XR-2 ascent and reentry tips

[george7378]

Hi all,

I am flying the XR-2 to the OFSS on Saturday, but I don't fly it much at all. I was wondering:

- What is the best method to ascend (with regards to fuel-efficiency)
- How do I re-enter?

Thanks.

 

[DarkEnergy]

The most fuel efficient way to ascend in the XR2 is by making use of the Scram engines. They have a separate fuel from the rocket fuel so you do not use up the majority of your fuel during the ascent phase.

To use them, ascend to about 30km using the rocket engines and then level off. At about mach 7-8, open the scram doors (Ctrl+G) and turn them on full power using the lever. Turn off the main engines once the scram engines start kicking in and begin to ascend at about 100 m/s with them. I usually wait until about mach 22 and the scram engines to begin to lose thrust before turning them off. After that you can finish your orbit with the rocket engines. (**Make sure to close the scram doors, or they will overheat! :lol:**)

To reenter I usually use Aerobrake MFD and BaseSync MFD to get the proper trajectory. There is documentation with these that you can look at that will show you their usage (if you don't already know). My usual angle of attack is about 40 degrees but it changes depending on the slope of my descent. For the deorbit burn, you usually need very little slowing down; lower your periapsis to only about 40 or less km above the ground. This gives a good slope for a low orbit (<400km).

I hope this helps you. It took me a while to get my first successful XR2 reentry without burning up (I was so sick of hearing "Hull breach at mach xx!" :rofl. But once you get it, it becomes much easier, especially with the help of those excellent MFDs.

 

[PhantomCruiser]

I do just about the same... It really helps if your sound is loud enough for you to hear the pitch of the scram engines change as they kick in and you really start to tear through the sky. As the pitch (noise) increases, slowly back off the "mains".
I too typically use the attitude-hold autopilot and maintain about 100-110 m/sec positive. The XR's are a dream to fly once you get used to them.

 

[dbeachy1]

For an ascent tutorial, take a look at page 28 of the XR Flight Operations Manual in the section titled, "Walkthrough: Launch from KSC to Low-Earth-Orbit".

 

[Shadow Addict]

For an ascent tutorial, take a look at page 28 of the XR Flight Operations Manual in the section titled, "Walkthrough: Launch from KSC to Low-Earth-Orbit".

I was just about to suggest this. Taught me how to do a pretty good SCRAM ascent.

Also, unless you plan to use them to extend your reentry, I'd recommend dumping your excess SCRAM fuel after MECO but before orbit insertion cause it's essentially dead weight by then.

 

[george7378]

Thanks - I'll try tonight. I would have looked at the operations manual, but I think I deleted it during a cleanout.

During reentry, is it just like the DG4 (i.e.- No rolling is required - just hold at a certain attitude)?
What is it like for interplanetary travel?

Thanks.

 

[PhantomCruiser]

Pretty much, if you can reenter an XR-5 the proceedure is pretty much the same. Use aerobrakeMFD and BasesyncMFD, and bring her on home. Last night I did a night recovery at Wideawake, when I went subsonic I was directly over the field at about 15KM altitude, that gave me plenty of room for an over-field break into the landing pattern for a sweet landing. I wish it were daylight, so I could have recorded it

 

[george7378]

Thanks. I tried the SCRAM ascent, but the Scramjets always seem to overheat...

 

[TSPenguin]

Unless you do an absolutely perfect ascent, your scrams will overheat before you use up all your scram fuel.
To get more out of them, I also open the doors only when I can make use of the scrams and only after I shut off the rockets.

 

[dbeachy1]

Thanks. I tried the SCRAM ascent, but the Scramjets always seem to overheat...

Don't worry, what you're seeing is normal: SCRAM temperature is directly related to velocity, and so once they reach their operational velocity limit they will overheat and lose thrust rapidly. (This is normal.) However, your SCRAMs will never overheat to the point of damage if you are only accelerating under SCRAM thrust. The reason is that SCRAM thrust decreases rapidly as the engines overheat, and so thrust will eventually approach zero (and hence, no more acceleration) before they pass their critical temperature threshold. So you don't need to worry about overheating your SCRAMs while you are just under SCRAM thrust. And if you still have SCRAM fuel remaining when SCRAM temperature hits the limit, congratulations! You just reached maximum SCRAM velocity and still had fuel remaining. :tiphat: At that point you can either dump the fuel or save it in case you need it to reach your base during a botched reentry.

 

[V8Li]

I used to do reentries with the autopilot but I did not find the settings "coarse" enough (I hope you know what I mean ). So I now do it manually and keep the hight AOA shifting the center of gravity manually. XR2 is the one for me...

 

[Tommy]

Here's a couple tips for the scram ascent, based on the method given in the XR manual.

1. At the appropriate time (about 300 seconds TN in align planes MFD for a trip to ISS or Mir, or as directed by another launch aid such as LaunchMFD, IMFD, etc) ensure that elevator trim is neutral and engage full throttle (main engines). When you hear the "Rotate" call-out pull back on the stick. Once you hear "Wheels up" reduce throttle to maintain about 200 m/s velocity and stow landing gear. Turn to your desired heading - maintaining subsonic speeds (around 200 m/s works very well).

2. Once heading alignment is attained, pull back on the stick and increase throttle. Your goal here is to attain a pitch (around 70 degrees, depending on vessel mass) that allows you to use full throttle while maintaining little or no VACC (in other words, maintain speed, a slow increase to 250 m/s is OK).

3. At 10k altitude, engage half to three quarters (nose) downward elevator trim. The exact setting will vary with load-out. You will typically break the sound barrier at around 15k alt. The goal here is to achieve zero VS (level flight) at 24k altitude. Use elevator trim to maintain altitude while you gain speed.

4. At mach 3.5, engage full scram thrust, reduce main engines to maintain about 5 m/s ACC until the scrams can maintain a positive ACC on their own.

5. Use elevator trim to maintain about 150 m/s VS until 30 k alt, then slowly reduce VS to about 70 to 100 m/s. Watch hull temps - you want a bit over 1000 degrees at the nose.

6. When the scrams overheat and no longer provide positive ACC, shut them down, close the scram doors, then engage main engines until a suitible apoapsis is established. Give yourself an extra 5k to account for losses from aerodynamic drag and gravity. MECO should be about 70k -80k altitude. Coast up to Apo and circularize.

I usually get the XR - 2 (at default settings - with CHM and cargo - no additional fuel) into a 205k parking orbit with 5 - 6% scram fuel remaining, and about 65% main fuel remaining.

 

[Shadow Addict]

I'd recommend using the airspeed hold AP (alt+s) while turning to your desired heading. It's the same as what Tommy said but you don't have to manually throttle it down.

 

[Ark]

I take off on my target heading and use the XR's built in attitude autopilot the whole way. Start the scrams at about 1000-1200m/s, maybe 20k in altitude. Keep a pitch of zero to five degrees to milk the thicker air for thrust, pulling up when you start to get ionization on the hull to prevent damage. You should be able to get the vast majority of your orbital velocity just from the scrams. When they overheat and thrust drops off, that's your cue to pull up and make the final insertion burn. Takes some practice, but my goal is to stay as low as possible on the scram ascent without getting ionization.

 

[Snake]

Start the scrams at about 1000-1200m/s, maybe 20k in altitude. Keep a pitch of zero to five degrees to milk the thicker air for thrust, pulling up when you start to get ionization on the hull to prevent damage.

I do exactly the same, adding that after the pull up I try to mantain the Angle of Attack as close to zero as possible, since that is the way the spacecraft generates less drag and you'll be generating a lot of drag while flying under dynamic pressures of more than 50KPa.