B9 and SSTO design questions

[How2FoldSoup]

Hey guys,

I'm not a novice spaceplane builder and this question is more tailored to people who build many spaceplanes as I do. Lately I've been experimenting with pure sabre powered SSTO's and I've found that with big or small designs I've hit a speedcap of about 1700m/s on Kerbin. Is anyone else hitting this?

I have a light 40ton Kargo SSTO to ferry Kerbalnaughts to my station and a heavy 160ton(fully loaded) SSTO for launching heavy payloads. Even with both of these craft I max out between 1700-1750 m/s at around 25km. I am just wondering if I am reaching the maximum efficiency I can with sabres in the atmo or if it's a design flaw.

I just realized that because I am using FAR it could be causing this speed cap. Does anyone else experience anything else like this? I still manage to have a decent amount of dV once they get into orbit but I'm trying to make a SST(another planet) without refueling if possible but capping out far below orbital speed is making this difficult.

Can anyone else share their experiences or perhaps provide an explanation?

Thanks!

Soup

Edit: Grammars and stuff :S

[Hodo]

The issue is the altitude, to get much faster than 1700m/s you will have to go a little bit higher or have a much higher TWR ratio. SABREs are great for their power but are kind of lacking when you do the TWR math.

SABREs are horrible engines for trying to make an interplanetary SSTO from Kerbin. And honestly 1700-1750 isn't bad before switching over to rocket power if they are done right. I have more than a few SSTOs that use SABREs and quite a few that don't. May I see a picture of the craft so I can see how it is laid out and see if it is underpowered or using to many intakes.

[How2FoldSoup]

The issue is the altitude, to get much faster than 1700m/s you will have to go a little bit higher or have a much higher TWR ratio. SABREs are great for their power but are kind of lacking when you do the TWR math.

SABREs are horrible engines for trying to make an interplanetary SSTO from Kerbin. And honestly 1700-1750 isn't bad before switching over to rocket power if they are done right. I have more than a few SSTOs that use SABREs and quite a few that don't. May I see a picture of the craft so I can see how it is laid out and see if it is underpowered or using to many intakes.

Sure thing

This is my heavy lifter..I know there are still a few design flaws with regards to fuel balance and I was in the process of testing what works best when I took this screenie.

The 4 sabres under the wing also have the same intake combo as the engine blocks on top. It also has the intake adapter up front leading away from the cockpit.

This is the cargo plane

The only intakes I have are the ones you can see directly. This plane has 4 small sabre engine block combos and then 1 sabre M coming out of the fuselage.

side view

Hope this helps

[Hodo]

Ok the cargo lifter looks like it is to heavy to reach the speeds you want with the engines it has. Each one of those SABRE-S engines are heavier than a single turbojet engine by about 2x. They also are less fuel efficient than the turbojets. Here is my heavy lifter, the SP-400A, compare the designs and see the differences. Initially I had a problem much like you, where I had to many engines or relied to much on the SABRE, it is good but it is to heavy, and I found that the turbojet was just better.

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[How2FoldSoup]

What are your top speeds inside the atmosphere with that?

Another question, why use the D-30F7's?

I chose to use SABRE-S's instead of turbojets(especially the D-30F7's) was although they weighed slightly more(coming in at 1.5 mass compared to the D-30's 1.3 and the stock turbojet's 1.2) and I lost out on 10 units of thrust, they had greater Isp and I reasoned that greater fuel efficiency and control(due to thrust vectoring) was worth the small loss in thrust. Which leads me to ask does that 10 units of thrust matter? I have done a little testing with turbojets vs sabres and I did not find much of a difference in the speed in which I had to switch to rockets. In fact, I believe I had to use more fuel to break atmo due to my lower overall thrust due to lacking extra sabres to push myself out to 40km quick enough.

I will do some further testing in switching to mostly turbojets to see if that makes a difference on the big guy with regards to total dV left once in orbit.

Thanks for your response!

Edit: Having just tested the stock turbojets they lose all thrust at about 20km which won't help me very much. I'll test with the d-30's to see if they have a higher ceiling..

Edited November 12, 2013 by How2FoldSoup

[arq]

To go faster with jets, you must go higher to thin the air. If you can't fly higher without flaming out and have pretty much capped in speed, you should switch to rockets. If that's too low/slow, you need more intakes.

As soon as you need to start throttling back the engines, you are in the AIR-LIMITED regime. At this point, your max thrust is proportional to air intake, not the number of engines (in fact, more engines hurt because they add mass which adds drag). Of course, at some point you enter the realm of 'air hogging', which many players consider to be 'cheating' (enough intakes makes getting to orbit trivial), so do what you find 'fun'.

[Hodo]

What are your top speeds inside the atmosphere with that?

Another question, why use the D-30F7's?

I chose to use SABRE-S's instead of turbojets(especially the D-30F7's) was although they weighed slightly more(coming in at 1.5 mass compared to the D-30's 1.3 and the stock turbojet's 1.2) and I lost out on 10 units of thrust, they had greater Isp and I reasoned that greater fuel efficiency and control(due to thrust vectoring) was worth the small loss in thrust. Which leads me to ask does that 10 units of thrust matter? I have done a little testing with turbojets vs sabres and I did not find much of a difference in the speed in which I had to switch to rockets. In fact, I believe I had to use more fuel to break atmo due to my lower overall thrust due to lacking extra sabres to push myself out to 40km quick enough.

I will do some further testing in switching to mostly turbojets to see if that makes a difference on the big guy with regards to total dV left once in orbit.

Thanks for your response!

Edit: Having just tested the stock turbojets they lose all thrust at about 20km which won't help me very much. I'll test with the d-30's to see if they have a higher ceiling..

But the D-30Fs are to me slightly worse than the stock turbojet engines, but when I need brute force they actually do better on that craft. But to me it's like the difference between the LV-909 and the nuke engine, depends on the application.

That is actually the old version of that craft, I have changed it slightly to be more efficient. If you have B9, Kerbal Engineering, KSPX, and FAR, I can easily send you the craft file. I haven't flown it in a while but will take it up for a quick test flight and tell you what it does. I remember it was fast when empty and quite a bit slower when packing its full 72 ton cargo.

Ok just tested the SP-400A, I can get to without a cargo, up to 1750m/s on just airbreathers alone, at an altitude of 28km, before I have to switch over to rockets.

The SP-400A is not one of my most efficient craft, but it does quite nicely for what it is designed to do.

My fastest craft on jets as an SSTO is my SVO-11, SVO-12, and SVO-13. Both will do close to 1800m/s in atmosphere before having to switch to rockets.

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Edited November 12, 2013 by Hodo

[Hodo]

Edit: Having just tested the stock turbojets they lose all thrust at about 20km which won't help me very much. I'll test with the d-30's to see if they have a higher ceiling..

Oh one more thing, stock turbojets and D30s start to lose power around mach 2.5+ and really lose all power around mach 4. The stock turbojet makes power till around mach 5.5 then its dead. The SABRE I think pushes till Mach 5.5 in jet mode also.

The thing is I run at most 3:1 intake to air breathing jet ratio, climb at best possible rate till around 15km, or till my intake air percentage in the FAR readout drops to 300%, then I start to level off for speed. I then climb at no more than 10-15deg or keep it below 50m/s climb rate, till I can get to over 1650m/s, if I can ride the jet engines till 30km I am happy if not I try to keep them pushing till I am no longer accelerating, then I switch over to rockets.

[Virindi]

SABREs are horrible engines for trying to make an interplanetary SSTO from Kerbin. And honestly 1700-1750 isn't bad before switching over to rocket power if they are done right. I have more than a few SSTOs that use SABREs and quite a few that don't. May I see a picture of the craft so I can see how it is laid out and see if it is underpowered or using to many intakes.

I actually think SABRE engines are overpowered for long range vessels as well. The key to SABREs is that they have a huge amount of thrust in jet mode, which lets you not even bother with making high speed in the upper atmosphere. You can get a ridiculous amount of fuel into space by just using jet mode to punch upwards then circularizing with a small number of LV-Ns. That is, using SABREs in rocket mode should be limited as much as possible in favor of some sort of more efficient secondary engine. Adding an LV-N or two to the back also lets almost any plane reach interplanetary space with a refuel. Ideally you will only run your SABREs in rocket mode for a couple seconds as you leave the atmosphere.

Edited November 12, 2013 by Virindi

[How2FoldSoup]

But the D-30Fs are to me slightly worse than the stock turbojet engines, but when I need brute force they actually do better on that craft. But to me it's like the difference between the LV-909 and the nuke engine, depends on the application.

That is actually the old version of that craft, I have changed it slightly to be more efficient. If you have B9, Kerbal Engineering, KSPX, and FAR, I can easily send you the craft file. I haven't flown it in a while but will take it up for a quick test flight and tell you what it does. I remember it was fast when empty and quite a bit slower when packing its full 72 ton cargo.

Ok just tested the SP-400A, I can get to without a cargo, up to 1750m/s on just airbreathers alone, at an altitude of 28km, before I have to switch over to rockets.

The SP-400A is not one of my most efficient craft, but it does quite nicely for what it is designed to do.

My fastest craft on jets as an SSTO is my SVO-11, SVO-12, and SVO-13. Both will do close to 1800m/s in atmosphere before having to switch to rockets.

Gotcha. I have those 3 mods minus KSPX. So it seems as though between 1700 and 1800 m/s becomes a hard cap on speed as far as atmospheric traveling goes at around 25-30km.

I did another test flight with stock turbojets and pure sabres again after my last post and both seemed to cap out at about 1750m/s and I could run them until ~30km before switching to rockets same as you.

I actually think SABRE engines are overpowered for long range vessels as well. The key to SABREs is that they have a huge amount of thrust in jet mode, which lets you not even bother with making high speed in the upper atmosphere. You can get a ridiculous amount of fuel into space by just using jet mode to punch upwards then circularizing with a small number of LV-Ns. That is, using SABREs in rocket mode should be limited as much as possible in favor of some sort of more efficient secondary engine. Adding an LV-N or two to the back also lets almost any plane reach interplanetary space with a refuel. Ideally you will only run your SABREs in rocket mode for a couple seconds as you leave the atmosphere.

So if we're talking efficiency I should just ignore their rocket capabilities entirely. Hmmm - that's an interesting thought. I love the sabres for their aesthetics and capabilities but I may test some LV-N rocket versions of my planes and see where that takes me. I would feel like the sabres become wasted mass though if I am not using them for their rocket ability either but I suppose as a purely air-breathing engines they could have some use. Back to the drawing board I suppose

Thank you again for the replies and more input is always appreciated!

Soup

[Virindi]

Well, not exactly. Due to the low TWR you get from LV-N, the purpose of the SABRE rocket mode is to just kick you up so that you can linger in space long enough for the LV-Ns to pick up your speed. If you use only LV-Ns you will be unlikely to get enough TWR in your vehicle to get above the atmosphere at all like this. I'd call this the 'lazy' spaceplane ascent profile

[DrMonte]

By reading some Fastest Jet Chalenge forum I found that in parts cfg there is a top constant for an each engine, where its trust becomes 0. For example Turbo Jets has 0 trust at 2400m/s.

1800m/s might be your engines limit.

[Hodo]

I actually think SABRE engines are overpowered for long range vessels as well. The key to SABREs is that they have a huge amount of thrust in jet mode, which lets you not even bother with making high speed in the upper atmosphere. You can get a ridiculous amount of fuel into space by just using jet mode to punch upwards then circularizing with a small number of LV-Ns. That is, using SABREs in rocket mode should be limited as much as possible in favor of some sort of more efficient secondary engine. Adding an LV-N or two to the back also lets almost any plane reach interplanetary space with a refuel. Ideally you will only run your SABREs in rocket mode for a couple seconds as you leave the atmosphere.

This is true, SABREs are overpowered in some applications. Like if I have a 15 ton SSTO and I place a SABRE-M on it, yeah I am going to blow it to space with the least amount of fuel use. But on a 100+ ton application it loses a lot of that benefit, but that could be said for any engine at that point. I honestly have a love hate relationship with SABREs. I love them for the saving of parts on a craft, not having to place a second rocket or jet engine is nice, but I also hate them because they are so heavy and fuel hungry in rocket mode not to mention they are a bit overpowered.

Thank you again for the replies and more input is always appreciated!

Soup

I wouldn't ignore the rocket capabilities at all, they are good, but fuel hungry. On many of my other SSTO space planes that use the SABRE I use the rocket ability only to get into space, once I am there I usually use a more efficient engine to circularize. One thing I noticed about your two craft, you are using the intercooler between the engines and the hull. I have found they do nothing currently and this was confirmed by others in the B9 mod thread, so they are just wasted mass, unless you are using them as structure to move the COM back or space out your engines.

The only reason why I list KSPX as one of the mods, is because I don't remember what all I have used the KSPX parts on or what is stock and what isnt part of the KSPX parts list. That is one mod that should definitely be part of the stock game.