Stock KSP .90 intake comparo for SSTO turbojets

[GoSlash27]

I knocked this together for another thread. Figured I'd share it here.

So here's what I did:

I assumed enough surface area to adequately keep a turbojet lit at 43km altitude and 2.3 km/ sec velocity, where terminal velocity and aircraft velocity both exceed orbital velocity at 70km altitude.

I assumed an aircraft built to the standard rules of thumb; 13 tonnes of aircraft per engine and 1.0 lift coefficient per tonne of aircraft.

Finally, I assumed that the intakes themselves were granular. I computed the number of intakes required, the mass, the drag coefficient when running at high altitude and 2.3 km/ sec, and the cost.

Here's what I found:

Stock KSP .90 Intake Comparo for SSTO Turbojets

[TABLE=class: cms_table, width: 500]

[TR]

[TD]Type[/TD]

[TD]Number per engine[/TD]

[TD]Drag coefficient[/TD]

[TD]Mass[/TD]

[TD]Cost[/TD]

[/TR]

[TR]

[TD]Mk1 Fus intake[/TD]

[TD]2.67[/TD]

[TD]7.06[/TD]

[TD].320[/TD]

[TD]$1,922[/TD]

[/TR]

[TR]

[TD]Radial[/TD]

[TD]3.20[/TD]

[TD]10.6[/TD]

[TD].480

[/TD]

[TD]$5,280[/TD]

[/TR]

[TR]

[TD]Nacelle[/TD]

[TD]3.20[/TD]

[TD]10.6[/TD]

[TD].480[/TD]

[TD]$4,960[/TD]

[/TR]

[TR]

[TD]Ram air intake[/TD]

[TD]1.60[/TD]

[TD].354[/TD]

[TD].0160[/TD]

[TD]$4,288[/TD]

[/TR]

[TR]

[TD]Circular intake[/TD]

[TD]2.00[/TD]

[TD].442[/TD]

[TD].0200[/TD]

[TD]$1,360[/TD]

[/TR]

[TR]

[TD]XM-G50[/TD]

[TD]2.67[/TD]

[TD].589[/TD]

[TD].0267[/TD]

[TD]$2,670[/TD]

[/TR]

[TR]

[TD]Structural[/TD]

[TD]6.40[/TD]

[TD]1.13[/TD]

[TD].0512[/TD]

[TD]$5,760[/TD]

[/TR]

[TR]

[TD]Shock Cone[/TD]

[TD]1.33[/TD]

[TD].736[/TD]

[TD].0333[/TD]

[TD]$4,057[/TD]

[/TR]

[/TABLE]

The single most important consideration is drag coefficient. All of these intakes will allow the engine to make the same amount of thrust, but their drag will impact the top speed of the aircraft. Clearly, the fuselage intake, radial, and nacelle are unsuitable for spaceplane SSTOs.

Looking at the rest of them...

The ram air intake is the best choice in terms of sheer performance. On the downside, it's fairly expensive and it's clunky shape can make it a bit of a bear to plug into multiengine designs. Overall, an excellent choice.

The circular intake is an outstanding all-around choice. Low drag, low mass, and excellent price. The only drawback is that you have to fit 2 per engine. This is a problem with all stackable intakes, but at least this one has a low-profile shape. Overall, I was shocked to find out how good this intake actually is.

XM-G50: Unremarkable stats. Average in all respects. On the plus side, it attaches radially, allowing it to be placed nearly anywhere you can find room. This is a huge bonus for high-speed stability, as placing the intakes in the back is very helpful. I have had good success with these, and definitely recommend them.

The structural intake is a problem child. Poor performance at an outrageous price. You can make an SSTO spaceplane using these, but nearly any other choice will give you better performance. On the plus side, it does attach radially (see above) and it's low profile design can make for a more compact and pleasant design. Overall, I do not recommend these.

I was surprised to discover how good the shock cone intake isn't. Average stats, medium-high price. It doesn't quite have the surface area to feed an SSTO spaceplane on it's own, so it really holds no practical advantages over the other 2 stackable intakes. Overall, I do not recommend these.

Using the stackable intakes together in concert with XM-G50 radial intakes will provide a neat, compact design in multiengine spaceplanes. The Ram Air or shock cone will feed an engine fine when assisted by a single XM-G50. The circular intake will need 2 XM-G50s to assist it.

For a single engine design, 3 XM-G50s will do the job, and so will 2 circular intakes.

Best,

-Slashy

Edited January 5, 2015 by GoSlash27

[Dispatcher]

Thanks very much for this information, Slashy! Its interesting what some research can provide in the way of helpful information. Well done.

[Magniff]

Excellent analysis Slashy!

Just wanted to share the results of a test series I've recently run on the same subject.

Unsurprisingly, the outcome is pretty much in line with your analytical results.

To give a brief summary of the test conditions:

I used a test vessel flown by Mechjeb. The ascent-path was roughly adjusted with respect to the special requirements of an air-breathing main engine.

The vessel was equipped with a single rapier engine and the number of intakes was set up to induce roughly the same amount of dead-weight once the vessel arrives in orbit. Additional intakes were added if the vessel failed to achieve orbit in its original configuration (only true for fuselage and structural intakes).

In addition, the vessel was burdened by two tonnes of payload.

Mechjebs flameout prevention and intake management were enabled. Also the angel of attack was limited to 18° via Mechjeb.

Closed-cycle mode was manually engaged as soon as the vessels apoapsis started to drop.

Intake performance was rated by the dV left, after a circular 72.5 km orbit was achieved.

My Results imply the following ranking of intakes according to their effectiveness:

1. Ram Air (dV = 693 m/s @ 0.055 t of intake mass)

2. Circular (dV = 577 m/s @ 0.055 t of intake mass)

3. XM-G50 Radial (dV = 269 m/s @ 0.05 t of intake mass or alternativly dV = 404 m/s @ 0,0625 t)

4. Shock Cone (dV = 249 m/s @ 0.054 t of intake mass)

5. Structural (dV = 88 m/s @ 0.084 t of intake mass; Note: vessel failed to achieve orbit if less intakes were used)

6. Fuselage (vessel finally failed to achieve orbit)

Note: Maximum dV @ an intake mass of zero (no air intakes attached) was 888 m/s.

As already stated, the data generated in this test series is supporting your analytical conclusions very well.

Thanks for the additional insight your analysis has brought into the realm intakes

- Magniff

Edited January 5, 2015 by Magniff