Problem with terrier engines?

[Korthan]

I tried using a design from the web-based rocket optimizer, and got weird behavior. I have a ship with a total launch weight of 30 tons. The initial stage has LV-909 Terrier engines. According both to the Web and MechJeb, TWR is about 1.2. However, the rocket won't climb off the pad. Once the clamps release, it just sinks down, with throttle wide open.

Did the stats for the Terrier change? Should six of them be able to lift 30 tons?

Advice appreciated...

Edited August 26, 2015 by Korthan

[sal_vager]

It's probably a pre-1.0 design, as today you wouldn't want to make a craft with 909's on the first stage, the reason is that engine thrust is affected by air pressure now, where before it was ISP.

The Terrier is intended for use in a vacuum and is optimised for such, using it at sea level results in a massive loss in thrust.

You're going to have to swap them for other engines.

[Gaarst]

Terriers have terrible Isp asl.

In other words, they produce very low thurst in low atmosphere. Your TWR of 1.2 was probably predicted for vacuum use, and therefore completely false for atmospheric use.

EDIT: ninja'd

[GoSlash27]

I tried using a design from the web-based rocket optimizer, and got weird behavior. I have a ship with a total launch weight of 30 tons. The initial stage has LV-909 Terrier engines. According both to the Web and MechJeb, TWR is about 1.2. However, the rocket won't climb off the pad. Once the clamps release, it just sinks down, with throttle wide open.

Did the stats for the Terrier change? Should six of them be able to lift 30 tons?

Advice appreciated...

Korthan,

The stats changed a little, but what really changed is the physics. Engines now change their thrust with Isp. If you have crappy Isp (like the Terrier has at sea level), you will now have crappy thrust to go along with it.

The optimizer has evidently not yet been updated to reflect this.

Best,

-Slashy

[Korthan]

Its my first time playing with the optimizer, so it may be off. But MechJeb also predicts a 1.2 TWR, and I thought it was current. Off to the MechJeb thread....

[cantab]

Mechjeb might be showing vacuum figures.

[Red Iron Crown]

You might like this updated tool for finding optimal engines: http://meithan.x10.mx/KSP/engines/

[RoboRay]

MechJeb also predicts a 1.2 TWR, and I thought it was current. Off to the MechJeb thread....

In MechJeb's Delta-V panel, "TWR" is vacuum TWR. "SLT" is sea-level TWR.

[Korthan]

In MechJeb's Delta-V panel, "TWR" is vacuum TWR. "SLT" is sea-level TWR.

Aha! That's the answer!

Thanks!

[Ripper2900]

Dependant on the design of your ship, I suggest you add a first-stage. A SRB or a small liquid booster. You just want to bring your craft up a handful km. Then that terrier will start biting. It's wonderful in vacuum but it socks at sea level, so find some way of reducing the pressure (bring it higher up before you start using it)

[Sharpy]

Rule of thumb: at 10km altitude vacuum engines get 90% of their vacuum ISp.

It's not so important for rockets, but it means everything for spaceplanes: you don't need atmospheric rockets for them - carry them on jets to 20km and switch to vacuum engines and they will perform just fine despite still being 50km deep in the atmosphere. It's just the first few kilometers that kill the thrust.

[KerikBalm]

Which is one reason I wish rapier closed cycle was more optimized for vacuum thrust.

I suppose conceptually, its using the same nozzles that it uses in jet mode... but even those should be optimized for >10km

[Sharpy]

They are. Its atmospheric performance is worse at lower speeds and altitudes than that of Whiplash.

[Dispatcher]

Your engine choice(s) should be dictated by your mission and design, of course. That said, use the link to the graph (not the chart) in my sig line to view a comparison of engines performing under rather specific conditions. In this case, the test is of single stage performance from the KSC, using a single inline engine (or 3 radials).