Breaking 1700 m/s on Jet Engines

[Sivako Aerospace]

Ever since KSP2 came out I have been wondering what the fastest attainable speed that a jet engine powered aircraft could reach is. Because there is currently no atmospheric heating, I felt that an aircraft just powered by jet engines could reach far faster speeds than in KSP1 without having to use aero/ thermodynamic trickery. So, I decided to start the Hypersonic Research Program (with the original goal to hit Mach 5 but later ended up with the goal to hit 1700 m/s) to begin to learn what the current limits were. This project would end up spanning 9 months and numerous versions of KSP2. 

 

Early testing

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During my early testing, I began to discover several key variables that would prove to be useful when making my attempt to reach 1700 m/s. 

Altitude 

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The first parameter was the altitudes I would need to fly at to get maximum engine power. Through multiple tests, I discovered that high altitudes were not going to be helpful when making my attempt. I was getting lower speeds at high altitudes because there was less air for the engines to use. Although there is less air to cause drag at high altitudes, it was not worth the tradeoff. After a lot of testing, I found that the optimal altitude was going to be around 300m above the surface of the ocean.

Engine

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The second consideration was the type and number of jet engine. During my tests I was not sure originally if I wanted to use the Whiplash or Panther engine (because they have similar specs when the Panther is in Afterburner). After testing the Whiplash and Panther I determined that the Whiplash was the better candidate because it seemed to perform better at higher speeds. As a side note in earlier versions of KSP2 it seemed that adding more engines to a vehicle didn't necessarily mean that your top speed was going to be higher. This will become very important later on.

Airframe size

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The final parameter was the size of the airframe. During several test I found that the MK1 size aircraft parts seemed to give better performance than the MK2 parts.  So I ended up settling on this size for the airframe.

After a couple of failed attempts to include all the above parameters I had my first successful test.

HS3

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My first real attempt to break 1700 m/s was with the HS3 (short for hypersonic 3). The HS3 is powered by a total of 5 Jx-4 jet engines producing a combined total thrust of 650kN. This aircraft at the time was fastest aircraft I have designed in KSP2, but it was still not fast enough to break 1700 m/s.

As seen here the HS3 would top out at around 1682 m/s. The design would need some refinement to be able to reach 1700 m/s.

note: The image above was taken in the newest version of KSP2 (I couldn't find my old screen shot of this aircraft during the first few tests).

As mentioned in the Early development section, the number of jet engines was a limiting factor in earlier versions of KSP2. So, I abandoned the project for a few months because when I tested a newer design with more engines (this would become the HS4), I was getting less speed. I though 1682 m/s was a barrier that couldn't be passed, until last week.

Last week I found the save file with this project on it and I decided to have another go at reaching 1700 m/s.

HS4

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The HS4 was originally the second vehicle that failed to reach the HS3's top speed. It has a total of 9 Jx-4 engines producing a total of 1,170 kN of thrust. After finding it in the craft files I decided to take it out of a flight to see if I could determine why, it had originally failed to exceed the HS3's top speed.

 

During the flight I leveled out and began the attempt trying to follow the original flight plan that I had made months ago. I was surprised to see it was preforming better than it originally had. In a slight nose down attitude, I managed to reach 1700 m/s exactly before I had to pull the nose up to avoid the ocean. Having seen that it was possible to reach 1700 m/s on just jet engines I decided to do one final redesign.

HS5 (the final attempt)

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The HS5 is the final design. Like the HS4 it is powered by 9 Jx-4 engines producing a total of 1,170 kN of thrust. The main difference between them is the wings, canard, and vertical stabilizer were significantly reduced in size.

The Flight

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On the runway.

Breaking 1721 m/s in a straight line while flying away from the KSC.

Breaking 1729 m/s on the way back to the KSC (Note: this is my personal record for the fastest jet powered aircraft in any KSP game).

Arriving back at the KSC.

Due to the small size of the aero surfaces, I had to get creative with how to land safely.

 

Final results.

Closing thoughts

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Overall, I think this project was a success. I reach and well surpassed my goal of 1700 m/s and set a new personal record of 1729 m/s. However, now I have to wonder, if I thought that 1682 m/s was an unpassable barrier, I wonder if it is possible to go faster than 1729 m/s. Maybe I'll revisit this project at some point.

 

Edited October 18, 2023 by Sivako Aerospace

[Guest]

Hey I just posted this as a challenge!

I got to 1811 m/s on a RAPIER in airbreathing mode. Maybe I’ll see how fast I can go on a Jx-4.