Design a SSTO that uses propellers or rotors instead of jets during atmospheric ascent

[RoninFrog]

8 minutes ago, jinnantonix said:

I am new to chopper design.  I have a bacis design that I think meets the criteria, although I am yet to get to orbit.  Any hints on how to stop the blades from stretching out under centrifugal force?

Are you using Breaking Ground blades?  Breaking ground helicopter blades have an offset center of mass so they resist centrifugal force a lot better than elevons and fins.

[jinnantonix]

Quote

Are you using Breaking Ground blades?  Breaking ground helicopter blades have an offset center of mass so they resist centrifugal force a lot better than elevons and fins

I am using fins.  I'll try the BG parts to see if I get an improvement.

[xendelaar]

2 hours ago, jinnantonix said:

I am new to chopper design.  I have a bacis design that I think meets the criteria, although I am yet to get to orbit.  Any hints on how to stop the blades from stretching out under centrifugal force?

I only encounter blade stretching when I timewarp 2x...4x. so I can't help you out.

5 hours ago, It'snorocketscience said:

 

The same guy (Reddit user u/Chagaran) I linked in my post who built an orbital chopper SSTO actually built a Duna helicopter, but unfortunately it required orbital refueling. Now, he didn't use ISRUs, nor nukes or ion engines, and his design did have some aerodynamic flaws (mk2 fuselages used as a cool-looking cargo bay at the expense of creating negative lift), but I bet he came close. Link: https://www.reddit.com/r/KerbalSpaceProgram/comments/fi03d8/upgraded_orbital_helicopter_now_duna_capable/?utm_source=share&utm_medium=web2x

I just realized something about Duna props—Normally, on Kerbin, large props and rotors require large motors to turn at full power against the drag created by the blades. But in Duna's thin atmosphere, drag is very low, so you can get away with strapping 8 medium or large props on smaller motors, and then add duplicate the setup to create the necessary thrust. If your props and rotors can reach the maximum 460 RPM easily, you need to make them work harder (attach more/larger props) or downsize the motors to save weight. Of course, these overworked props and rotors will struggle on Kerbin, but if you add more of them (you'll need several for Duna, anyway) I think it might just work.

I'm getting close to succeeding with my sleek 23 ton MK1 design. It uses an ISRU and is very drag-optimized—the 2 Jr. drills, solar panels, radiators, and science parts are crammed inside 3 small service bays, thereby eliminating much drag. Plus, I've crammed one small converter, radial ore tanks, and fuel cells inside a fairing, eliminating even more drag. On top of that, I've got the wings angled up at 5 degrees so the plane can stay level without having to pitch up (which would expose the plane's body to the airstream and cause drag). So it's a really slippery plane—you guys should definitely try to minimize drag in your own builds as well.

But all the optimizations in the world can't replace the fact that I need more props. Two motors can't get anywhere near the necessary thrust. Forget "optimizations", It's time for MOAR BOOSTERS!

holy moly that's impressive! this is enough motivation te keep me going!

[It'snorocketscience]

11 hours ago, jinnantonix said:

I am using fins.  I'll try the BG parts to see if I get an improvement.

Not to mention that BG rotors and props are easier to adjust mid-flight than stock props. To get the most thrust, your prop/rotors must have adjustable pitch. (I don't remember the explanation in real life, but for KSP, just experiment with linking blade pitch to throttle or two keys using the KAL-1000 controller and use advanced tweakables to view rotor/prop thrust so you can fine tune the blade pitch at different speeds).

Stock props (that use control surfaces) can adjust their blade pitch mid-flight, but not as much as BG props and rotors can.

Edited April 24, 2020 by It'snorocketscience

[sturmhauke]

The "apparent wind" experienced by the prop blades changes angle based on plane speed and RPM, which effectively alters the angle of attack. The faster the plane is moving, the smaller the angle of attack becomes, so you need to change the blade pitch to compensate.

[It'snorocketscience]

Found this handy forum post for Duna planes:

Also, here are some tips gathered from my own experience and from the thread above:

Lift:

Spoiler

Build large wings if you want to fly to Duna. The thread above suggests 1.4 wing area per 1 ton of weight, which apparently will allow you to take off at 50 m/s on Duna (!!). Now, large wings do create lots of drag on Kerbin, but prop planes can't fly that fast to begin with, not unless you use some math witchcraft. (That linked video by Brad Whistance is rather advanced... I'll stick to my normal props...)
Aerodynamic Control: Control surfaces struggle at low speeds - don't spam control surfaces when reaction wheels do the job better. Two control surfaces weigh as much as if not more than one medium reaction wheel. Find a good balance.

 

Drag:

Spoiler

Reduce it—cram everything that isn't a wing, rocket engine, or cockpit inside cargo bays, or fairings if you're crazy. (Beware, fairings create lift and can unbalance your plane. If you do use fairings, enable interstage nodes to attach stuff inside.) Angle wings up by 5 degrees from the body so your plane produces lift without having to pitch up. If you're using Breaking Ground props, you could hide motors inside fairings to reduce drag. Offset the props outside the fairings so they can actually work.

 

Thrust:

Spoiler

• Use a KAL controller setup to adjust blade pitch to get the best thrust as you fly.
• For Duna, use many motors, and make them work hard. A small motor with big props can reach max RPM on Duna as there isn't much drag or atmosphere to fight it.
• (Untested) Try offsetting your props further away from the motor shaft so the blades travel further with each revolution and create more thrust.
• TWR: The old spaceplane rule of thumb "60 KN of thrust (one nuke engine) per 15 tons of craft" seems to work, given enough rocket fuel, with prop planes flying at lower altitudes on Kerbin. Just try to get above 10K.
• Set up thrust reversers if you're flying to Duna—landing at high speeds and on bumpy terrain isn't safe at all, so it's important to stop fast. Set up thrust reversers by reversing motor direction or using negative blade pitch. I don't recommend air brakes nor drogue shuts, as they struggle at low speeds and add weight. Wheel brakes tend to skid in low gravity. Your best bet to have props capable of reversing thrust. Or just build a VTOL, lol.

 

Edit: Don't use normal props—ducted fans are actually a game-changer. They produce a lot more thrust, significantly increasing effectiveness on Duna and max operating height.The shrouds also come in handy for protecting your props during landing.

 

Edited April 27, 2020 by It'snorocketscience
Added a note about ducted fans.

[Guest]

On 4/22/2020 at 1:09 AM, Klapaucius said:

@Brikoleur?  Your Eve craft?

Not an SSTO.

[KerikBalm]

On 4/23/2020 at 11:27 PM, sevenperforce said:

I have not built any prop planes with the new parts -- do they need the KAL-3000? How do you start them?

KAL not needed, I almost never use it. I just use axis groups, one group for prop pitch is all that is needed. Action group the motors on and off, map torque to throttle like any other engine.

I only use the KAL for varying the torque on my quad rotor designs to give pitch, roll, and yaw control.

Its been a while since I did SSTOs on stock size kerbin.

Like Brikoleur, I wasn't able to make an Eve SSTO, but I was able to make a proof of concept reusable eve ascent vehicle (Upper stage got to orbit, lower stage was recovered, I never further refined it to allow the upper stage to reenter and be reattached to the lower stage)

I've made plenty of rotorcraft that work on Eve and Duna, but they were always payloads for an SSTO, and not SSTOs themselves.

He asked for a:

Quote

type of SSTO that can explore atmospheres with props or rotors, thereby saving fuel and enabling the exploration of oxygen-free bodies, like Duna. This would enable a craft to biome-hop for science or transport resources much more effectively than slow rovers and fuel-guzzling rocket planes/landers can in said atmospheres. While these kinds of biome-hopping and transport crafts do exist, they don't seem to exist in reusable SSTO form, it seems

I make craft that can explore atmosphere with props or rotors, enabling the exploration of oxygen-free bodies, enabling biome hopping and resource transport much faster than rovers (and over bodies of liquid water, as on Eve), in completely re-usable (at the destination) form. They are payload to the destination, and remain at the destination... but they do everything else requested. I make surface to orbit shuttles, and a separate rotor craft as a surface to surface shuttle.

Basically, I'd take something like this (but with more blades for Duna) if I don't need to much payload capacity:

Spoiler

Its a VTOL tilt rotor on Kerbin that can lift an orange tank, but doesn't lift on Duna)

or this if more payload capacity is needed (note, its a modded duna with more gravity and a thinner atmosphere:

Spoiler

and to get payloads to and from the surface I use this:

Spoiler

again, on a modified duna, also scaled up 3x.

Again its a VTOL, but a VTOL tailsitter.

But how does it deploy cargo from its cargobay? robotics are great:

The FAT-455 wings are hinged to act as airbrakes on descent when it comes for a vertical landing, and swing back alongside to act as vertical stabilizers and prevent side slip. The center cargobay is on servos and has telescoping pistons, allowing it to place the cargobay nicely on the ground:

 

But none of that meets the challenge requirements of SSTOing from Kerbin. Even at 1x scale, I never did that much, I don't see the point in taking deadweight along for the ride when you can leave it in LKO and still recover it. I always aimed for reusable over SSTO... to the point that I stopped making SSTOs, and instead made re-usable 2 stage launchers (at 3x rescale, SSTOing is still possible with >10% payload fraction, but really long LV-N burns are needed. Time to orbit and payload fraction were much improved with re-usable 2 stage launchers).

Edited April 27, 2020 by KerikBalm

[xendelaar]

After a week of designing and testing I finally came up with a working version!
I made a Single Stage to Duna vessel with  ISRU capabilities.

HHere are its specs:

• Name:  Honey bee
• Pilot capacity: 1
• Tourist capacity: none
• Weight: 127 tonnes (wet) / 37 tonnes (dry)
•  Delta v budget: 4900 m/s
• Rocket engines: 2 vectors, 1 nuke
• Helicopter: 2 large  shift-able motors with 8 propellers each
• ISRU: yes
• Launch cost: 176k
• Payload: nope      

 

 

It was an awesome challenge, but I did come to a conclusion that Heli-SSTD ships are very impractical and bulky. It is soooo much easier just doing a multiple stage to duna design. But that wouldn’t be a challenge, now would it!

I made a small video of the mission. I hope you guys like it!

 

Edited April 27, 2020 by xendelaar
added some stuff

[It'snorocketscience]

1 hour ago, xendelaar said:

After a week of designing and testing I finally came up with a working version!
I made a Single Stage to Duna vessel with  ISRU capabilities. [...]

Congratulations @xendelaar! Your entry is the first to make it on the list.

Quote

It was an awesome challenge, but I did come to a conclusion that Heli-SSTD ships are very impractical and bulky.

Well, I might just be able to prove you wrong . I was about to give up on my sub-30 ton plane design when I discovered ducted fans. They are so much better than the normal props I've been using.

[xendelaar]

4 minutes ago, It'snorocketscience said:

 

Well, I might just be able to prove you wrong . I was about to give up on my sub-30 ton plane design when I discovered ducted fans. They are so much better than the normal props I've been using.

epic! I can't wait to see your design! My expierence with ducted fans is that they work perfectly on kerbin, but not so much on Duna. Guess I was wrong!

 

Edited April 27, 2020 by xendelaar

[It'snorocketscience]

2 minutes ago, xendelaar said:

epic! I can't wait to see your design!

By the way, does your craft have a name? Also, what's the launch cost? I just want to know for leaderboard ranking purposes.

Is it capable of carrying any tourists (I see it can carry one Kerbal in a capsule, but does it have a probe core if that 1 kerbal is a tourist)? I assume it's not capable of carrying a payload, which is fine (I'm beginning to suspect a payload-carrying prop SSTO is a bit too much to ask for).

[xendelaar]

2 minutes ago, It'snorocketscience said:

By the way, does your craft have a name? Also, what's the launch cost? I just want to know for leaderboard ranking purposes.

Is it capable of carrying any tourists (I see it can carry one Kerbal in a capsule, but does it have a probe core if that 1 kerbal is a tourist)? I assume it's not capable of carrying a payload, which is fine (I'm beginning to suspect a payload-carrying prop SSTO is a bit too much to ask for).

I'll edit my first post for this.

darnit... knowing that you made a vessel 4 times less heavy than mine, makes me want to redo the challenge! haha! thanks for the idea dude! I'm loving this challenge!

[It'snorocketscience]

2 hours ago, KerikBalm said:

I've made plenty of rotorcraft that work on Eve and Duna, but they were always payloads for an SSTO, and not SSTOs themselves. [...]

I make craft that can explore atmosphere with props or rotors, enabling the exploration of oxygen-free bodies, enabling biome hopping and resource transport much faster than rovers (and over bodies of liquid water, as on Eve), in completely re-usable (at the destination) form. They are payload to the destination, and remain at the destination... but they do everything else requested. [...]

Well, an all-in-one prop/rotor SSTO can use its rotating wings to assist landing from and ascend to orbit. I'll admit that that point is kinda moot on Kerbin (jets work) and Duna (barely any atmosphere), but on Eve or Jool, I think it's safe to say that props and rotors one of the most efficient ways out of the lower atmosphere. 

But even in Duna's case, rotating wings could help save dV. I need to test my 24~30 ton design to measure savings on ascent (probably not much), but I know that a suicide burn on Duna costs a few hundred m/s of dV as the thin atmosphere and parachutes don't slow you down a whole lot. Meanwhile, my winged prop SSTD can land without burning any fuel—it lands like a plane and then uses thrust reversers to stop before the bumpy terrain kills it.

Quote

I don't see the point in taking deadweight along for the ride when you can leave it in LKO and still recover it. I always aimed for reusable over SSTO

Fair point, although a reusable/recoverable stage isn't perfect as it requires extra mass in the sense that it requires another engine, probe core/batteries, reaction wheel/RCS setup, landing gear/chutes, and docking ports. It also adds piloting time/complexity to a mission, which is another moot point, but some people really like the "cool factor" and design challenge of a spaceplane that drops nothing except for its payload.

 

But ultimately, the goal of my challenge, besides being a challenge, was to encourage players to push the limits of rotary-wing crafts, and to develop a Eve/Jool SSTO, if rotating wings can make such a feat possible. That would be quite the achievement.

[It'snorocketscience]

Dang, it's embarrassing to not have the first entry in my own challenge. But I'm getting close. Here's a sneak peek of the UNFINISHED "Duna-Phantom IIX":

Spoiler

 

 

I didn't just name it "Phantom" because it sounds cool—the craft file corrupted and disappeared twice during my tests, like a ghost! Thank goodness I made backups. After three days of work and 8 (IIX) iterations, I would've lost my mind.

An ISRU, radial ore tank, and fuel cell array are crammed into that middle fairing (see spoiler). Those three bays at the front hold one little drill, solar and radiator panels, science experiments, and one Kerbal in a chair. It's rather cozy (read: cramped as hell), but I managed to fit everything in with almost no part clipping. The floating landing gear is fixed in the next iteration.

Click the spoiler below for more specs and tricks I used (they might help you in getting your crafts to work):

Spoiler

The Duna-Phantom IIX earned its namesake from how light it is—Iteration Eight (IIX) weighs 20.5 tons on the runway. Its large, inclined wings give it a low takeoff speed of 35 m/s on Kerbin and about double that on Duna, making takeoffs safer and raising its flight ceiling. One aerospike provides an excellent combo of thrust and efficiency for ascent, while two overworked medium rotors provide just enough power to escape Kerbin's lower atmosphere (the rotors have no trouble on Duna and in Kerbin's upper atmosphere). 32 big fan blades provide just enough thrust to reach a top speed of 130 m/s on Duna and a flight ceiling of over 4000m.

But the problem is, despite being nearly half fuel, the craft has only 1650 m/s of dV, and it's not ready for the challenge. But even though I've played nearly every trick in the book to squeeze out more efficiency from this thing, I still got a few tricks left for Iteration Nine before I resort to "moar boosters".

Firstly, I'll cut down aero surfaces. Ironically, the plane has too many wings and control surfaces for Duna—the center of lift is balanced like a coin, so one medium reaction wheel turns the plane faster than the wings can handle. Shaving off extra control surfaces, a fairing, and two wing boards will lighten the plane by about one ton for almost no downside. If that's not enough, I can take out the unnecessary solar/radiator panels, science, probe core/antenna, and batteries to shave off another ton, enabling me to use a lighter, more efficient LV-909 "Terrier" engine instead of the aerospike.

Secondly, there's gravity assists off of Duna's and Kerbin's moons.

I'm trying to squeeze out more delta-V from my design - anyone know how much dV you can save with a Mun, Minmus, or Ike slingshot?

Edited April 28, 2020 by It'snorocketscience

[mystifeid]

43 minutes ago, It'snorocketscience said:

I'm trying to squeeze out more delta-V from my design - anyone know how much dV you can save with a Mun, Minmus, or Ike slingshot?

I think you might be better off refueling on Minmus then doing an Oberth dive back down around Kerbin.

[KerikBalm]

13 hours ago, It'snorocketscience said:

Fair point, although a reusable/recoverable stage isn't perfect as it requires extra mass in the sense that it requires another engine, probe core/batteries, reaction wheel/RCS setup, landing gear/chutes, and docking ports. It also adds piloting time/complexity to a mission, which is another moot point, but some people really like the "cool factor" and design challenge of a spaceplane that drops nothing except for its payload.

Well, depending on the required dV, the added mass of the components for control and recovery is more than offset by the savings you get by not hauling airbreathing engines, intakes, and the now emtpy fuel tanks (that had the fuel used to get to orbit) all the way to Duna/Eve/Jool.

SSTOs do have a cool factor, but I find that reusable boosters/ carrier spaceplanes also have a cool factor.

For a long time I used SSTO cargo planes to lift large payloads to orbit, but the SSTO doesn't go past orbit. You aren't talking about a SSTO really, but a SSTD/SSTE/SSTJ... as you want them to go to Duna/Eve/Jool without staging.

I already didn't want to do that in stock ksp, and its out of the question in 3x ksp. Craft at the destination have more parts and are not optimized for their destination.

Quote

But ultimately, the goal of my challenge, besides being a challenge, was to encourage players to push the limits of rotary-wing crafts, and to develop a Eve/Jool SSTO, if rotating wings can make such a feat possible. That would be quite the achievement.

Yes, I understand the challenge. I haven't tried rotors on jool yet (always viewed it as a one way trip, and rotors wont keep the craft up as soon as you switch vessels, so they don't allow flying bases, as cool as that would be).

I have tried rotors on stock Eve... and I only ever tried an eve sea level to orbit shuttle with no isru. I had to do a 2 stage design with a recoverable fan using first stage. It would have required a base to refuel it (the electric fans would allow it to fly back to base). At the base it would also require some sort of robotics to grab and reattach the orbital craft.

Duna on the other hand, rockets easily allow getting to and from orbit with a payload.

I did try making Duna SSTO rotorcraft to allow VTOL and flying short trips from the landing site to surface bases - but again, they didn't SSTO from kerbin, and they flew pretty bad/had terrible payload, so I made rockets with giant airbrakes, and a dedicated rotorcraft to fly from the surface base to the landing site (I try to land nearby, but its normally at least a few km away)

 

[chargan]

Reading through this thread has been interesting. I had fun messing around with that Duna helicopter concept. I think I can definitely optimize it with a tilt rotor design + nerv engines + ISRU.

Does this count? Made an Eve SSTO a while back using props to gain altitude. Had to refuel in Eve orbit to get back to Kerbin though. https://redd.it/f39x5q

Also search for the Eve SSTO posted by realseek. I think his is the most optimized design I've seen.

[It'snorocketscience]

10 hours ago, mystifeid said:

I think you might be better off refueling on Minmus then doing an Oberth dive back down around Kerbin.

I checked the community delta-V map, and if you start from LKO, landing on Minmus costs about 1270 m/s of dV. Meanwhile, it costs only 1080 m/s of dV to fly on a trajectory through Duna's atmosphere, at which point you can aerobrake to a landing. My prop design can land in Duna's atmosphere without burning fuel, so flying straight to Duna and refueling there is the cheapest way.

 

4 hours ago, chargan said:

Reading through this thread has been interesting. I had fun messing around with that Duna helicopter concept. I think I can definitely optimize it with a tilt rotor design + nerv engines + ISRU.

Oh hey, I actually tried implementing a tilt rotor on your helicopter that you posted to Reddit, and it's quite hard. Mounting rotors on top of a hinge will cause it to buckle and bend under aerodynamic forces, but I bet the alligator hinges won't flex as much like the normal ones do.

4 hours ago, chargan said:

Does this count? Made an Eve SSTO a while back using props to gain altitude. Had to refuel in Eve orbit to get back to Kerbin though. https://redd.it/f39x5q

No, unfortunately, as it requires undocking/refueling. But you can tow asteroids in this challenge, so it might be possible (albeit insane) to tow one to low Eve orbit for refueling. That would count as a self-sufficient SSTO design.

 

7 hours ago, KerikBalm said:

Well, depending on the required dV, the added mass of the components for control and recovery is more than offset by the savings you get by not hauling airbreathing engines, intakes, and the now-empty fuel tanks [...] all the way to Duna/Eve/Jool.

I guess it really depends on the craft. In my minimal 21~24 ton build, empty fuel tanks add up to about 1.25t, about 9-12% of my craft's dry weight, so it's not that bad. My prop setup, while about as heavy as a jet setup at ~4.75 tons, is not dead weight on Duna. The craft doesn't need to burn any fuel on landing (I think a Duna suicide burn costs a few hundred m/s of dV), and the props have saved me at least 300 m/s during my sloppy Duna ascent test. Even then, it's probably not as efficient as reusable stages, but I'm going to settle this matter by building a reusable stage mission after I finish this SSTD craft.

Another small benefit of building an all-in-one Duna SSTD instead of a detachable helicopter is that I can haul the craft's ISRU module to a location with high ore concentration. It's not super useful as I could just use a scanning satellite to pick a good landing/refueling site, but if I was too lazy or poor to launch relay/scan sats ahead of time, or didn't have the scanning tech, there's that.

Edited April 28, 2020 by It'snorocketscience

[Lt_Duckweed]

So a while back @Stratzenblitz75 and I did a collab project where we flew a 2 Kerbal part clipped monster of an SSTO from Kerbin to Jool "surface" and back via refuel pitstops at Pol and Laythe.  Looking through the challenge rules I think it qualifies.  I wouldn't consider this an entry though, just want to show that it is possible (or at least was, props got nerfed in 1.9 so this craft would need a serious redesign)

 

Edited April 28, 2020 by Lt_Duckweed

[It'snorocketscience]

1 hour ago, Lt_Duckweed said:

So a while back @Stratzenblitz75 and I did a collab project where we flew a 2 Kerbal part clipped monster of an SSTO from Kerbin to Jool "surface" and back via refuel pitstops at Pol and Laythe.  Looking through the challenge rules I think it qualifies.  I wouldn't consider this an entry though, just want to show that it is possible [...]

Of course, leave it to him/you guys to have done everything possible in the game already. Nice work. I haven't watched it yet but I wonder how far the part clipping goes...

Quote

I wouldn't consider this an entry though, just want to show that it is possible [...] (or at least was, props got nerfed in 1.9 so this craft would need a serious redesign)

Does the craft use the R-25 ducted fan blades, or regular props? It's hard for me to tell which type is used on my mobile screen, but if you guys didn't use fan blades then you're missing out on a lot of performance.

Edited April 28, 2020 by It'snorocketscience

[Lt_Duckweed]

40 minutes ago, It'snorocketscience said:

Of course, leave it to him/you guys to have done everything possible in the game already. Nice work. I haven't watched it yet but I wonder how far the part clipping goes...

Does the craft use the R-25 ducted fan blades, or regular props? It's hard for me to tell which type is used on my mobile screen, but if you guys didn't use fan blades then you're missing out on a lot of performance.

We used the largest fan blades, 48 of them in fact!  As for part clipping... there are 200 of the little mk0 liquid fuel tanks crammed under the fairing along with plenty of other goodies!

[xendelaar]

14 hours ago, Lt_Duckweed said:

So a while back @Stratzenblitz75 and I did a collab project where we flew a 2 Kerbal part clipped monster of an SSTO from Kerbin to Jool "surface" and back via refuel pitstops at Pol and Laythe.  Looking through the challenge rules I think it qualifies.  I wouldn't consider this an entry though, just want to show that it is possible (or at least was, props got nerfed in 1.9 so this craft would need a serious redesign)

 

holy crap that's an impressive design! Man that must have taken a lot of time to build...and fly!?! What is the mass of that baby?

[Mephisto81]

This should work on Kerbin as well:

https://kerbalx.com/Mephisto/XE-01-APEX-Mk-VI-Eve-SSTO

[Lt_Duckweed]

8 hours ago, xendelaar said:

holy crap that's an impressive design! Man that must have taken a lot of time to build...and fly!?! What is the mass of that baby?

Right about 134 tons when totally full. And yeah it took a lot of time to get the design refined, and we even ran the mission all the way up to the laythe landing before realizing the design didn't work for the water takeoff and had to adjust the design and start the mission over.