# Flight QuestionXR-2 Engine Specific Impulse or Exhaust Velocity?

#### Wedge313

##### Member
I'd like to perform some calculations using the rocket equation, and I'm trying to find either the Specific Impulse or the Exhaust Velocity for the XR-2 engine. The XR-2 .cfg file shows a Specific Impulse value of 25962. Is that correct? It seems to be very high, as most engines are in the 300-400 range. If that is not an accurate value, I'd appreciate the correct number or the Exhaust Velocity. Also, is there a Specific Impulse or Exhaust Velocity value for the SCRAM engine? The XR-2 flight manual shows a Thrust value but I don't know how to apply that. Thank You!

#### dbeachy1

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Yes, the Specific Impulse values listed in the comments in the XR2 configuration file are correct. As for the SCRAM engines, the logic is very similar to the default DG-S SCRAM engine logic; i.e., the thrust and ISP values vary constantly based on atmospheric density and the vessel's velocity.

#### Wedge313

##### Member
Let me ask this: when I look for information on the dV required to get into earth orbit, the value seems to be 9.3-10 km/s., which gives an orbital speed of around 7 km/s allowing for loss due to atmospheric and gravity influence. Is there a ballpark way to calculate the fuel required to get into orbit that models using both rocket and SCRAM XR-2 engines and accounts for the drag loss? A sort-of "effective specific impulse" I could use in the calculation that would model the dV gained using both rocket and SCRAM that would allow me to calculate the fuel required to get into orbit, if I specify how much fuel I want to have remaining once I get there? Thanks.

#### BrianJ

##### Addon Developer
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The XR-2 .cfg file shows a Specific Impulse value of 25962. Is that correct? It seems to be very high, as most engines are in the 300-400 range.
Be careful not to confuse different conventions of ISP units.
300-400 would be correct for aeronautical/NASA convention of ISP in seconds (s)
Orbiter uses (a more rational?) SI unit convention for ISP in terms of Impulse per unit mass of propellant (Ns/kg)
To convert: ISP(Ns/kg) = ISP(s).g
(where g = standard acceleration due to gravity ~ 9.81 m/s^2)

#### dbeachy1

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Let me ask this: when I look for information on the dV required to get into earth orbit, the value seems to be 9.3-10 km/s., which gives an orbital speed of around 7 km/s allowing for loss due to atmospheric and gravity influence. Is there a ballpark way to calculate the fuel required to get into orbit that models using both rocket and SCRAM XR-2 engines and accounts for the drag loss? A sort-of "effective specific impulse" I could use in the calculation that would model the dV gained using both rocket and SCRAM that would allow me to calculate the fuel required to get into orbit, if I specify how much fuel I want to have remaining once I get there? Thanks.

The challenge with trying to calculate an "effective specific impulse" that covers both the main and SCRAM engines is that it very much depends on the pilot's skill using the SCRAM engines during ascent: the efficiency of the SCRAM engines is highly dependent on how the pilot flies the ship during ascent (ship velocity vs. atmospheric density, etc.). There may be a way to come up with some number for an "average" pilot, but I would have no idea how to calculate that. Maybe some of the math gurus on the forum do, though -- the XR SCRAM engines are very similar to the default DG-S SCRAM engines except for the width of their performance envelope.

#### Wedge313

##### Member
Thanks for the info, that should get me started. I'm still new to this, and while I'm at a point where I'm able to get from A to B, I'd like to do it as efficiently as possible. To date I know I've been lugging around too much fuel. Let's get that corrected! Thanks again.

#### N_Molson

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The XR-2 .cfg file shows a Specific Impulse value of 25962. Is that correct? It seems to be very high, as most engines are in the 300-400 range.

The XR-2, like the DeltaGlider or the Imperial Star Destroyer, is a sci-fi spacecraft.

As said above, a realistic velocity exhaust value in Orbiter for conventional chemical rocket engines is expressed in m/s and should be in the 3000-4000 range (and even 4000+ for LOX/LH2 space-only engines like the RL-10).

#### Marijn

##### Active member
I've done quite a bit of advanced mission planning for the XR series. Basicly, the process is to create a delta-v budget with estimated delta-v values. If your estimate to get into orbit is 9.3km/s, then enter that. Then you test-fly that part of the mission and you measure the actual delta-v change. If it is significantly different, then this is the new estimate. You repeat this process for each stage of the mission. I have been doing this for many years and when you gain experience, you develop a better first estimate.

#### Marijn

##### Active member
My pursuit of this question: After using eyeballed spreadsheets for some years and getting tired of all the manual steps and text editing of .scn files to prepare a mission, I made a webbased XR mission planner, which allows the user to set all XR specific settings and create a flight plan, optionally based on results of the NASA Trajectory Browser (using web scraping as there is no API, but there's a lot more information in the html source of NASA's pages than the UI shows including state vectors and the MJD's of each step of the mission).

And it includes the Asterank browser to provide a gameplay element expressed in dollars. You can search for a big\$ asteroid, then check if NASA has prepared a trajectory for this asteroid and if so, select your XR vessel, add payload and select a mission type.

So it became a mashup application. It allows the user to vary all config and re-run the rocket-equation on each change and see what the implications are. Once all the boxes for fuel and lox tick green, you can download the scenario at every step of the mission including associated files and install it as any add-on by unzipping the download in Orbiter's root folder.

Unfortunately, as a self-taught developer the project wasn't bug free to begin with. I would have shared it earlier if it was ready. And then Google changed the environment where it is running (Google Apps Script). That introduced more hassles. I need to fix things to get it running properly again. But I like to show this vid showing some of the features: