Escape Velocity: The Hypersonic Race

[AdmiralTigerclaw]

If you thought Pod Racing in star wars was intense, go grab your airsick bag.  It's time to challenge KSPers to a racing event that makes SR-71 time trials look like the [REDACTED].

The name of the event is the Escape Velocity Hypersonic Race, or Escape Velocity 3800 km challenge.

In this race, the goal is to circumnavigate the planet Kerbin in as short a time as possible.  And that means traveling at velocities faster than orbital.

You may ask yourself, 'How do I go faster than orbital velocity?  Won't I be ejected into deep space?'.

 

Well, here's how you fly an E.V. 3800 Challenge:

1: Build a spaceplane with excessive engine power you can maintain for a long period of time.  Air breathing or pure rocket.  (No Orion Nuclear Pulse Engine, however.)

2: Fly into the upper atmosphere and attain speed.

3: Roll upside down and use reverse lift to keep your ship in the atmosphere as you throttle up to insane.

4: Find the sweet spot altitude where you don't fly off into an orbital ejection, and you don't get so low that you burn to a crisp.

5: Circumnavigate the planet and track your time.

 

Remember, best time is your goal.  More Speed = Better Time.  But beware.  The faster you go, the more narrow your window altitude becomes.  Too low, too fast, and you fry.  Too high, too fast, and you fly... off.   Into space.  Really, I have to send rescue ships for those who leave the atmosphere.

 

Okay, so the idea came to me from a video I watched someone do a while back in orbiter.  They performed an ultra-high-speed record flight from Canaveral to Wideawake Island by flying an XR-2 raven, upside down, on full scrams in the upper atmosphere, and then burned the rocket engines until they were above ejection velocity, and then maintained the flight at an astounding cruise velocity across the Atlantic.  The guy had to maintain an insane amount of focus and control to keep both 'bad things' from happening for the entire flight.  So this race is more than just design, it's piloting prowess.

 

 

Edited June 8, 2016 by Snark
Redacted content

[Pax Kerbana]

This is a brilliant and original challenge. I may or may not participate, depending on what your pointing scheme is.

Shall there be rules?

[Pds314]

Hmm.. Interesting. It's like upside down unstable hypercruise. Reminds me of my 6.2 km/s infiniglide missile back when we could make those.

Also reminds me of the so-called "coffin corner." too much negative lift and you quickly lose speed and upward force, as well as descending. Not enough negative lift and you quickly gain altitude, and above a certain altitude, you just stall and fly into interplanetary space.

Edited May 12, 2016 by Pds314

[AdmiralTigerclaw]

I don't have much in mind as far as rules go at this time.  Though some common-sense in competition should be expected.  (Don't hack in some kind of absurdly overkill engine and the like.)

 

For some basics, let's begin with something along the lines of the following:

1: No 'invulnerable' equipment, mods or cheats.  If you can't fry, it's not a challenge.

2: No infinite fuel.  And nothing that makes absurdly unrealistic specific impulse values.  (And no Orion Drives.)

3: Use FAR.  I know it's hard for some, but the (more) accurate aerodynamics physics are a must.

4: No multi-stage vessels.  You are allowed your racer, and a booster to get it up into position.  The booster may have multiple stages, but all booster parts must be jettisoned prior to starting your timed run.

5: Starting Altitude is 20 km or higher.

6: Starting position is KSC.  The course is the equator, prograde direction.  Deviation from the course doesn't matter because any deviation slows your time.  The 'Start/Finish line if it comes down to that kind of precision, is the centerpoint of the LVL 3 launch pad.

7: Absolute Shortest flight time is the goal.  Track it.

8: The onus of proof is on you.  Take screenshots or videos of your flight (Videos are best, we want to see the madness).  Take screenshots of F3 reports during your flight, that'll be useful*.

9: Bonus street rocket cred, mad props, and extra kudos to any vehicles that can run multiple laps around Kerbin in race profile.**

 

*Kerbin's circumference is a little shy of 3800 km.  So your ground distance coverage for the 'mission' should meet or exceed that when all is said and done.

** Race profile is between 20 and 70 km altitude, equator prograde, as fast as you can go.

 

 

Additionally, anyone who executes this race utilizing 6.4 Kerbin,  Real Solar System Earth, or any other realistic system size get the street cred to end all street cred.

[Pax Kerbana]

Bummer.

I tried it out last night and I successfully flew upside down with a negative angle of attack (relative to the surface) with a 4x whiplash powered 30 Kerbal SSTO I'm working on.

Shame I don't have any mods. I am thinking hard about getting them tho... the RO pack at least. I'm afraid if I start on mods I'll end up hating the game (too many options, I'll get lost figuring out what I want/don't want).

[Van Disaster]

'Tis the ancient Circumnavigation challenge, only with FAR - so you might want to look over the rules of that but I think you covered it. I haven't done this since atmo heating came along - a hypersonic aircraft  made it in under an hour, so you're probably looking at 40 mins or so. Presumably mods which hold the craft on course like SmartASS or Pilot Assistance are ok? my fingers aren't up to using a keyboard like that for 40 mins.

Did you do a run yourself, btw? generally the thread starter is meant to kick things off. You might also find it comes down to angle of thrust rather than flying inverted... but that's still a challenge.

Edited May 13, 2016 by Van Disaster

[AdmiralTigerclaw]

Manual control is desired to test your mettle.  Though control programs for the joystick impaired are okay to fool around with.  Nothing is being taken too seriously here.

 

Also, if you take 40 minutes, you're not racing the Escape Velocity 3800.  AT 120 km, the orbital period for a circular orbit is 34 minutes and some change.  Lower will just be faster.  So if you can't get your time under 30 minutes, you're not even making it through qualifications.

[MedwedianPresident]

The concept of exploiting lift in order to achieve great speeds in the upper atmosphere is a very interesting one. However, I doubt whether negative lift is possible in KSP (because airfoils are usually vertically symmetric), with or without FAR, but I will try.

 

I think that my "Supercruiser" design will involve a SRB booster for ascent and acceleration and then a relatively low-thrust rocket or jet stage for prolonged flight at altitudes of 30-40 km. However, speeds in the 3000-4000 m/s range probably involve heavy aerodynamic stress and heating even at those altitudes, just as known from interplanetary missions returning to Kerbin. I think that there is some sort of an "altitude corridor" between 35 and 47 km where heating has not yet become a major factor that can end the mission quickly but there is still enough drag and lift to aerodynamically control "pseudo-escaping" objects. Go lower, and either drag becomes too strong and you lose speed or you explode from overheating; go higher, and there will be not enough negative lift anymore, resulting in the need to abort the mission using a deceleration burn.

 

If FAR treats wings as vertically asymmetric and negative lift is possible, another solution may be rotating the wings using robotic parts.

 

 

EDIT: Getting the thing down will be a challenge too, probably. Just point downwards and you'll burn up - powered deceleration or some intricate aerobraking system up to separating the cockpit, decelerating using a SRB and landing on parachutes may be needed.

Edited May 14, 2016 by MedwedianPresident

[MedwedianPresident]

Update: I'm currently working on a design that utilizes a standard suborbital glider powered by a RAPIER which is accelerated using a special multistage SRB arrangement.

 

Edit: It is relatively easy to achieve 4000 m/s using SRB's, especially using the larger ones from NovaPunch, but the we need 4000 m/s of HORIZONTAL velocity. And we all know that trying to point a SRB-powered rocket horizontally during ascent will result in loss of control.

 

Edit 2: I will try a Shuttle-style stack. As usually, SRB's just keep misbehaving.

Edited May 14, 2016 by MedwedianPresident

[QuesoExplosivo]

Why even bother with lift? Couldn't you just angle your ship downwards and let engine thrust push you towards Kerbin? This way, you could fly high enough to not need to worry about heating (until landing, of course).

Or is using "reverse lift" required for the challenge? If it is, how will that be enforced?

[max_creative]

This looks fun. And extremely dangerous... But It's possible, and if it's possible and you get to fly really fast, IM DOING IT!!!

[AdmiralTigerclaw]

18 hours ago, QuesoExplosivo said:

Why even bother with lift? Couldn't you just angle your ship downwards and let engine thrust push you towards Kerbin? This way, you could fly high enough to not need to worry about heating (until landing, of course).

Or is using "reverse lift" required for the challenge? If it is, how will that be enforced?

 

There are two ways to answer this question.

The first is to answer why...  or rather, why not.

And that is that every second of thrust spent pushing your ship 'down' is a second of thrust not spent Going Faster.  IoW: Wasting fuel, being heavier to carry said fuel, that kind of thing.  The negative lift is all but a free counter to an ejection trajectory compared to the fuel consumption needed to stay the course.

The second answer is simple, when you turn in your shots that have your F3 screenshot, your maximum altitude achieved will be shown.  If you're trying to play the system, the first thing that's going to be obvious is an altitude of 70 km or greater:  Proving you either violated the spirit of the challenge, or you're a poor pilot who can't keep their ship under control.

 

Of course, if you want to use your method for the challenge and you succeed, good for you.  Though you succeeded in the most inefficient manner possible...  I'm not going to harp on you about it.

[MedwedianPresident]

I built my first semi-functioning space shuttle, and the ascent stage seems to be very good. Is it a good idea to use a "tuned" space shuttle able to reach escape velocity for this? By "tuned" I mean like doubling the amount of SRB's, replacing OMS with real engines, etc...

[MedwedianPresident]

I also wonder whether the lift surfaces must be large or whether the normal skimpy airfoils a shuttle has would be enough.

[Cunjo Carl]

Hm. My initial attempts have me topped out at about 2000m/s for long haul (round Kerbin) flights at ~20km. I haven't seen any actual numbers, but What kinds of speeds are we anticipating? It sounds like 3500m/s, given the escape velocity. If so, I think I'll switch up my design rules!

(Edit)

    So, I made some serious improvements to the craft and noticed it didn't speed up any, which was frustrating. So I looked in the config files, and it turns out there's a hard cap on the speeds you can attain using air breathers- set just about at 2000m/s, which is gratifying! It's in an array called velCurve. I took some time and tried to tease some bugs into the engines so they'd forget how fast they were going and keep going faster, but they seem to not be spoofable, unlike the intakes. :/ Worth the shot, though! So that takes air breathers out of the running for propulsion options.

   The only trouble is, even at very high speeds (3.5km/s), this'll take atleast 10 minutes to run round the planet, and 10 minutes of rocket fuel for a single stage is very.... big. Unpleasantly big, in fact.

   So I made an ion probe to run at about 3.2km/s just above the atmosphere, but then noticed the OP was not interested, understandably. It'll work great for another project, though.

   All that said, I think I'm out of options for propulsion! Does anyone else have a plan for overcoming these hurdles? If so, I'd love to help work on them!

Edited May 23, 2016 by Cunjo Carl
Add'l information, next day; not worth the dreaded double post.

[Cunjo Carl]

Of course! Nervs. I always forget about them. They're kindof an awkward in-between for the pleasant TWR of chemical rockets, and the lovely ISP of ions. That said, I think they're the rocket for us, because they're the only thing with enough thrust to make a difference and enough deltaV to last the trip. Here's the starters!

The craft travels at 2500 m/s and rides around Kerbin perpetually at periapsis using the negative lift of its wings. It keeps from overheating by using an inflatable heat shield as a nose cone, which shields the sensitive pods and fuel tanks behind it. The negative lift is obtained through 6 big delta wings and six big elevons to control attitude.  It has a tendency to explode, but I think it's the spark of an idea that can be refined into a success! I'll keep plugging, but if others want to try, it seems NERV + shuttle parts will be the way to go.

 

Edited May 23, 2016 by Cunjo Carl

[Cunjo Carl]

Success! It took a few tries, but I made it round in 27:41. The primary challenge was with my keyboard controls, disengaging the launcher at mach 7.5, and a UI glitch which caused me to not transfer some fuel correctly. All said though, I'm quite pleased! I'll probably be uploading some of the math that helped me conceptualize the optimal design rules, as well as a video in my style tomorrow. The next step (though I'm not sure I'll tackle it), is to make a plane which optimizes for parasitic drag and L/D ratio. Does anyone here have experience with mesospheric hypersonic flight in FAR?

Edit: As a note, I think we can push a bit faster heading the retrograde direction round Kerbin. Would that be ok, OP?

 

 

Edited May 25, 2016 by Cunjo Carl

[Cunjo Carl]

I'll probably be switching to another challenge soon here. I think the slowest plane challenge is calling now! Between my first and final designs went a lot of mulling and fingernail chewing, whose results I thought I'd share. It should provide some points of interest for others designing the fastest planes yet to come! I think the first 3-4 equations are nice for tuning intuitions, the later ones I found interesting but ultimately less useful. I hope they help! I'll post the video soon as an edit to this post so I don't have 5 posts in a row. I'm a bit embarrassed by 4 to be honest. But onwards!

 

Spoiler

 

As a rocket guy, visualizing the design challenge was tricky, and in a few cases counterintuitive. For that reason I started from simple principals and thought of what the race is actually accomplishing. The race is more or less about maintaining a fast but ‘steady state’ of flight. Because the race is 20 minutes long, any additional speed we have in the beginning just doesn’t last. These equations are based around the concept of maintaining flight conditions across the 20 minute race, so things are easy and steady the whole way through.

 

Terms are on the bottom!

 

Eq. 1: Maintaining Altitude
We are going waaaaay faster than we should (orbital speed), so centrifugal force is trying to throw us off of Kerbin and into space. To prevent this from happening, we’ll use our plane’s lift and its gravity. When these three forces balance out, we maintain our altitude, which is our goal. The equation below shows the lift and gravity working together to balance centrifugal force.
F_C = F_L + F_G

 

Eq. 2: Relating Lift and its troublesome tag-along induced drag.

The lift we’re creating to hold us down to Kerbin comes with a nasty tag along. The lift generates ‘induced’ drag on our craft. The amount of induced drag scales with the lift, but is surprisingly much less than the lift. The ratio between the two is called the L/D ratio, and it seems to be about 4-5 for my wings. It seems odd though, how can wings possibly make more lift than drag, isn’t that making free energy? It feels like it, right? However, the drag is pulling directly backwards against your velocity, lowering your energy. Meanwhile the lift is working at a right angle to your velocity, which doesn’t change your energy at all, only your direction. @QuesoExplosivo asked ‘why bother with lift’, which was an excellent question! The L/D ratio is a measurement of how much better our craft will hold to Kerbin using lift, rather than just by pointing engines down to Kerbin. Now that said, it comes with its own troubles. Humorously, the ‘enhanced orbit’ ion probe I spent 7 minutes on is still way faster at circumnavigating Kerbin than the hypersonic plane I spent 7 hours on, but you get the idea!
F_L = ^L/_D F_ID
 

Eq. 3: Maintaining Speed
Our craft is slowed down both by induced drag and parasitic drag, which is just drag we get naturally by going through the air quickly. We need to generate forward thrust to counteract them or we’ll slow down and fall. When these three forces balance out, we maintain our speed, which is also our goal.
F_T = F_ID + F_PD
 

Eq. 4: Putting it Together, Relating Thrust to Velocity

Putting these together, we can see the two forces we need to counter are centrifugal and parasitic drag. You can consider the (F_C-F_G) term to mean our the force caused by our excess in speed over orbital. These scale with the square of your velocity, so a little faster can make a big difference. The (F_C-F_G) term depends on your mass, in the familiar way of orbiting for KSP rocketeers. The F_PD term doesn't depend on your craft's mass, only its shape and speed, in the familiar way of cruising for KSP aeronautics folk. This distinction makes a big difference in how they use fuel in your craft. As a note, if F_PD is greater than (F_C-F_G) (1- ^L/_D ), then we're loosing more to parasitic drag than we're gaining from our lovely ^L/_D. We're better off with an ion probe! This is the case for me, but hey who's counting!

F_T = (F_C-F_G)/ ^L/_D + F_PD

 

This part is skippable! We can plug in the familiar values for centrifugal force, gravity and drag.

F_C = m* (v_plane+v_rot)²/r_tot

F_G = m*g(r_tot/R_Kerb)²

F_PD = c_p* v_plane²

F_T = (m/ ^L/_D )*((v_plane+v_rot)²/r_tot - m*g(r_tot/R_Kerb)²) + c_p* v_plane²    or, a little nicer for rocket minded people... F/mg = TWR so

TWR_plane = (1/ ^L/_D )*((v_plane+v_rot)²/(g_o*r_tot) - (r_tot/R_Kerb)²) + (c_p/mg_o)* v_plane²

So what does this tell us? Well, the v_plane+v_rot term near the beginning of the later two equations says that the centrifugal force will require more lift (=thrust) to counteract when traveling in the prograde direction around Kerbin. Also, the m in the  (c_p/mg)* v_plane²  term for parasitic drag in the last formula says that if you can make your craft more massive while maintaining its parasitic drag and TWR, you will go faster!

 

Eq. 5: Fuel Guzzling

We use an amount of fuel governed by the engine's Isp. This relationship is important because it infers how much fuel we'll need to bring with us. My ship was 70% fuel by weight at the start, I think!

^dm/_dt = -F_T/I_sp

 

Eq. 6 Mass of a craft over time given a steady velocity and altitude

Using a bit of ODE calculus (or the physicists' favorite, guess and check!) We find the solution for Eq. 5's integral. This equation tells us how much gas we'll use going a certain speed given a certain wet (starting) mass. So what's interesting in here? For starters, the important I_sp term in the denominator of a and b means that you can make up for a low Isp by getting a high ^L/_D and also a low c_p -> low F_PD. If a is high, then most of our thrust is going in to fighting centrifugal force (=going fast) which is good! Unfortunately, when a is high, the mass decreases exponentially with time, which is painfully fast in the start. When a is low instead, most of the thrust goes in to fighting parasitic drag instead, and the  ( e^-at - 1)/a term approaches simply (-t). In other words if our velocity and thus a is low, the mass decreases linearly with time, which is nice but means we're mostly wasting our time fighting parasitic drag.

m(t) = m_o*e^-at + b*( e^-at - 1)/a  where

a = (F_C-F_G)/ (^L/_D * I_sp)   

b = F_PD/ I_sp

 

Eq 7: How much fuel should we bring. Also where I stop!

Thank you wolfram alpha for solving the Eq. 7 for time... The equation below will tell you how long your fuel will last for a given flight conditions, wet mass and dry mass. You'd probably never use it  I never did! Can we tell anything interesting? Hm. It approaches ( m - m_f )/b for low a, so linear flight time for additional wet mass, but we knew that. I feel like there's a gem of a goodness factor hidden in there somewhere, but heck if I can find it! If anyone else like's an engineering challenge it might be a fun one to look in to.

t_f = ln((m_o+b/a)/(m_f+b/a))/a

m_f = m_oTWR_plane /TWR_engines + (1/9)( m_o -  m_f ) + m_{coreparts         The dry mass is the weight of the engines plus the weight of the fuel tanks, plus the weight of the core parts.}

 

Terms:

F_C  The centrifugal force trying to pull us away from Kerbin into space

F_L   Our force of lift trying to hold us down to Kerbin

F_G  The force of gravity holding us down to Kerbin

F_ID The force of induced drag caused by the lift we're making.

^L/_D  The lift over drag ratio. For our purposes it's a constant somewhere in the 3-6 range. The bigger the better!

F_T  The force of thrust exerted by your engines. For the sake of sanity, we assume it's straight forward rather than tilted.
F_PD The force of parasitic drag, constantly slowing us down. It scales with the square of velocity and linearly with pressure (so exponentially down with altitude)

^dm/_dt How much fuel we use. (kg/s)

I_sp   The familiar value of rocket efficiency

v_plane  The velocity of the plane relative to the surface of Kerbin

v_rot   The velocity Kerbins surface as it rotates

r_tot   Your altitude plus the radius of Kerbin

R_Kerb The radius of Kerbin

a        A convenience term used to describe fuel used due to fighting centrifugal force

b        A convenience term used to describe fuel used due to fighting parasitic drag

m_o     The initial 'wet' mass of the plane

m = m(t) The mass of the plane as it lowers over time

t_f        The time at which our fuel runs out

m_f       The final 'dry' mass of the plane

 

 

 

 

 

Edited May 26, 2016 by Cunjo Carl
Editor didn't handle 0 as a subscript so I swapped them with Os...

[cantab]

Seems similar to the old

 But since that IS an old old challenge, nothing wrong in doing a new one. The winning entries there were rockets using downthrust to stay in orbit, but a spaceplane did it in 30 minutes. That was back in the old intake-spam-friendly but soup-drag aerodynamics though.

[AdmiralTigerclaw]

Nice run @Cunjo Carl So I guess that puts you in the top slot as the fastest... and only completion of the challenge. 

27:41 and you held an altitude around 33 km and about 2500 m/s?

 

 

Anyone think they can go higher and faster?  I'm thinking if you can hold it around 40 km, you might be able to get 3 km/s.

[AlexSilver]

@AdmiralTigerclaw, what about procedural wings and parts?

[AdmiralTigerclaw]

3 hours ago, AlexSilver said:

@AdmiralTigerclaw, what about procedural wings and parts?

Give it a shot.

[Cunjo Carl]

@AdmiralTigerclaw Any word on going the retrograde direction around Kerbin? It speeds things up by a few hundred m/s, which makes a pretty big difference, both for speed and for how much fuel needs to be brought along.

[AdmiralTigerclaw]

Doesn't that only speed it up because you're counter-direction to a moving object (kerbin's rotation)?

[Cunjo Carl]

9 minutes ago, AdmiralTigerclaw said:

Doesn't that only speed it up because you're counter-direction to a moving object (kerbin's rotation)?

Yep. My favorite way to look at it is, if you're going prograde, the finish line is moving away from you at 175m/s, but if you're going retrograde it's coming towards you at 175m/s. So's the wind, though- Parasitic drag will still be felt on the craft's extra speed, but drag caused by fighting centrifugal force will be lowered by the 350m/s.

  I should note, I think the challenge is nice both ways, so I'm not pushing one way or the other! If it's prograde, it's more about maximizing your L/D ratio and packing enough fuel. If it's retrograde, it's more about being fire-resistant and having nice thrust. But, retrograde is faster, so of course I we gotta think about it!