Building very heavy rockets

[SunlitZelkova]

Hello,

 

I have been trying to build a replica of the UR-700. A problem has arisen where due to its massive size, it is virtually several different rockets (not boosters, full fledged rockets) packeted together. As a result, it is too heavy to lift off.

 

It has six Rhino engines right now and weighs 1524 tons. My question is-

 

Is there a way to build such heavy rockets that will fly?

 

 

[Liam_The_Great]

That's too much fuel per engine. Each engine can only lift a finite amount of fuel, so you need to either take out some fuel or add more engines without adding more fuel.

[FleshJeb]

In addition to Liam's advice, Rhinos are vacuum-optimized. They produce relatively little thrust at sea level. I will avoid writing a paragraph and just say, use the Mammoth for your side boosters.

[paul_c]

What is the TWR of the first stage?

You want something like 1.2 - 1.5

Too low? Add more engines. Nowhere to attach them? Make the rocket shorter and use 2x, 3x, 4x, 5x or more fuel tanks joined side-to-side, then attach the needed engines under each fuel tank. And/or use SRBs too, firing them alongside main engines. 

[VoidSquid]

And in addition to what @FleshJeb and @paul_c already said:

1st stage has almost 1700 m/s (I'd put the launch clamps directly into the 1st stage too), ASL, probably way more vacuum dv as those are Rhino engines. That's quite a lot of dv.

I recommend for the 1st stage between 1000 and 1300 m/s (vac), using engines that are build for ASL operations like the Mainsail or SRBs.

2nd stage then 2200 to 2500 (depends on the dv for the 1st stage), engine-wise something between an ASL and a vacuum engine, e.g. the Skipper or the Mastodon or the Skiff, that stage should bring your rocket out of the atmosphere but not fully into orbit (avoiding space junk).

Finally, in space use any vacuum engine as it fits.

Edited March 3, 2021 by VoidSquid

[GuessingEveryDay]

Hmm... Looking at pictures of the UR-700, I can see why you're using Rhinos. However, the measurements of the boosters say that they are 3.75, but in Kerbal scale, that means they are 2.5m. So Mainsails or if you have MH, Mastodons are the way to go.

[swjr-swis]

4 hours ago, SunlitZelkova said:

Is there a way to build such heavy rockets that will fly?

Yes, there are ways. That and more, in fact. Just one recent example I can find in a quick search on the KerbalX craft share site: the Titan v2.

Replicas in (stock) KSP are not easy: due to the limited set of parts and different scales of kerbals, vehicles and solar system, you will find yourself struggling to find the right combination of size, looks, and functionality. For replicas, looks are often the most important parameter (size following closely), in which case you'll inevitably have to compromise on functionality. It's generally accepted that replicas require using special techniques, like clipping and craft file editing, to compensate. No one will fault you for that.

For this project specifically: if you wish to hold onto the relative size and power of the entire rocket, there really isn't a stock engine that looks much like the RD-270 on the first stage of the UR-700, or that delivers the relative power required. The Rhino, as others have already mentioned, was re-engineered at some point by Squad to be specifically a vacuum engine, and won't give you the raw ASL lift-off power that you would need.

The properly powered ASL alternatives, like the Vector and the Mammoth, when placed by themselves, change the engine looks a lot. Building a notch smaller (2.5m), you have the Mainsail at your disposition, which is a proper ASL engine, but looks a little less like the RD-270. You'll have to decide what is important to you in this project.

If you want to hold onto the looks and sheer size of the original, you could choose to clip a Vector into each Rhino, in such a way that the bells end at the same level (to avoid them obstructing each others' thrust). You'll end up with very powerful ASL thrust, and unless someone looks directly into the bell, you should get a very convincing effect.

 

[Anonymous49]

9 hours ago, SunlitZelkova said:

Hello,

 

I have been trying to build a replica of the UR-700. A problem has arisen where due to its massive size, it is virtually several different rockets (not boosters, full fledged rockets) packeted together. As a result, it is too heavy to lift off.

 

It has six Rhino engines right now and weighs 1524 tons. My question is-

 

Is there a way to build such heavy rockets that will fly?

 

 

that happens if you have a too small twr. Don't use rhinos use a bigger engine like the mammoth.

or if not the mastodon engine 

[Spricigo]

12 hours ago, swjr-swis said:

Yes, there are ways. That and more, in fact. Just one recent example I can find in a quick search on the KerbalX craft share site: the Krakatoa

Some people have no shame.

Spoiler

Kataklysm

None at all!!  

 

 

13 hours ago, swjr-swis said:

The Rhino, as others have already mentioned, was re-engineered at some point by Squad to be specifically a vacuum engine, and won't give you the raw ASL lift-off power that you would need.

Food for thought: The Rhino happens to have more thrust ASL than all but 6 stock engines. (Mammoth, Clydesdale, Twin Boar, Thoroughbred, Mainsail and Mastodon)

[VoidSquid]

36 minutes ago, Spricigo said:

The Rhino happens to have more thrust ASL than all but 6 stock engines

Hmm... as per the Wiki, thrust atm (kN):

Mainsail 1 379.03
Twin-Boar 1 866.67
Clydesdale 2 948.936

Rhino 1 205.88

 

 

Edited March 4, 2021 by VoidSquid

[Anonymous49]

37 minutes ago, VoidSquid said:

Hmm... as per the Wiki, thrust atm (kN):

Mainsail 1 379.03
Twin-Boar 1 866.67
Clydesdale 2 948.936

Rhino 1 205.88

 

 

mastodon?

[si2504]

jumping on this thread, although i don't have an issue with getting the correct TWR for my rockets, my issue is actually getting the damn things on the launch pad without them bouncing about and blowing up!!! I'm using RSS and attempting to put a heavy payload into orbit. this requires a BIG rocket. I'm talking a rocket with a starting mass of 9000 tonnes. I've tried adding struts, struts and more struts. nothing will hold it stable on the launch pad, it just bounces a little then blows up.

[VoidSquid]

@si2504 Maybe this mod can help you 

 

[Spricigo]

41 minutes ago, si2504 said:

my issue is actually getting the damn things on the launch pad without them bouncing about and blowing up!!!

 

If it is just at loading you may try to activate Unbreakable Joints and No Crash Damage, load it, wait for it to stabilize and deactivate the options.

But, is not unlikely for further problems when you try to fly the thing. Beside the aforementioned KJR, I recommend mods with big parts (reducing part count helps a lot, as stronger joint also do) and patience, lots and lots of patience.

 

[si2504]

58 minutes ago, VoidSquid said:

@si2504 Maybe this mod can help you 

 

yeah already using it, unfortunately it does very little for real sized/very large rockets. I've also tried KJR cont.

6 minutes ago, Spricigo said:

 

If it is just at loading you may try to activate Unbreakable Joints and No Crash Damage, load it, wait for it to stabilize and deactivate the options.

But, is not unlikely for further problems when you try to fly the thing. Beside the aforementioned KJR, I recommend mods with big parts (reducing part count helps a lot, as stronger joint also do) and patience, lots and lots of patience.

 

also tried activating the above with no success. The amount of launch clamps and struts etc I've tried also is ridiculous. Patience is also something that is wearing very thin, I'm wasting my evenings just trying to get one rocket to sit on the launch pad, never mind lifting off. 

[king of nowhere]

1500 tons is not all that big as far as large rockets go.  here is a small guide for big rockets

1) if the rocket does not lift off, add moar boosters!

2) if moar boosters aren't doing the trick, add moar engines!

3) if your rocket is falling apart during the flight, add moar struts!

 

the only limit is when your pc lags too badly because the struts increase part count. My biggest launch was some 700 parts and 5000 tons, i used 30 mammoth and it flew like a dream.

Edited March 4, 2021 by king of nowhere

[Spricigo]

12 minutes ago, si2504 said:

Patience is also something that is wearing very thin,

Been there...In fact lack of patience with those issues is what is preventing me to build anything bigger than the Kataklysm. Your craft is already 50% more massive than my current limit. Still, quite small limit as demonstrated by others.

I'm not sure if the best advice at this point is about how to build massive crafts or about how to accommodate your goals to something not so big. Only you can judge that. But Is good to remember that  you don't play the game to get frustrated.

[jimmymcgoochie]

The biggest rocket I ever launched was about 7 kilotons on the launchpad with a payload of 1kt to low orbit of JNSQ’s Kerbin. Big parts are essential- I used Near Future Launch Vehicles for its 5m and 7.5m fuel tanks along with 43 engines in total on the first stage core and boosters; it lagged the game extensively due to the sheer size of the thing and the number of parts involved, but it worked.

Put fuel tanks on top of your solid boosters and enable crossfeed on their decouplers, this gives you more fuel for the core stage without adding dead weight as the empty tanks get dropped with the boosters. This works best with bigger boosters, but make sure the fuel in the tanks gets drained before the boosters run out of solid fuel or you’ll either throw away usable fuel and delta-V by decoupling them, or end up dragging the dead weight of the boosters along with you until the fuel runs out in the tanks and lose delta-V that way.

[king of nowhere]

15 hours ago, si2504 said:

I'm wasting my evenings just trying to get one rocket to sit on the launch pad, never mind lifting off. 

for that you need launch stabilizers. if the rocket is still unstable, add more launch stabilizers.

[si2504]

4 hours ago, king of nowhere said:

for that you need launch stabilizers. if the rocket is still unstable, add more launch stabilizers.

see my post above. I've tried adding launch clamps, and more launch clamps and even more launch clamps

8 hours ago, jimmymcgoochie said:

The biggest rocket I ever launched was about 7 kilotons on the launchpad with a payload of 1kt to low orbit of JNSQ’s Kerbin. Big parts are essential- I used Near Future Launch Vehicles for its 5m and 7.5m fuel tanks along with 43 engines in total on the first stage core and boosters; it lagged the game extensively due to the sheer size of the thing and the number of parts involved, but it worked.

Put fuel tanks on top of your solid boosters and enable crossfeed on their decouplers, this gives you more fuel for the core stage without adding dead weight as the empty tanks get dropped with the boosters. This works best with bigger boosters, but make sure the fuel in the tanks gets drained before the boosters run out of solid fuel or you’ll either throw away usable fuel and delta-V by decoupling them, or end up dragging the dead weight of the boosters along with you until the fuel runs out in the tanks and lose delta-V that way.

I use mainly Procedural parts on my crafts. Especially tanks & fairings, Its worth noting that I also use SMURFF as a RO alternative.

[king of nowhere]

16 minutes ago, si2504 said:

see my post above. I've tried adding launch clamps, and more launch clamps and even more launch clamps

sorry, i missed it.

very strange. i gave a better look at the image, it's a pretty simple rocket, and it's not particularly big. besides the lack of thrust for launch, everything else should be fine. and there are way more struts than necessary. my Dream Big was bigger, had a more complex shape with plenty of stuff attached hapazardly, and it got away with less struts. I'm thinking perhaps there is some specific problem, maybe some clipping gone wrong...

i would suggest you share your craft on kerbalx and i could try and give it a good look, but we have different mods so i don't think it's possible

[Spricigo]

2 hours ago, king of nowhere said:

i gave a better look at the image

oops! which image? @si2504 is not the OP.

si2504, how about starting a new question to avoid further confusion? I think we already provided the 'generic advice' but with further details maybe there is some specific we can provide help.

[si2504]

So the main issue was with building large rockets to deliver heavy payloads. No matter what I did, the rocket would not sit or remain stable once I actually managed to get it off the launch pad.

 

Since, I've been having a play about in the VAB and taking note of each stages COM with a full tank, and an empty tank. Thus here lied my problem. Due to the sheer size of my rocket, by the time the first stage was empty, and taking boil off of other tanks into account, the vessels COM was way too far forward, this was causing flips, joint instability and inevitably, constant launch failures.

 

I'm now sure to check each stages COM and COT with full and empty tanks.

[swjr-swis]

9 hours ago, si2504 said:

the vessels COM was way too far forward

, said no successful rocket designer ever. Are you sure you meant to say CoM (center of mass)?

You *want* the CoM to be as far forward (the end pointing towards space) as possible with rockets.  Heavy stuff at the front, draggy stuff at the back, basically.

Flipping mid-flight happens when the CoM is -or moves- too far down/back (the end where the fire comes out), because the drag forces become overwhelming and 'pull' everything back that isn't powering through by sheer momentum (making aerospace engineers everywhere cringe right now, but the mental image works for me). 

[paul_c]

si2504, this is a bit confusing because you've jumped on someone else's thread, and also not really given much info apart from your big rockets wobble. Then you've misattributed a shift of Centre of Mass forwards, with some kind of instability. I dare say, what is actually happening is that the weight/mass balance isn't the issue, but the drag is higher at the front than the rear. As speeds build, the aerodynamic forces destabilise the rocket (as swjr-swis mentions above).

It would help to post some pics of the design.

Of course, stage 2 needs to have its aerodynamics balanced as well as stage 1 (and stage 3, etc).......

Also, AFAIK KSP doesn't simulate "boil off" of fuel over time, tanks are modelled as perfectly sealed (unless you have a weird mod in place.....)