I am grumpy but elated (yep, it's another spaceplane comment)

[James_Eh]

So... I have spent the last 2+ months watching my giant spaceplanes which were designed to be SSTL (without any refueling stops) get to altitude hdeath (where hdeath = 20,000m + the square root of the log of the effort that I put into improvement of the design) before careening into oblivion. I must have killed poor Gus 100 times.

Today I caved and decided that it was morally OK to stop in at a gas station in LKO and refuel the beastie before heading out to Laythe.

So I spent, dunno, 10 minutes (?), building a massive fueling station in the VAB, launched it, wound up with about 3 SSTL's worth of fuel (yes, that is a unit, look it up) in orbit with plenty of docking bits and such for easy refueling. 0.23.5 parts make that a breeze.

Flipped back to Gus (one has to wonder why he keeps climbing into these things) on the runway, realized that I should oughta put some kind of docking port thingy onto the front of the plane (plus some mono and those wee RCS things), so I went back to the SPH, moved a couple of bits around, and headed out. I figured while I was at it, I would try to optimize my ascent path a bit more. So I spent a little bit more effort than usual keeping my pitch balanced by moving fuel around.

Upshot - here I am in 100 km LKO with 8000 dV left, prior to the visit to the refueling station. Best I ever got out of this kludge previously was 3000 dV at LKO, and I made the craft a bit heavier with the changes. dV is measured assuming that I don't use the Skipper any more, nor the rams. (What? You think Skippers are crazy for spaceplanes? I scoff at you.) The space station beckons, but I'm pretty close to what I was aiming for dV-wise.. Am thinking of just heading out.

Thoughts? Seriously, can slight adjustments to your ascent path gain 4000+ dV?!?!?

(Pure stock except MJ BTW, no FAR which I hear I oughta use...)

Do I have to post a picture? It's kind of embarrassing. Not overly... picturesque. How about you look at my previous posts and extrapolate?

Edited July 30, 2014 by James_Eh

[Vanamonde]

This report of a mission has been moved to Mission Reports. Carry on, and good luck.

[James_Eh]

Thoughts? Seriously, can slight adjustments to your ascent path gain 4000+ dV?!?!?

Well, OK, except in there I had a real question. Should I have posted it somewhere else? Too much background? I was hoping for help.

[Vanamonde]

I was hoping for help.

Ah, I see. Off to gameplay questions then.

[Laie]

Thoughts? Seriously, can slight adjustments to your ascent path gain 4000+ dV?!?!?

In principle, yes. Going in a degree steeper or slower can have a tremenduous effect, but that's kind of a snowballing or butterfly thing where small beginnings lead to a very different course of events. I doubt that you'd call this a slight adjustment, even though it may have started as such.

Other than that, can it be that you still have an active jet engine somewhere? At least in Mechjeb, they will always show insane dV, it doesn't know or care that they won't work in space.

[LordFjord]

What exactly did you do differently to gain that much dV? And you are talking about skippers, so it surely ain't a small SSTL. Wanna share pics of the bird?

In general, the higher and faster you can go on jets alone, the more dV you can save.

[impyre]

The theoretical maximum dV that can be saved by using a spaceplane is the dV attributed to change in elevation only. Considering that for a normal rocket launch it tends to go around 4500 dV, and orbital velocity at 70km is somewhere around 2200 m/s, this means that the maximum you could save with optimal ascent profile for space plane over a rocket would be somewhere around 2300 m/s dV. This is because of all the fuel a rocket spends attaining orbit, only the horizontal component adds to orbital velocity. In the spaceplane, you use lift to counteract gravity and provide increased altitude while still gaining horizontal velocity for relatively low fuel cost. Your change in dV must be related to some other change unless you were previously flying your spaceplane in a manner that allowed it to be less efficient than a rocket... which is really hard to imagine.

[Laie]

The theoretical maximum dV that can be saved by using a spaceplane is the dV attributed to change in elevation only. [...] this means that the maximum you could save with optimal ascent profile for space plane over a rocket would be somewhere around 2300 m/s dV.

Im principle, you're right. In practice, mixing dV from jets and rockets is difficult to do and most people just don't. It is entirely possible to get into an 80x30km orbit on jets alone, requiring only ~40m/s for cricularization. The spaceplaner will then claim that he went into orbit with only 40m/s -- which is obviously baloney, but a very practical benchmark.

As I understand it, the OP required considerably less rocket power to get into orbit than he is used to, and wonders why. But yes, just to be sure: do i understand that correctly?

[Red Iron Crown]

I think your dV calculator may be confused a bit. The theoretical maximum dV for a single stage chemical rocket in KSP is 8415m/s, and it takes a lot of fuel and a very low dry mass to approach that number. I suspect it's calculating using your air-breathing engines (possibly RAPIERs still in air breathing mode).

Don't need a pic, but can you list the engines on your ship aside from the skipper?

[James_Eh]

Other than that, can it be that you still have an active jet engine somewhere? At least in Mechjeb, they will always show insane dV, it doesn't know or care that they won't work in space.

Now that's a thought, although I'm pretty sure that only the two LV-Ns were lit up in the staging list. Will double check and post orbital pix when I get home.

[Red Iron Crown]

8km/s of dV from nuclear engines would require your ship to be 63% fuel by mass. Does that sound close?

[James_Eh]

Phooey. It's a MJ reporting issue. I got all excited when I executed a 100 m/s burn and the total dV available only dropped by 100 - thought I had hit the jackpot. Then I started transferring fuel around and noticed the remaining dV as reported by MJ was changing dramatically. Despite the fact that only the two LV-N's are activated, it is obviously assuming (based on fuel flow I would wager) that at some point the ramjets will need to be included in the picture. Ah well. Off to the fuel depot.

(Red Iron - nope not even - at 100x100KM orbit: 59.44T total mass, 18.2T fuel. I should have cranked those numbers through my spreadsheet myself, it sure seemed fishy...)

Another spaceplane dream ruined. Gotta love this "game".

[James_Eh]

...unless you were previously flying your spaceplane in a manner that allowed it to be less efficient than a rocket... which is really hard to imagine.

Although the mystery is solved, I have to answer this by saying you should come over and watch me fly a spaceplane some day. (WDDAASSQEQEQEQQQEESSSAAAAWWWWWWWWWW!!!!!WW!!!!WW!!!! <crash>) - and so forth

[Red Iron Crown]

Still about 2800m/s remaining, that ship has some legs. Many of the stock spaceplanes I see are pretty much limited to Kerbin's SoI. Where are you taking it?

[James_Eh]

After the visit to the fuel depot, Gus is off to Laythe. He likes to live on the edge. I kind of sort of think we might be able to squeak out a return to Kerbal without refuelling again. And if not, well, there are hitchhiker cans in orbit around Laythe left over from my Jool-5 mission that he can hang out in for a while.

[O-Doc]

8km/s of dV from nuclear engines would require your ship to be 63% fuel by mass. Does that sound close?

Yep. That's 7.8km/s dV. The best dV I've gotten in a solid SSTO design is 7.3km/s dv@LKO. That's pretty close to your theoretical max right there. So, I would question the legitimacy of 8km/s dV@LKO.

You can make huge dV savings on the way you pilot your craft. The best way to save fuel is to get to the limit of your air intake and be throttling down as you climb out of your intake threshold. Air resistance is what chews up the fuel but, you should makes sure you hit the max power range of the turbojet engine so that you are putting 250kN of energy into your ascent for as long as possible but, not lose that energy to air resistance. You can only max out your power range with adequate air supply at 1000m/s.

[Red Iron Crown]

Yep. That's 7.8km/s dV. The best dV I've gotten in a solid SSTO design is 7.3km/s dv@LKO. That's pretty close to your theoretical max right there. So, I would question the legitimacy of 8km/s dV@LKO.

The theoretical limit for nukes is much higher than for chemicals, over 17.2km/s. The number became far more feasible when James_Eh revealed his use of nuclear engines.

You can make huge dV savings on the way you pilot your craft. The best way to save fuel is to get to the limit of your air intake and be throttling down as you climb out of your intake threshold. Air resistance is what chews up the fuel but, you should makes sure you hit the max power range of the turbojet engine so that you are putting 250kN of energy into your ascent for as long as possible but, not lose that energy to air resistance. You can only max out your power range with adequate air supply at 1000m/s.

That's new advice to me, are you saying you should increase your climb rate to stay at 1000m/s and maximum thrust for as long as possible? My jet-assisted ascents spend only a moment at 1000m/s, they continue to accelerate and climb past there. Generally I slow the climb rate to almost nothing over 25km altitude and let the ship build up speed as it slowly climbs to flameout altitude. The thrust is far below maximum during that high altitude acceleration run. Am I doing it wrong?

[O-Doc]

The theoretical limit for nukes is much higher than for chemicals, over 17.2km/s. The number became far more feasible when James_Eh revealed his use of nuclear engines.

Yeah, but you have to be effective in atmo and I've found 7.5km/s dV@LKO is around as effective in atmo as you can be. I'd be happy to checkout anyone's crafts who claim a better than 7.5km/s dV@LKO. That's why I would say it's the theoretical limit. This is after thousands of test flights with hundreds of designs.

That's new advice to me, are you saying you should increase your climb rate to stay at 1000m/s and maximum thrust for as long as possible? My jet-assisted ascents spend only a moment at 1000m/s, they continue to accelerate and climb past there. Generally I slow the climb rate to almost nothing over 25km altitude and let the ship build up speed as it slowly climbs to flameout altitude. The thrust is far below maximum during that high altitude acceleration run. Am I doing it wrong?

You can't "stay" at 1000km/s but you can control the rate at which each turbojet on your craft gets to 250kN and then trails off. The most efficient method I've found is to get vertical from take off. Always use as much runway as you can. IE; don't pull of the runway until you are at terminal velocity. That's free vertical velocity converted by your wings. Then, go vertical to the point where you are running out of air. Your target speed is 750m/s(that's my basic jet cutoff target) to jump onto the turbojet "power train". Don't run out of air or you will move through 1km/s with your jets only putting out around 160kN. Remember, it's all an energy equation.

Once your power starts to decline past 200kN you are not going to see a huge power differential at varying air supplies until your engines start to cut out, so, get as high as you can after that. My ultimate target is 2130m/s at 40km. I don't need to be going at 2175m/s or more at 35km, that's a waste of fuel. But, you do need to balance your intake air to keep power down to get that final atmo "exit" velocity. That comes down to your craft design.

So, get to the max turbojet curve as quick as possible and stay there for as long as possible. Then, get the kell out of atmo asap.

[EdFred]

So Vx and Vy don't exist in KSP?

[O-Doc]

Yes they do. Velocity is in relation to prograde, everything seems calculated against that in terms of air intake, engine power and wing aerodynamics(that pea soup everyone describes it as). Until you are in space it's best to keep your navball on surface to compare your engine performance against atmo. So, when it switches to orbital, manually switch back to surface for your benchmark numbers.

As per my previous post, it's absolutely essential to have good Vy prior to your turbos max power range so you can convert that energy into orbital speed, not vertical.

[James_Eh]

I have to try the suggestions above. With a smaller craft methinks. Meanwhile:

Heading to the Fuel Depot. Here be the vehicle in question. This is basically what I wound up with in 100x100. Note the MJ confusion regarding dV. I am happy that it generated some discussion, though.

[Red Iron Crown]

The calculator is definitely confused, it looks like stage 1 is your current, correctly calculated nuclear stage and stage 0 is something fueling the turbojets. Is there some fuel tankage that can only feed the jets? Or maybe it's calculating what the jets can do with the leftover liquid fuel after you run out of oxidizer.

[James_Eh]

There are two teeny fuel tanks not visible in this picture on the underside of the "plane" which only serve the purpose of providing a halfway point for fuel lines because they wouldn't reach as far as I needed them to. (There are an identical pair visible in this screenshot, just inside the LV-N's. Well, you can only see one of then actually, and then just barely.) They are full of fuel and I suspect are only feeding the rams - that's why the MJ calculator has decided to split up the dV calcs. Usually there is only one line showing total dV and it changes dramatically depending upon which engines you have engaged. It's too late for an easy test because I have begun to refuel after a rather eventful docking procedure.

Thanks for not commenting on the... slapdash design.

[Red Iron Crown]

Thanks for not commenting on the... slapdash design.

I'm hardly one to criticize, most of my spaceplanes end up closely resembling a pile of parts spread around the apron at the end of the runway. Intact and in orbit has a beauty all its own. Besides, it's very difficult to make spaceplanes that both work well and look good with stock parts, most of the pretty spaceplanes are done with mod parts.

[James_Eh]

Yah, the one thing I have learned is keep the wheels tucked in just a bit so that they don't strike the two spikes of death at the end of the runway. (But not TOO far, else you veer one way or the other on your runway acceleration.) Anyway, Ritier is safely off to Jool, now I want to fiddle with a smaller plane and the suggestions above about optimizing the ramjet performance. I've always viewed that as a derivative of everything else I am doing, not as a control variable. I feel a bunch of backflips into the VAB coming up.