Flight Question What's the optimum altitude for SCRAM engines

[Pipcard]

in kilometers?

 

[SandroSalgueiro]

I'm not much into theory, but, in Orbiter, I've obtained best performance between 25K and 75K (above which it's practically nonoperational) with speeds beyond MACH 3.5.

 

[Tommy]

It depends on the vessel. The stock DG-S operates at much lower velocities than the XR series (and there's some variation within that series as well).

 

[Moach]

it's not as much an optimal "altitude" you get - is more an optimal "ascent profile"

depending on mach number, too low can get too hot or too hight can get too thin, so you should plan your ascent so you keep a balanced dynamic pressure as you accelerate :thumbup:

 

[Hielor]

SCRAM jets work best at an optimal dynamic pressure, which is dependent on both altitude and airspeed.

The short version is that the faster you're going, the higher you should be in order to obtain maximum efficiency (up to a point, obviously).

 

[Pipcard]

Yesterday, I did my first successful SCRAM ascent with the XR2. I followed the operations manual, but I didn't do the elevator trim stuff because that's hard to control. The attitude was controlled manually. I got to Mach 20+ at about 60-70 km altitude. It does save a lot of your main fuel!

 

[IL2windhawk]

congrats.
trim is your friend

 

[Pipcard]

I just watch my hull temps. No trim stuff.

Tex says 400-600°C. (3:00)

 

[rcd463]

SCRAM ascent with XR1

Yesterday, I did my first successful SCRAM ascent with the XR2. I followed the operations manual, but I didn't do the elevator trim stuff because that's hard to control. The attitude was controlled manually. I got to Mach 20+ at about 60-70 km altitude. It does save a lot of your main fuel!

I am flying the XR1 and just reaching Mach 18 at 63Km the temperature at SCRAM engines reach the operational limit. At 62Km the performance drops spectaculary.

Is it possible to go faster or/and higher?

 

[Tex]

I just watch my hull temps. No trim stuff.

PART 2: XR2 Wideawake to Brighton Beach Tutorial (Launch to parking orbit)

Tex says 400-600°C. (3:00)

I really should re-film that because it's not the most efficient ascent profile, but it works. Generally speaking, if you don't have at least 400C heat on the vehicle, then you're going too slow for the altitude you're at to effectively use the scrams. There is a scram efficiency gauge in the cockpit which the manual tells you how to read. You'll find that keeping a moderate amount of heat on the vehicle during your ascent keeps the scrams working pretty efficient.

Example ascent profile:

1. Pitch up to 45-50 degrees using main engines until you pass 9km.
2. Begin pitching down fairly quickly to 10 degrees or so and hold it around there until you reach mach 3.5 to mach 4. At this point, you just want to climb enough that you're not seeing plasma on the vehicle, so play with your ascent profile until you find what works best.
3. When you start using the scrams, you'll probably need to pitch down more in order to maintain 600-800C heat on the vehicle. That should give you a nice ascent to near orbital velocity around 6200m/s by the time you either run out of scram fuel or they overheat
4. By now you should be around 55-65km. Use your main engines to complete the orbit and your engines will likely be shut down by the time you reach 70-75km.

Coast up to your ApA and circulize the orbit.

 

[rcd463]

Interesting. I am reaching 65Km at Mach 18.2 burning about 20% of main fuel and about 2% of SCRAM fuel remaining. I think it is a good efficiency climb. (With XR1 of course)

 

[Pipcard]

When I keep it between 400 and 500 degrees, I can reach just over Mach 20 before it overheats.
(Maybe even 300-400 during the final parts of the scram burn?)

(I didn't change any of the scram settings in the config.)