Question Realistic settings for XR2 Ravenstar

[Sky Captain]

I have decided to configure my XR2 to the most realistic setup what would possible with today`s and very near future (say 10 - 15 years) tech, so I`m asking for help.
Here is parameters what I`m interested in

Empty Mass - default value is 16080 kg. Is this realistic and if not what would be the approximate mass of the XR2 if it were built from best materials available today?

Main Fuel Tank Capacity - default value is 13396 kg for main and 3350 kg for scram fuel. What I`m interested in is the supposed volume of internal fuel tank so I could adjust the mass of fuel according to density of fuels used in today`s space launchers. Also it would be good to know the volume of optional payload bay fuel tanks and the payload bay itself.

Scram Engines - are their performance on default settings realistic or not and what tweaks would be necessary to make them perform as realistic as possible. Or it might be better (from the realism point) to ignore scrams and simply use the freed internal volume for more main engine fuel.

 

[Usquanigo]

Some good info in this thread - http://www.orbiter-forum.com/showthread.php?t=6107

It depends on what you choose to mean "realistic" really. One could argue that something of it's size would never be SSTO. But then that brings up the issue of launch vehicle. The TX Winged Launcher is a great concept, but it's orbital capable on it's own. And suddenly you're faced with - if it can carry the XR into orbit by itself, why bother with the XR then? Why not replace it with pure cargo?

So while I think the TX is absolutely bad ass, it, like the XR, is just wildly overpowered.

As I mentioned in that thread, I tweaked the XR2 down to the point where you could make it to the ISS with 10% fuel left in the mains. If I were to tackle this now, I'd take it further so that it would be left just short of the ISS from a runway takeoff, requiring something like a DragonFly to ferry cargo and/or passengers up to and back down from the ISS. Even THAT is a bit on the outlandish side for something so small, but it would at least "feel" better than the default settings (which is what I figure you are after).

Alternately, I might even be inclined to make it require either Energia or velcro rockets. The downside being that you almost render the scrams redundant by doing so (although they would be good for the return home if you are off and still high and fast enough to do something about it).

 

[Sky Captain]

I`m thinking of setup where you need some kind of booster to get XR2 above the densest part of atmosphere and up to some 1.5 - 2 km/s speed so the XR2 can finish the ascent under it`s own power, but with only just enough fuel left to reach max 1000 km high orbit and rendezvous with orbiting space station.

 

[TSPenguin]

There is an addon that straps an XR1 to a shuttle SRB wich brings it to scram start.
I think there was still some dV left, so it might work for the XR2 in a similar fashion.

 

[loading]

I had the same idea a while ago.

I used an B2 from orbithagar and Universal Cargo Manager by kulch to attach the Xr2 to it.

The B2 was good to an altitude of about 20 km and mach 3 - perfect to start the scram. (i know the B2 is not capable to do this, but it looks kinda good with the stealth skin )

I was able to get an orbit using the "scram and main tank neccesary"-tank config and an ISP of about 4500. (reduced RCS and O2 tank as well)

One thing about the scram bothers me:
In the config-file the Scram fuel heating value is much higher than the value for hydrogen.
when i tested the same config with the H2-value i missed orbit by about 1.5 km/s, if i recall correctly.

another thing:
If the Xr2 was an real ship im quite sure there would be no retro or hover engines, and the payload bay might also be replaced by an internal fuel tank. (weight savings)
Eventually could dbeachy add an additional option to the config file to disable the payload/retro/hover doors. The tank could easily created using the payload system.

 

[dbeachy1]

Eventually could dbeachy add an additional option to the config file to disable the payload/retro/hover doors.

Actually you can already do that right now by making the following changes to your scenario file in each XR2Ravenstar section:

RCOVER -1 0.0000
...
HOVER_DOORS -1 0.0000
BAY_DOORS -1 0.0000

The first number is the door status: -1 = "failed", and so the doors will not be able to move. Or if you'd rather not edit the scenario file, you could always just choose to not open those doors...

 

[Usquanigo]

I had the same idea a while ago.

I used an B2 from orbithagar and Universal Cargo Manager by kulch to attach the Xr2 to it.

The B2 was good to an altitude of about 20 km and mach 3 - perfect to start the scram. (i know the B2 is not capable to do this, but it looks kinda good with the stealth skin )

I was able to get an orbit using the "scram and main tank neccesary"-tank config and an ISP of about 4500. (reduced RCS and O2 tank as well)

One thing about the scram bothers me:
In the config-file the Scram fuel heating value is much higher than the value for hydrogen.
when i tested the same config with the H2-value i missed orbit by about 1.5 km/s, if i recall correctly.

another thing:
If the Xr2 was an real ship im quite sure there would be no retro or hover engines, and the payload bay might also be replaced by an internal fuel tank. (weight savings)
Eventually could dbeachy add an additional option to the config file to disable the payload/retro/hover doors. The tank could easily created using the payload system.

What do you mean about the scram temp and H2? I didn't really follow that.

When you say the B2, you mean the B2 Spirit (flying-wing stealth bomber)? :-o

I'm pretty sure no Wing will ever go trans-sonic. The center of lift moves backward when you go trans-sonic, this requires both leverage and control authority, which is why supersonic craft have "all flying tails". Plus yaw instability is a real issue past M1.5 as well, and a Wing is already handicapped in that area. ....unless it could morph itself somehow, but then it would be of questionable point I suppose.

Pity too because I LOVE flying wings. There is just something about that shape. :hotcool:

I would think using that as a M3 carrier makes the hover engines look minor in comparison in the realm of "realistic". hehe

 

[loading]

the B2 was just the first thing i found and it worked quite well as a proof of concept.
In my opinion when doing things right, an derivate of the XL-70 (the hyperdart launcher) maybe an UAV version of it, sould be used.

I dont have an installed XR2 right now, but i think the Scram value was within the Cheatcodes-Section at the bottom of the config.

Another value to think about might be the thrust of the main engine, because we dont want to melt the nozzle. But i have no idea about realistic values for this one, or to what existing engine we could compare the XR2 ones. (Launcher engines like the SSME are much to big, and existing on-orbit engines might have a hard time getting the XR-2 from 60km alt to orbit.

 

[Usquanigo]

Sorry, I didn't mean where were the scram values, I meant, what does 'scram fuel heating being higher than hydrogen' mean?

Regarding engine comparisons, what about the booster engines on the Energia? they are liquid fueled and seem smaller than SSMEs.

 

[Mindblast]

When you burn all Oxygen of a certain amount of air with Hydrogen you will get a certain energy that is capable to heat that amount of air (and resulting water vapor) to a certain temperature and not beyond. If i'm not mistaken this is somewhere around 2400K temperature rise in the case of H2 and Air.

 

[Sky Captain]

I just compared the physical parameters of XR2 to SR 71 on which XR2 design is based and found the XR2 is about 2 times lighter than SR71 when empty so I think 16080 kg is a bit unrealistic especially because XR2 would require much stronger heat shielding than SR 71. I think a realistic dry mass for XR2 would be ~30000 - 35000 kg

I find the volume of payload bay in the manual and it is 30 m3 It turns out even if whole bay was fuel tank it would hold only 8400 kg of LH2/LOX fuel. Still have no idea how to get accurate the volume of internal fuel tank.

Perhaps dbeachy1 could shed me some light on this very important parameter.

Still haven`t decided what to do with SCRAM engines - use them or not. I read the Wikipedia article on scramjets and there seems to be various technical problems with them, also it seems scram engines have only 2 : 1 thrust to weight ratio. XR2 scrams have much higher performance, It seems unlikely scram engines with such performance could be possible in very near future.

 

[Sky Captain]

So here is the values I have come up with.

EmptyMass=32080

MainFuelISP=4530 (RL 10 engine)

MainTankCapacity=16796.0 (I assume payload bay is in fact a fuel tank)

MaxMainThrust=110000 x2 (2 RL 10 engines)

ScramFHV=1.42e8

However I encountered a strange bug, when I put an XR2 as a payload to shuttle SRB some sort of weird thing appears. When payload is jettisoned it reverts back to normal XR2

Here is my ini file.

[MISC]
cog=26.5
GNC_DEBUG=1
FOCUS=1

[TEXTURE_LIST]
TEX_1=Exhaust2

[STAGE_1]
MeshName="Atlantis_SRB"
Height=38.5
EmptyMass=86183
FuelMass=503487
Thrust=11519800
BurnTime=124
PITCHTHRUST=2500000
YAWTHRUST=1500000
ROLLTHRUST=1500000
angle=0.
off=(0,0,0)
ENG_TEX=Exhaust2
ENG_PSTREAM1=exhaust
ENG_PSTREAM2=contrail
ENG_1=(0,0,-19.25)
ENG_DIAMETER=3.0

[PAYLOAD_1]
Mass=54400
off=(0,3.75,-6.)
Name="XR2-01"
MeshName="XR2Ravenstar\XR2Ravenstar"
Module=XR2Ravenstar
SPEED=(0,15,0)

 

[dbeachy1]

When payload is jettisoned it reverts back to normal XR2.

Do you mean the spawned XR2's mass, fuel capacity, etc. do not match the values set in the config file? As a test, can you try spawning an XR2 via the scenario editor? Any config settings (including cheatcodes) should be applied normally. I just re-tested that here and the spawned XR2's mass matches the value in the 'EmptyMass' cheatcode, as expected.

In technical terms XR vessels parse their config files as the very first thing when each vessel is instantiated, so config-file-wise there should be no difference between XR vessels that are created via scenario files and XR vessels spawned dynamically by the scenario editor or other vessels.

 

[Sky Captain]

Do you mean the spawned XR2's mass, fuel capacity, etc. do not match the values set in the config file? As a test, can you try spawning an XR2 via the scenario editor? Any config settings (including cheatcodes) should be applied normally. I just re-tested that here and the spawned XR2's mass matches the value in the 'EmptyMass' cheatcode, as expected.

In technical terms XR vessels parse their config files as the very first thing when each vessel is instantiated, so config-file-wise there should be no difference between XR vessels that are created via scenario files and XR vessels spawned dynamically by the scenario editor or other vessels.

No you get me wrong. What I mean is when I start the scenario with XR2 strapped to srb an XR2 appears as some weird shape (see the attached image in my previous post) and turns back to normally looking XR2 at the moment of jettison from srb.
As far as config file goes there everything is OK.

 

[Sky Captain]

Here it is - XR2 before and after jettison from SRB

 

[Sky Captain]

I just tried to put XR2 as a payload on velcro rocket and there is the same problem and Orbiter also crashed on jettison which didn`t happen with multistage rocket.

 

[dbeachy1]

Ah yes, now I see what you mean. That is because the XR2's global mesh is decrypted when the first XR2 is instantiated, not when its global mesh is simply loaded by another vessel. As a workaround, you can just add a standalone XR2 at the top of your scenario file. It doesn't need to be in visual range -- you can stick it on Mars if you want.

The reason is that the XR2's mesh can only be decrypted by code in the XR2's DLL, and that cannot occur until the DLL is loaded by the Orbiter core when the first XR2 is instantiated.

 

[Sky Captain]

I just tried and it works!!!

Thanks very much.

---------- Post added at 08:51 PM ---------- Previous post was at 08:39 PM ----------

Tomorrow I will try to see what happens if I use fuel with more energy density per volume than hydrogen or more powerful launcher because with current setup I was unable to reach orbital velocity.

 

[Sky Captain]

Now after trying different settings and launchers I have come up with what works quite well. Since the XR2 has relatively little internal volume I decided to find out what happens if I use more dense fuels than LH2. I decided to use methane for scrams and kerosene+LOX for main engines. It turned out it worked much better, especially for scram engines.
XR2 was still capable of reaching from 1.050 to 6.2 km/s with scrams and with methane 0.54e8 FHV scram engines produce only 0.2 - 0.3 G acceleration meaning the ascent is looong.
I also find out the best launcher for XR2 is XL70 supersonic jet from Hyperdart addon which is capable of boosting XR2 to Mach 3.5 and 25 km altitude - perfect to start scram engines.

So here is my config and scenario files for realistic XR2. Required addons are Universal Cargo Deck and Hyperdart.

 

[Sky Captain]

Now I just uploaded my modified XR2 cfg file and scenario with XR2 strapped to XL 70 jet to Orbithanger.