SLS Block 2 core stage TWR?

[FennexFox]

I made a replica of SLS in Realism Overhaul in very simple method.

Put a engine, stack procedural tanks, stretch it to match figure, and so on.

The problem is, when I detatched SRB,

core stage TWR is dropped to 0.8 in the middle of ascent,

which make it impossible to circularize.

I wonder whether it`s real or not.

If it`s real, I should know proper ascent profile.

If it`s not real, I want to know what is real figure.

Edited November 2, 2014 by FennexFox

[Dangerous_Beans]

~0.8 T/W for the core stage looks about right, based of the stats on wikipedia.

try a steeper ascent profile so you have more vertical speed when you stage the boosters. also don't worry about always having positive vertical velocity, you can still get to space if you start dropping back down provided you are gaining horizontal velocity fast enough.

[Taki117]

You might actually have better luck just using the Stock Parts. I made an SLS replica using stock parts and SDHI.

[NathanKell]

You can calculate it: get the vacuum thrust and vacuum Isp of the SLS core. Get the burn times of the SRBs. Calculate how much fuel would be burnt as tons fuel = burn time in s * thrust in MN / IspV / 9.80665. Subtract that tonnage from the stack, then calculate TWR.

[LethalDose]

The problem is, when I detatched SRB,

core stage TWR is dropped to 0.8 in the middle of ascent,

which make it impossible to circularize..

WTF does TWR ~ 0.8 have to do with "impossible to circularize"? The only time you need TWR > 1 is to get off the pad. After that it's gravy. It's easier if you get your Apoapsis above the atmosphere while you still have a "high" TWR, but even that's not required.

[FennexFox]

WTF does TWR ~ 0.8 have to do with "impossible to circularize"? The only time you need TWR > 1 is to get off the pad. After that it's gravy. It's easier if you get your Apoapsis above the atmosphere while you still have a "high" TWR, but even that's not required.

Since it was a middle of ascent, as I stated. 5 segment SRB burns for only 2 minute, which means I`m yet in the atmosphere.

That`s why "WTF" it`s impossible to circularize.

The other guys: Thank you for all your reply. I found some figures wrong.

[LethalDose]

Since it was a middle of ascent, as I stated. 5 segment SRB burns for only 2 minute, which means I`m yet in the atmosphere.

That`s why "WTF" it`s impossible to circularize.

The other guys: Thank you for all your reply. I found some figures wrong.

Circularization is a function of horizontal speed, which requires no minimum TWR to increase. Increasing Horizontal speed is also the MORE efficient way of raising your apoapsis.

A TWR < 1 means your vertical speed will be decelerating, which is fine because your vertical speed has to decelerate to zero when you circularize. The TWR also increases as the burn goes on, so it's no big deal if the rocket goes sideways for a few seconds after staging.

I'm also stumped trying to figure out which SRB you're talking about. None of the stock SRBs have a 2 minute burn time.

The point here is that you can circularize even if your TWR is 0.8 2 min into your launch (there is no minimum TWR at any point after launch required to circularize), which is supported by what everyone else has said here.

[NathanKell]

LethalDose: consider it possible that a person is playing modded KSP

(in this case RSS + RO, so the *actual* SLS Block 2 with the *actual* SRBs. Going to a TWR of 0.8 when you're only two minutes into ascent is Very Bad News on RSS.)

[LethalDose]

LethalDose: consider it possible that a person is playing modded KSP

(in this case RSS + RO, so the *actual* SLS Block 2 with the *actual* SRBs. Going to a TWR of 0.8 when you're only two minutes into ascent is Very Bad News on RSS.)

Okay, first, I acknowledge I missed that it was stated that he was using Realism Overhaul in the OP. My bad, mea culpa.

That being said, my original point stands: Having a TWR < 1 during ascent does not mean it is impossible to reach a circular orbit as was stated in the OP:

The problem is, when I detatched SRB,

core stage TWR is dropped to 0.8 in the middle of ascent,

which make it impossible to circularize.

The reason that this is an issue is that I've seen this myth that "kicker" stages need a TWR > 1 propagated in other locations. It's simply untrue. In these cases, having a TWR < 1 at staging, as I stated above, means that your vertical speed will decelerate, but thats okay. This stage just acts as a sustainer stage that keeps the rocket moving upward, albeit a bit slower, even in the atmosphere! This helps the rocket get outside the atmosphere where the upper stages can do their work.

If you need a real-life example of this, look at the Saturn V: The first stage (S-IC) cut out after 6 minutes at 67 kilometers, which is still in the Earth's atmoshere. If my math is right, the S-II, S-IVB, and CSM had a combined mass of ~ 630,000 kgs, and the S-II could produce 4,400 kN of thrust, meaning the vehicle had a TWR of ~ 0.71.

So the Saturn V's TWR dropped even below 0.8, and the CSM + S-IVB were able to go to space in a circular orbit.

If having a SRB that only burns for two minutes is screwing it up, then fine, but say that's the problem! The core stage's TWR has nothing to do with this at all, like the OP is complaining about.

Edited: "Core stage" used instead of "second stage"

Edited November 2, 2014 by LethalDose

[LethalDose]

LethalDose: consider it possible that a person is playing modded KSP

(in this case RSS + RO, so the *actual* SLS Block 2 with the *actual* SRBs. Going to a TWR of 0.8 when you're only two minutes into ascent is Very Bad News on RSS.)

~0.8 T/W for the core stage looks about right, based of the stats on wikipedia.

Well, if what Dangerous Beans is saying is true (I didn't do the math ~ 1 million kg & 8000 kN = TWR 0.8, so looks right), and the wiki is also correct where it explicitly states that's the boosters burn for 2 minutes, then NASA is planning on having "Very Bad News" during the ascent of every block II SLS launch if RSS is appropriate balanced.

/whsipers PSSSSST... I think that means there's a problem with the RO/RSS balance, not the TWR.

Edited November 2, 2014 by LethalDose

[Jouni]

The point here is that you can circularize even if your TWR is 0.8 2 min into your launch (there is no minimum TWR at any point after launch required to circularize), which is supported by what everyone else has said here.

There is always a minimum TWR required to reach orbit after launch. If the TWR is too low, the crashes into ground or burns in the atmosphere before it has gained enough horizontal speed to circularize.

[NathanKell]

There is no "balance" in RSS/RO, just real numbers. Also, FennexFox mentioned above that s/he was using wrong stats. (If 0.8 is the right stat after SRB burnout, then presumably the core is getting lofted very high indeed.)

You're 100% right about the necessity of that myth dying. Consider circularizing a 20t payload from Atlas V on the anemic SEC they use. To do that, though, you need the main stage to loft you quite high, and you'll be reaching orbital velocity well after apogee.

As for Saturn V, you're indeed correct. Here's an acceleration vs. time image though it's worth noting that S-IC burned for ~161 seconds, with quite possibly a higher TWR than the SSRBs provide SLS (since their fuel is shaped and grained to provide precise thrust throughout ascent, whereas F-1s were unthrottleable).

The problem as described is exactly as the OP says: burn time is too long on the core for the velocity it must produce for the ascent path it was flown. Yes, it's very true that it's possible to "circularize" on 0.8 TWR. It's possible to do it on 0.01, depending on how much velocity you need to make up and the time you have to do it in...

(And the excess delta V to make up for additional gravity and steering losses)

EDIT: Oh, speaking of Saturn V, here's a cool image showing a lot of ascent parameters.

Edited November 2, 2014 by NathanKell

[FennexFox]

Thank you LethalDose, that`s what I`m very familiar with.

I launched most of my sats & crafts with Delta IV replicated rocket, which has only <0.4 TWR upper stage.

This case it was problem only because it was * Middle of Ascent * which means my apogee is still inside of atmosphere(>130km, in RO).

Plus, I made a incorrect replica of SLS block 2, which has significantly large * Common Core * then block 1, was to fail.

(I read some figures that indicate only block 1 of SLS has significantly small * Common Core* then others,

but I think that was wrong. It makes much sense that every SLS has that small common core.)

There`s no problem with RO/RSS or WIKI figures.

Edited November 3, 2014 by FennexFox

[Borogove]

Are you burning the core stage at the same time as the SRBs? SLS does/will; if you aren't, then you'll have a full (heavy) core stage tank at SRB burnout and will be going that much lower and slower.