Mach 5+ below 30 km Altitude

[derbleifuss]

I've recently taken up the Challenge of the Mach Ribbons from the Final Frontier Mod. While some of my Rocket Planes can reach Mach 4 and land safely and reach Mach 5 (seems to be something like 2.5 km/s at 25 km) with serious overheating Issues I never even came anywhere near Mach 6. Looking for advice or Craft Files to check out and reverse-engineer them.

[Dr. Omicron]

use one of the small heat shields at the front of the nose and off set it and turn it so its not a brick

[Laie]

On 1/17/2020 at 11:56 AM, derbleifuss said:

While some of my Rocket Planes can reach Mach 4 and land safely and reach Mach 5 (seems to be something like 2.5 km/s at 25 km)

If you play with stock physics, it's much slower than that. Mach 6 is about 1800m/s@24km.

Perhaps FF requires that you not only reach a certain speed, but maintainit for some time for some duration (10 seconds, one minute, whatever)? Then get yourself a mod that will show your current Mach number (Kerbal Engineer will do nicely) and stop accelerating one you're fast enough.

Regarding heat: the simple solution, sufficient most of the time, is to not use a cockpit as your nose cone. The nose is the part that gets the hottest, by far, and you want to use something that can stand more heat than a cockpit. Intakes are the obvious solution.

The above screenshot was taken during the Around the World in 80 Minutes Challenge. The original craft has been lost, but here's something very much like it: this vessel is able to go around the world at a little short of Mach-6. An inline cockpit survives the heat for about 20 minutes or so, but the flight takes longer -- the only way to protect the crew was to stick them into a cargo bay.

[derbleifuss]

I am currently working on a Video where you can see my recent Concepts. All three of them use the Mk 1 inline Cockpit and the Advanced Nose Cone Type A or this longer, Nose-Cony Adapter Tank (1.25 m-0.625 m) which's Name I forgot and the Small Nose Cone. Along with the Video I will also upload the Crafts to the Steam Workshop.

My best so far is 2.4 km/s at 28 km Altitude. But Final Frontier still considered that as Mach 4. The only Time I got the Mach 5 Ribbon is when I accidentally de-orbited an Interplanetary Ship that was coming from Duna and hit the upper Atmosphere at over 3 km/s. Surprisingly, all six Kerbals on Board survived that Inferno thanks to EVA Parachutes

[swjr-swis]

2 hours ago, derbleifuss said:

My best so far is 2.4 km/s at 28 km Altitude. But Final Frontier still considered that as Mach 4

Well, FF is calculating incorrectly then, probably with a fixed speed of sound as measured at sea level. It probably doesn't take into account that altitude has an effect on Mach numbers (because the speed of sound is slower in the lower density gas found at higher altitudes).

 

9 hours ago, Laie said:

hen get yourself a mod that will show your current Mach number (Kerbal Engineer will do nicely) and stop accelerating one you're fast enough.

No mod needed. In the stock game, open the debug menu (default is Alt-F12 or Mod-F12 on PC), click on Physics/Aero, and enable "Aero Data in Action Menus". Rightclick almost any part on your craft, and you will notice it will be showing some extra data, including the Mach number. Pin that menu and you have a continuous display to keep an eye on and to use in screenshots for proof.

[derbleifuss]

2 hours ago, swjr-swis said:

Well, FF is calculating incorrectly then, probably with a fixed speed of sound as measured at sea level. It probably doesn't take into account that altitude has an effect on Mach numbers (because the speed of sound is slower in the lower density gas found at higher altitudes).

Only horizontal Velocity counts. Turns out that even a slight vertical Speed has some considerable Effect after doing some Trigonometry

[Raptor kerman]

Two ways of approaching this. Make it accelerate hard enough that it reaches its design speed and slows back down again before heating becomes an issue. To do this with air breathing engines, you're going to need some sort of lite™ heat sink. Even a properly designed rapier missile in my experiences can't get past 1500 m.s^-1 before 

a. running out of athmosphere

b. running out of  acceleration for this to be the case

That is unless you're copying Brad whistance and strap ~40 rapiers to one fuel tank. 

The second, more viable option is to design the plane for maximum heat resistance and minimal drag. Heat shields are out of the question unless you do stuff to them. In my many experiences (mach 5+ planes are my fav' design challenge) your best bet is this:

-minimal aero surfaces, they're the first to go bang from heating. Big-S wing strakes are a good idea as they carry a lot of fuel so you can make a smaller fuselage. No need for loads of flaps, elevators etc. 3 control surfaces max. period. If you can work out a solution for roll then 2. (don't rely on thrust vectoring), powerful reaction wheels are worth looking into. 

-tiny fuselage. Go drone. If you need a kerbal for the challenge, go command chair stuck in 1.25m payload bay. (plus's include better heat resistance and free airbrakes!). Put drone core or kerbal as far back as possible for heat protection. Do you need all that fuel? Heck no. Get it right and a few mk0 tanks work fine. Go Mk1 size fuselage, anything else has too much drag. If you can just get rid of the fuselage by using wet wings. 

-tiny wings. Strakes are the way to go, you're going so fast and high you won't actually need that much lift. Strakes have enough lift for take off, but have low drag. Minimise size, it is possible to just slap one rectangular wing thing, centre it and call it a day, but this is hard. Cause: higher drag. If you're not using wet wings use a pre cooler instead of a normal intake. it works amazing at high speed and carries fuel as a plus!

-heating. THE MOST IMPORTANT ONE. Think of the whole thing as a battery that accumulates heat. Not Just, heat resistant nose as a shield and that's it. Medium sized landing gear are a good way to go for the nose, just remember to remove it from the action group. Long and thin payload bay nose is a good idea, or stacking 2 or 3 short and stubby ones. Test at high speed and see which parts are the hottest and deal with the problem. Engine and nose tend to be the hottest, auxiliary parts such as batteries are usually the first to fail. First thing to look at when selecting parts is max heat. Any important part that's under 2200k is nope. That or above, heating can be manages. 

 

And remember: Remember part clipping is a thing!

[bigcalm]

I was curious about this myself, so I looked into the code.

Firstly, it's horizontal only, any vertical component gets squashed before calculations, but here's the relevant piece for Final Frontier -- 

         public static double ApproximateMachNumber(CelestialBody body, double atmDensity, double altitude, Vector3 velocity)

         {

            if (altitude < 0) altitude = 0;

            // a technical constant for speed of sound appromixation 

            // experimental resolved; feel free to make better suggestions

            double c1 = (altitude / 16000);

            double c2 = (altitude / 39000);

            double c = 1.05 + (altitude / 15000) * (1 + altitude / 10000) + Math.Pow(c1, 4.15) + Math.Pow(c2, 5.58);

 

            return velocity.magnitude/(300 * Math.Sqrt(atmDensity))/c;

         }

A quick punching of numbers through this formula suggests somewhere around the 16k altitude mark you have the best bet of getting the record, as speed of sound is lower around this point, whereas it's pretty high at 30k.  If you can get a plane up to 16k altitude in level flight, to hit mach 6 you'll need a little over 1900ms^{-1}, which should be do-able but perhaps a bit toasty.

At altitude 10.0 ,mach 1 is: 348.8626928292286
At altitude 2500.0 ,mach 1 is: 311.63503674654066
At altitude 5000.0 ,mach 1 is: 313.54556356096964
At altitude 10000.0 ,mach 1 is: 318.82155693104175
At altitude 15000.0 ,mach 1 is: 332.9383851070413
At altitude 16000.0 ,mach 1 is: 324.0240388068514
At altitude 20000.0 ,mach 1 is: 360.43249133697435
At altitude 30000.0 ,mach 1 is: 531.2891953543435

 

 

 

 

[Klapaucius]

@Stratzenblitz75 managed 2032 m/s at 988 metres of altitude. Scroll down on page one of this old challenge, and you will see his entry.

 

 

 

[TheTripleAce3]

9 hours ago, Klapaucius said:

@Stratzenblitz75 managed 2032 m/s at 988 metres of altitude. Scroll down on page one of this old challenge, and you will see his entry.

 

 

 

With cheats he can reach escape velocity at sea level iirc.

[TheTripleAce3]

I'm not sure if Final Frontiers changes any aerodynamics in the game or if you run 120% heating, but running 100% heating I'll tell you that mach 6.5 (ballpark guess for 2,000m/s) below 30km is definitely doable with a decently aerodynamic design, good part choice for heating, 2 Whiplashes, and an Aerospike. 

 

Have fun,

Ace.

[pmborg]

Hello what about this one?

https://kerbalx.com/pmborg/Concord1-Mach48

Edited January 28, 2020 by pmborg