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RO / Procedural Fuel Tanks / Real fuels . I'm losing Delta-V more quickly that I expected.


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Hi,

I have been trying to do a SSTO with RO, but I have some problems. I need some advice, tips for this :)

I have some issues with my Delta - V. At VAB my Delta-V is about 12500m/s.

Doing the ascent let say when my speed is about 2000m/s I still having delta-v 7400 m/s, but I don't know why the faster I go the quicker my delta v is consumed (this is not linear at all) and finally I finish my ascent at i.e 6900 m/s (very sad haha)

Details of the SSTO:

- I'm using Balloon Cryogenic as Fuel tank type

- HydroLox fuel used 86% utilization (4 tanks in total)

- 76 tons dry more than 1000 tons wet.

- I'm limiting the acceleration to 18 m/s2 and switching off engines to maintain a reasonable thrust throughout all the ascent.

Thanks

Edited by jrodriguez
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But is not a bit here. I am losing more than a 30% I think.

When you say SSTO do you mean a SSTO rocket, or a spaceplane?

That is roughly what I'd expect, if you did not account for gravity and especially drag losses. Also, engines have worse ISP the lower you are in the atmoshpere.

Just think about it like this, to get to LKO you need roughly 4500 m/s of dV if I remember correctly (I don't use stock aero so I'm not sure) however, orbital velocity in LKO is only ~2km/s.

--> ~50% losses

All of this depends largely on how efficient your staging is, and more importantly your ascent profile. As soon as you exceed terminal velocity your drag losses go through the roof. If your design is suboptimal [to low L/D] and you have to reach very high speeds while still fairly low in the atmosphere you will loose even more due to drag.

If you can turn off engines and still have enough thrust you have too many engines, they way a lot and if you are not running them at 100% thrust, some fraction (depending on thrust levels) is dead weight which means more drag and more gravity losses.

Hope this helps a little, if not it would help us help you if you could give us some more info about your ascent profile.

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@TheXRuler: He's using Realism Overhaul. Based on the dV numbers he's quoting, that also includes FAR and Real Solar System. So he needs ~10 km/s dV into low Earth orbit - by kerosene engine numbers - out of which about a quarter will be lost.

@jrodriguez: is that 12.5 km/s dV just vacuum dV, or atmospheric? If you're running with pure hydrogen engines, you're going to have high atmospheric losses, because atmospheric performance of hydrogen engines is very bad. They go from 450s Isp down to 350s or something, whereas a kerosene engine would only drop from 330s to 300s or thereabouts.

As a result, your vacuum dV number with kerosene engines is a close enough approximation that you can handwave the lower atmospheric Isp away and still make it to orbit, but with hydrogen engines the vacuum dV number is in no way, shape or form usable as a number for planning ascents out of an atmosphere.

Considering how close you are to orbital velocity - you should need about 7500 m/s, and you make 6900 - I wager that a large chunk of your "missing" dV is simply lost because your engines have so much Isp loss at liftoff. A good indicator for this: most of your dV disappears early on, while late in the flight you pretty much get what you think you should get. By then, the atmosphere is gone and your hydrogen engines run at full efficiency.

This is, incidentally, why pure hydrogen rockets are very unpopular IRL. In fact, there's only one single pure hydrogen rocket that's flying right now: the Delta IV Heavy. And even the launch provider itself calls it overpriced and inefficient and says that they will never repeat that particular experiment. The rocket only still flies today because there's no alternative to replace it with. All other hydrogen rockets have solid rocket boosters that do the majority of the lifting off the pad (Space Shuttle, Ariane 5, Delta IV medium, H-IIA etc). The famous Saturn V was all hydrogen... except the first stage, which was kerosene in order to give it a kick into the regions were hydrogen was efficient.

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Thanks guys for you answers.

@TheXRuler: My ascent path is 50% aiming to a 220x220km LEO with Gravity turn beginning at 2200 m.

@Streetwind: I'm going to provide more figures:

dV Vacuum = 12.6 km/s .TWR = 1.52(25.59)

dV Atmospheric = 9.95 km/s TWR = 1.20(20.19)

I have 8 x RD-0120 engines with ISP 359/455

Thanks for your explanation of the ISP but actually what I found strange is that when I see that my Delta - V is consumed more quickly is when I'm finishing the ascent!

I will provide some pictures and a chart with my speed and the remaining delta-v but as an example it could be something like this:

Altitude Speed Delta-V

-------- -------- --------

18000m 360m/s 9500m/s

25km 600m/s 9000m/s

60km 1400m/s 8000m/s

120km 3500m/s 5000m/s

180km 5500m/s 3000m/s

200km 6500m/s 1000m/s

220km 6900m/s 0

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Hmmm. With that kind of atmospheric dV you should be set, technically.

If your dV is evaporating at the end of ascent... my first impulse would be to say that one or more of the engines you're running at that point is occluded by something and is not generating thrust, only consuming fuel. You say that you start with all engines running and then turn them off one by one to maintain acceleration. If you have at least 2 still running by the time you run out of duel, then one of them might not be generating thrust. That would also explain why the effect becomes more pronounced the further you go up, because you keep deactivating working engines yet the occluded one keeps running.

Easily confirmed by hyperediting the craft into orbit and checking each engine individually for correct thrust output.

Of course, if you're only on one engine at that point then you shouldn't be accelerating at all anymore, and from your reports you're still getting some sort of thrust. In that case I got nothin'. It perhaps might be something best asked in the Realism Overhaul discussion thread over in the add-on affairs section.

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At the risk of being a party pooper, is it even possible to do a SSTO in RO? Nobody has yet figured out how to do one in real life - everything is at least 1.5 stages, and most are 2 or more. Turning off engines as you ascend means they turn into dead weight. Maybe cut the number of engines down to 6 and then add some SRBs for the initial kick up to speed?

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Warmer engines lose efficiency. Take gravity loses and aerodynamic drag into consideration as well. The more Gs that you endure, the more loses you take from density-volume ratio [That one's complicated] Also, don't run full thrust in the atmosphere. You'll hit terminal velocity like that, and take more loses. Well, not Delta-V loses, but loses in end velocity.

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My guess is your fuel is boiling off due to convective heating.

Convective... heating...?

You mean like, the engines are not properly insulated from the tanks or something?

...is that even something the player can influence? *confused*

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At the risk of being a party pooper, is it even possible to do a SSTO in RO? Nobody has yet figured out how to do one in real life - everything is at least 1.5 stages, and most are 2 or more. Turning off engines as you ascend means they turn into dead weight. Maybe cut the number of engines down to 6 and then add some SRBs for the initial kick up to speed?

haha fair point. I think this is the reason why I'm trying to do it :)

I think is do-able. I'm not sure if my goal is possible though. I'm trying to do is a full reusable SSTO with heatshields that can move 180 degrees to cover engines in order to survive to the deadly reentry!

- - - Updated - - -

My guess is your fuel is boiling off due to convective heating.

hmm sounds interesting and something that I need to check. In fact I have a procedural tank covering 2/3 parts of the engines (just because aesthetic I have to say) . But I didn't think that this can be causing a increase on the boiling off rate, but I guess that even a small increase on the temps can be causing this.

Do you have any advice Nathan? If I change the type of the tanks this will be any better? Maybe if I make shorter the tank that is covering my engines this will be better.

I will test this this evening - I'm at the office now :)

Thanks.

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This is normal and expected, even in the stock game. As fuel is burned the spacecraft gets lighter, so each second of burn provides more change in velocity.

Well, not in my case. I have limited the acceleration to 18 m/s2, so my TWR is constant and Thrust gets reduced as my SSTO gets lighter, so I'm sure that in stock game you will be experiencing a linear decrease of your dV.

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Well, not in my case. I have limited the acceleration to 18 m/s2, so my TWR is constant and Thrust gets reduced as my SSTO gets lighter, so I'm sure that in stock game you will be experiencing a linear decrease of your dV.

In atmosphere it still won't be linear. Drag force is increasing with the square of speed, so the thrust needed for 18m/s2 is always increasing. Or for that matter, gravity drag is a factor even if you're out of atmo (though it is usually only a factor early on in an ascent).

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My guess is your fuel is boiling off due to convective heating.

Hi Nathan,

I have been doing a bit of testing. Please find a picture below. I'm wondering if this could be a bug? Suddenly, the ambient temperature has increased from -200C to 1.15C (BTW I'm using the WIP Procedural Heat-shields)

lBdW2Nz.png

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I find it basically impossible to believe that ambient temperature was ever -200C in the atmosphere; that's incredibly cold. Certainly the temperature curve in RSS for Earth doesn't have -200...If it did show as that, that's definitely a bug. Can you describe when ambient does show as -200C?

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I find it basically impossible to believe that ambient temperature was ever -200C in the atmosphere; that's incredibly cold. Certainly the temperature curve in RSS for Earth doesn't have -200...If it did show as that, that's definitely a bug. Can you describe when ambient does show as -200C?

Hi Nathan,

Find below some screenshots with the ambient temp at -200 C. As you can see there is a extreme temp change between 185km and 198km from -200C to -1.15C. However, I'm not sure this can be related with my delta-v problem. Also you can see there that my speed is about 4100 m/s and my remaining dV is about 4277m/s

Du6HNLC.png

xj0b9xH.png

Edited by jrodriguez
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Ah, I see what's going on I guess--it's switching to a "vacuum" temperature, the air is no longer shielding you from solar radiation. Sorry, didn't realize you were in space at both times.

That's all totally screwy of course, and I believe 1.0 will allow us to do much, much better.

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Thanks Nathan.

I did some testing yesterday. I swapped the WIP Procedural Heat shield for the standard DLR Heat shields (same sizes but less weight due to the amount of ablative material) .

And......I MADE IT! (And I still having about 500 m/s for landing). Besides, I didn't see the extreme temp change from -200 C to -1.5 C but I can't assert yet that the WIP Heat shields is causing this (I will do more testing and send you feedback about this)

Taking into account Nathan's answer I guess that there is a lot of things that can be causing this, some of them expected (ISP, ascent path, boil off) but other caused by limitations on the current KSP version or possible issues.

Thanks all for your comments, I will post my SSTO on the spacecraft exchange as soon as I finish the optimization phase.

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A couple of pictures of my SSTO landed and the payload (sorry I'm a happy man today haha). 33 tons of payload capacity, that is 17% more than the Delta-IV Heavy, however my SSTO is about 40% more heavy. Would it be profitable if it is partially reusable? Is this completely SciFi haha?

zHR8UM0.jpg

LaU0R5t.jpg

Edited by jrodriguez
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