Space Plane construction Question

[seaces]

Hello,

I finally made some Mk2 space plane in 1.02 that has one large cargo bay which when loaded with one full tank can go to LKO so I guess this can be used for Sat contracts those Mk2 planes.

Now I went to Mk3 and things are completely different here. I think to use Mk3 maybe for bigger satellites that should go further.

So here are my questions:

1) How to mix those fuel tanks? Usually I am having issues of either running out of OX or LF when I switch to rocket and it seems really difficult to balance fuel to have enough to get up set orbit and land. Somehow I balanced this for Mk2 but still I want hear some tips about it since I know I will have this issue in future.

2) Some general tips about building Mk3 space plane?

3) I have Mk3 space plane in work and currently I am having issues of plane that after 20km has nose down tendency. CoM is always a bit forward of CoL checked that in SPH. Tried with forward canards but not much of help.

[MaverickSawyer]

First off, get the RCS Build Aid. It'll show you your dry and average CoM, which can help you figure out why you're encountering the nose-down issue.

For finer tuning of your fuel tanks (and everything else), get the Tweakable Everything mod. It's been a godsend for properly balancing fuel loads for several versions now.

[DeMatt]

1) How to mix those fuel tanks? Usually I am having issues of either running out of OX or LF when I switch to rocket and it seems really difficult to balance fuel to have enough to get up set orbit and land. Somehow I balanced this for Mk2 but still I want hear some tips about it since I know I will have this issue in future.

I'd have to say "trial and error". You might consider the Big-S wings... they can hold LF for your jet phase, while the fuselage tanks are reserved for rocket fuel.

3) I have Mk3 space plane in work and currently I am having issues of plane that after 20km has nose down tendency. CoM is always a bit forward of CoL checked that in SPH. Tried with forward canards but not much of help.

You can try pumping fuel rearwards.

[Kagame]

1. Just use one Mk. 1 LF fuselage per RAPIER, then have the rest be Mk. 3 LFO. Make the intake balancing easy on yourself and just create a fake engine nacelle built around an LF fuselage that has both a ram air intake and the RAPIER attached to it. Make sure to run fuel lines from the rocket fuel tanks to the RAPIER! (It's happened to me...)

2. Sure. Mk. 3s are all about determining how much cargo you'll need, then building the plane around the cargo bay. (Well, you build it using the cockpit as the root part per usual, but I get what you mean.) One CRG-50 will take one normal rocket fuel fuselage and eight to ten RAPIERs. One CRG-100 will take one long rocket fuel fuselage and twelve to sixteen RAPIERs. Any more and you're getting into very difficult waters.

3. This is somewhat normal. I usually turn on RCS whenever I start to lose control authority...

[seaces]

I am not sure but I think I have already tried with RCS to stabilize the plane however I will retry.

How much is Mk3 plane without cargo heavier then Mk2? I am asking this just to check how much there is increase in required number of jet engines.

I have that Mk2 plane that has 2 long tanks with OX and LF one longer cargo bay between those tanks and at the end one shorter part for terminating the construction that has one spike engine for orbit. In atmosphere it is powered by 2 turbo jets attached under delta wings on engine precoolers which also have on them those bigger ram intakes forgot their names.

Now I am trying to build on similar design but with just Mk3 parts so how much I will need to increase the power?

Also to me look like that Mk3 could get away with less tanks because those are huge.

[Red Iron Crown]

Make sure to run fuel lines from the rocket fuel tanks to the RAPIER! (It's happened to me...)

Fuel lines are not required for RAPIERs in either airbreathing or rocket mode. They draw from all tanks in a stage evenly and ignore crossfeed rules.

[Geschosskopf]

1) How to mix those fuel tanks? Usually I am having issues of either running out of OX or LF when I switch to rocket and it seems really difficult to balance fuel to have enough to get up set orbit and land. Somehow I balanced this for Mk2 but still I want hear some tips about it since I know I will have this issue in future.

I find it helpful to build spaceplanes using the same sort of mental process used to build a rocket---think in terms of "stages". Even though the spaceplane itself is an SSTO, switching from jets to rockets is analogous to staging boosters.

So as with a rocket, first I build the essential mission payload package (cockpit, docking port, RCS fuel, SAS, probe core, antenna, electrical stuff, plus whatever cargo and/or passenger space). This wad of parts needs sufficient rocket dV and TWR to reach orbit from where the jets die and then either rendezvous with a station or de-orbit, so now I start adding the LFO tanks and rocket/RAPIER engines to the payload. I use either MJ or KER to make sure I have the TWR and dV necessary for this "stage". If you assume the jets will give you about 1200-1500m/s, then the rockets will need to add about 1000m/s to your speed to reach LKO, plus say 500m/s more for orbital maneuvering. However, given atmospheric drag and Isp effects, and the fact that I haven't yet added jet fuel tanks, wings, landing gear, etc., I need at least 2000m/s for this "stage" at this point in the build. More is helpful. Also, TWR with full LFO tanks needs to be at least 1.5.

NOTE: If using RAPIERs, you'll probably have to right-click on them and change them to rocket mode so you get the proper numbers in MJ/KER. Don't forget to switch them back to jet mode before proceeding further.

While building the payload/rocket "stage" of the spaceplane, I also use RCS Build Aid to manage my CoM. Ultimately, I want the full and empty CoMs to correspond so that trim doesn't change during flight and RCS translation when docking doesn't cause rotation. Also, it would be nice if the CoM was near the middle of the cargo bay so that moving things in or out of the bay doesn't hose the trim. So I rearrange the order of the parts in the core until I get close to what I want, bearing in mind that when I add the jet fuel tanks, wings, and whatnot later I can fine-tune CoM position.

OK so now it's time to build the jet "stage". This consists of LF-only tanks, intakes, wings, and (if needed) separate jet engines, plus landing gear. Also lights, ladders, and whatever other doodads you want. You don't need a whole lot of jet fuel but you do need a TWR of at least 1.3 with full tanks (more is better), and remember the "precooler" intake part holds some. In general, for a Mk2 or smaller spaceplane, a single Mk1 fuselage LF tank + 1 precooler + 1 shock cone intake per jet/RAPIER is sufficient for both ascent and any atmospheric flying you need to get back to the runway. Again, use RCS Build Aid to manage your CoM during construction, then adjust wings to put the CoL where it needs to be.

The final touch, once all the major components are in place and the CoM is how you want it, is to add the RCS thrusters. Again, use RCS Build Aid to balance them out so you don't get undesired roll when translating. That's about it.

2) Some general tips about building Mk3 space plane?

I build them exactly the same way as Mk2 and Mk1 spaceplanes. It's just more tedious because you need to spam a lot of small engines. We really need a 2.5m RAPIER .

3) I have Mk3 space plane in work and currently I am having issues of plane that after 20km has nose down tendency. CoM is always a bit forward of CoL checked that in SPH. Tried with forward canards but not much of help.

Edited June 18, 2015 by Geschosskopf

[Old Foxboy]

One big problem the games still has that it doesn't let you switch vessels in a convenient way and doesn't provide autopilot capabilities for a second vessel.

If they implement this in a next version, the "Sänger" approach might be the way to go: Have one atmosphere bound supersonic carrier plane lift a smaller "SSTO" into upper atmosphere where it will be able to reach the orbit. It's basically a split stage approach, but both stages would be fully reusable.

[A_name]

1) How to mix those fuel tanks? Usually I am having issues of either running out of OX or LF when I switch to rocket and it seems really difficult to balance fuel to have enough to get up set orbit and land. Somehow I balanced this for Mk2 but still I want hear some tips about it since I know I will have this issue in future.

I do this by trial and error. Launch with full tanks and once you're in orbit see how much surplus LF or OX you have left. Then revert to SPH and take out that amount from your tanks to achieve balance.

2) Some general tips about building Mk3 space plane?

Use lots of engines . What I do is attach intake-LF tank-jet combos to the sides of the fuselage. This helps keep the dry COM from moving too much.

Also, since your craft will be big heavy, you want to minimize drag as much as possible. I've found that in 1.0.2 you generally need less wing. Use the least wing as will get you off the runway. For example, in one of my designs I was able to lift a Mk2 plane with only its body lift, the small wing strakes and the rear stabilizer fins + control surfaces. Especially if your TWR at takeoff is higher than 1 you can definitely pull off a design that uses very little wing, therefore less weight and more importantly less drag.

3) I have Mk3 space plane in work and currently I am having issues of plane that after 20km has nose down tendency. CoM is always a bit forward of CoL checked that in SPH. Tried with forward canards but not much of help.

What I've found is that, for some reason, no matter how close your COL is to your COM, in heavier designs the nose will always want to point down and you need to counteract this with control input. In your particular case, if you're not able to keep the nose up and your COL and COM are already vlose enough, you probably just need more control surfaces.

[ajburges]

Clarify nose down issues: are you having nose down issues when you attempt to pitch up or at all times? (IE can you dive quickly) If you only experience issues when diverging from prograde, you need to either increase control authority (add more surfaces) or decrease craft stability (move CoM back and/or CoL forward). The other less common cause is torque caused by engines, check CoM alignment with thrust vector (RCS aid does this well)

For air breathers conservative planning dictates 150 LF units per engine. For accent and landing. My gut says a real number is half that. The amount needed is dictated by flight profile. Note landing LF and tweak downward as needed. Budget an extra 40-50 units for cross range on landing if desired.

Mk3 is an interesting beast.

Modules are heavier than they appear. Plan the Mk3 body to be at least 30 m long. Those parts are draggy and short craft have bad drag to mass ratios. That kind of mass requires a lot of thrust.

You can mitigate part of the engine count issues by supplementing your design with a rocket engine larger than 1.25 m and optimising your aerobic engine configuration more. You can also leverage that large node at the tail for a more efficient OMS engine and use RAPIERs for supplementary thrust. Regardless, you will need a bunch of aerobic engines.

Control is a constant issue for Mk3 craft. Similar to the volume vs surface area scaling issue, they require a lot of control surface in atmo. In space, despite being near empty, there is still a lot of mass there. Plan to supplement the command module with at least 2 large reaction wheels or use RCS for attitude control. You may want to upgrade RCS translation thrust.

I'm testing out my first career Mk3 plane to deploy payloads. It uses 8 turbo jets and 1 mainsail for thrust. I recon it has about 32 lift from the wings. It flies nicely which is good because it is a bit laggy in atmo.The numbers suggest that it won't be capable of delivering a full orange tank to LKO, but that's a future test. It was intended to deliver "small" nuclear Mk2 space craft to LKO with lower total launch costs.

In a true tragedy, I design and fly space planes better than rockets. Once you learn their accent profile, they are more forgiving than executing an optimal gravity turn from surface due to the crazy efficiency of jet engines.

Do RAPIERS actually ignore cross-feed rules in rocket mode? I thought it was only for aerobic mode.

[seaces]

Nose down issue solved. was due the engine torque.

I have managed to make some tinny Mk3 that put in LKO 9t of payload. Tried with 18t but couldn't make it.

Definitely Mk3 is more difficult to design. It took me 4 hours of testing. Mk2 in hour was ready.

Which engines are good for Mk3 use turbo jet and then switch to rocket or all way rapiers? This one that put 9t in orbit had 6 rapiers and 1 turbo jet it can do with just 7 rapiers too.

Btw 9t of payload how does that sound too little for medium mk3 cargo bay? I don't have that sense for payloads still.

Edited June 19, 2015 by seaces

[Nothalogh]

We really need a 2.5m RAPIER .

Damn straight, we do

[Mic_n]

Biggest difference I've seen so far with Mk3s is in heating. Even flying the same basic profile as with Mk2 designs, these things seem to get way hotter, to the point that I've actually blown up probe cores tucked in a 2.5m service bay on ascent. No "Firey Phoenix" stuff going on, either.. not even getting past 900m/s at 20km.. the heat seems to be coming in higher up, in the acceleration to orbital speeds. Seems like these might need a much steeper profile and to have a much more defined "ascend, then circularise" profile that keeps most of the acceleration right up high in the atmosphere. Similarly, much harder & higher aerobraking while coming back down.

[ajburges]

Biggest difference I've seen so far with Mk3s is in heating. Even flying the same basic profile as with Mk2 designs, these things seem to get way hotter, to the point that I've actually blown up probe cores tucked in a 2.5m service bay on ascent. No "Firey Phoenix" stuff going on, either.. not even getting past 900m/s at 20km.. the heat seems to be coming in higher up, in the acceleration to orbital speeds. Seems like these might need a much steeper profile and to have a much more defined "ascend, then circularise" profile that keeps most of the acceleration right up high in the atmosphere. Similarly, much harder & higher aerobraking while coming back down.

What's your design?

I haven't designed many Mk3 SSTO planes thus far, but the ones I have needed a fuel tank between the cargo bay and the cockpit for balance reasons.

In almost all my designs, if the front tank is 30% or less of my accent dV I disable flow of front tank fuel to enhance accent stability. Keeping the fuel in there makes it a reasonable heat sink for the compression heating the nose receives.

I also avoid using direct node attachmentof my cargo to the body. A reaction wheel is connected to the body and batteries and docking port to that.

The thermal mass of Mk3 is high, if you see a heat bar on them, you need some measure to avoid heat redistribution before you can cool off. What gives a 2.2 k part heat bars melts 1.2 k parts!

[Mic_n]

That's kinda what I'd expect, which is why it's surprised me as much as it has. With a very similar design/layout with mk3 parts to the very successful mk2 planes I've made (which admittedly do wind up putting a temp bar on my probe core which is tucked inside a service or cargo bay), the heating seems quite considerably stronger. I guess the larger, heavier plane is going to be generating more heat to begin with, but I'd hoped it'd all stay fairly proportional.

As you say, I have a tank (actually, a Mk3 -> 2.5 adapter), a 2.5 service bay with probe core and some batteries, then the 2.5 -> 1.25 adapter, then a ram intake on the nose. I actually wound up putting a pre-cooler in there in an attempt to soake up a bit more of the heat, with very little impact. Main body is 3x cargo bays, some fuel scattered around the back.. power comes from 8 ramjets and 5 spikes. It's basically a delta wing config.

I similarly like to keep fuel up the front to keep the COM stable.

I'll put up some screenies and a .craft if it gets annoying enough and I can't sort it out

Edited June 22, 2015 by Mic_n