before even taking off can help getting on orbit quicker and so sparing even more main engine fuel, as it should make the shuttle a bit less heavy.

 

[Tommy]

The Attitude Hold Autopilot is NOT your friend here. If you attempt holding a fixed pitch, you will get some "rollercoaster" action - and when it bottoms out you can burn up the scrams.

The things to watch are Vertical Speed, Hull temp, and Dynamic Pressure. You want about 150 m/s VS while climbing from 25k - 35k, and be down to 100 m/s by 40k. Keep an eye on temps here - if hull temps climb too rapidly increase the VS a bit, or throttle back a touch.

After 40k, keep an eye on Dynamic Pressure - it should be slowly falling. If it's rising, increase VS. If it's dropping fast, reduce VS. Use Elevator Trim to control the pitch - and never use more than 50% upward trim. If you can't maintain altitude at half trim, then you are too high for your velocity - let the ship settle down a bit and it will begin climbing when the speed increases enough.

You will need to make a lot of trim adjustments during the ascent - watch the VACC and keep it low or you will end up with problems overcorrecting.

 

[dbeachy1]

BTW the SCRAMs can never never overheat as long as you don't burn the main engines at the same time: the computer will throttle down the SCRAMs automatically just before they reach 8000K, so you can run them right to the edge and not worry about it. Personally I like to run Attitude Hold during ascent -- I don't see any "roller coaster" action, but YMMV.

 

[Robsoie]

The "roller coster" effect with the Attitude Hold should only occur if you accelerate time (even only at X10), but in real time i didn't observed this problem during my ascents.

It's a very very usefull autopilot to me.

Tried a few more attempts at an orbiting flight starting from KCS with some variations in my manoeuvers, but have not yet succeeded to save more than my best "59% main engine fuel saved" from using my ascent method.

Someone can describe a method to get over 60% main fuel saved ?

 

[n122vu]

Any SCRAM fuel left is dead mass. If you don't run it to depletion, you're doing a sub-optimal ascent (provided you won't use more main engine fuel to get from SCRAM cutoff to LEO).

Which is why I always dump the remaining SCRAM fuel before engaging main engines after scram ascent. I've never needed them on reentry, so I figure why carry the fuel?

 

[Grover]

ive used it once or twice, on a couple of my poorer re-entry attempts, so i might choose to keep a little, but if youre always fine after re-entry, you shouldnt worry

 

[jangofett287]

BTW the SCRAMs can never never overheat as long as you don't burn the main engines at the same time: the computer will throttle down the SCRAMs automatically just before they reach 8000K, so you can run them right to the edge and not worry about it. Personally I like to run Attitude Hold during ascent -- I don't see any "roller coaster" action, but YMMV.

Why isn't this in the manual? *Runs off to try it*

 

[Grover]

well, i can assure you its true:

just run the scrams at full tilt with no payload, when the woman starts going crazy, dont touch the scram throttle, and watch the scram fuel flow drop

 

[Robsoie]

That's great to know the XR-2 is regulating the Scram temperature automatically, when the XR-2 voice always started to warn me in panic, i was always shutting down Scram as soon as possible to avoid disaster.