SRB-powered landers

[ammonia_ocean]

So I've done my fair share of landings on bodies with atmospheres, and one of the things I've noticed about my attitude towards them is that I'm always nervous about the ascent – running out of fuel, etc. So what I've done a few times is strapped solid-rocket boosters onto my landers, and then fired them at the same time as the main engines in order to provide an initial boost in the lower parts of the atmosphere. (I've actually only done this for Duna and Laythe landers; the Duna one was all-stock, nuclear-powered, and had four RT-10s; the Laythe one used three RT-10 boosters and the HOME radial aerospikes for the first stage, then an LV-909 for the second stage). So what I'm wondering is: is this actually effective? Has anyone else done this? Does the quicker, easier ascent outweigh the weight costs of toting several solid-rocket boosters hundreds of thousands of kilometers and the difficulties incurred in landing them safely, or would I be better off just going with liquid boosters or a bigger lander?

[MalfunctionM1Ke]

Eeeerrrrrr... well I could only guess and I am not pretty good when it comes to crunching the Numbers

But in my opinion the SRB's are only a cheap way (If Money will be implemented) to get some additional thrust on take off at Kerbin.

I dont really use them anymore because Liquid Fuel Engines are much more efficient and controllable.

Besides this, feel free to do what you want.

[AlphaFish]

I think that this is definitely an interesting possibility! I myself have used two seperatrons on a Minmus lander to do a suicide burn right before landing. It seems that you would need Kerbal Engineer or similar to calculate the TWR, in conjunction with precisely planned numbers for velocity. In short, it would take a LOT of precise planning, and it may not be worth it in the long run. I can see this being useful on atmospheric bodies if you don't want to tote around a lot of parachutes, especially for Duna or Laythe, which have thinner atmospheres.

[Motokid600]

It really depends on the mission. A manned land and return science trip to Duna for instance is best done Apollo style. Leave the lander/lab behind and have a small, efficient ascent stage launch to rendezvou with a return stage.

If your taking all your science back to Kerbin however then an all-in-one lander would still be better off with liquid boosters. They'll get you much farther. Unless your taking off from Eve ( god forbid ) then maybe the extra thrust would be needed. But for bodies smaller then Kerbin.. no.

Edited December 3, 2013 by Motokid600

[capi3101]

Well...there's almost always a trade going on between thrust and delta-V; increasing one tends to come at the cost of the other (in general - there may be cases where this is not true).

To judge if it's a useful technique in your case, I'd need some data on the lander - its mass, liquid engine, fuel capacity, etc.

Without that data, I'm going to have to give you a solid "maybe". It certainly sounds like it might've been a good choice for that Duna lander, though I have to question the need for four boosters...

[Kegereneku]

Don't do that !

Solid Booster have an HORRIBLE weight-efficiency, whatever you gain carrying solid-booster around will be less than if you used drop tank for longer burn or even drop tank with actual LV-T30 engines.

[Psycix]

A 3 man capsule can return from the mun with one keypress using the RT10 SRB. (Of course, this only works from the Kerbin-facing side in a somewhat retrograde angle.)

This makes a very simple and reliable escape pod to bring everybody home ASAP.

@Kegereneku

While that may be true, the SRB's also have a very low dry weight. If your payload is small and requires a high TWR, it is actually one of the most efficient engines.

Edited December 3, 2013 by Psycix

[DrMonte]

... So what I'm wondering is: is this actually effective?

- No.

There will never ever be the case when SRB will be more effective than LF. Although there are plenty of other reasons to use them. Like - "I'm too lazy to modify my nice lifter to lift few extra tons so I'll just add more boosters".

[Seret]

Don't do that !

Solid Booster have an HORRIBLE weight-efficiency, whatever you gain carrying solid-booster around will be less than if you used drop tank for longer burn or even drop tank with actual LV-T30 engines.

This. SRBs have a lot of thrust, but terrible efficiency. For a given amount of extra ÃŽâ€v you'll be able to do it for less weight by just adding more liquid fuel. If you're worried about having enough fuel for a landing and ascent get either Mechjeb or Kerbal Engineer and see what the actual ÃŽâ€v of your lander stage is. Or just do the sums by hand, you can get the weight of all the parts from the wiki. If you want to be extra sure you could always design it to drop some of the empty tanks before ascending.

[Psycix]

- No.

There will never ever be the case when SRB will be more effective than LF.

I challenge that statement!

http://imgur.com/a/cbbjG#61

[Skorpychan]

I'd advise against using them. The lack of throttle control makes it insanely likely you'll end up somewhere you don't want to be. Pack liquid-fuel drop tanks, either with or without their own engines.

[Starwhip]

Don't use them for braking, unless you calculated it beforehand. I use Sepatron motors for ascent stages, giving a small boost to separate landing from ascent.

[700NitroXpress]

Duna is easy to land on and easy to get off of. Laythe is a little different, but the low gravity makes it easier than Kerban. For a Duna lander, I'd say it was a waste. For Laythe I'd also say it was a waste there too because Laythe supports air intake engines and jet engines are more efficient than SRB's by a long shot and far more effective for getting back into orbit. It's better to just have a lightweight SSTO lander for every planet except Eve. SRB's are basically useless.

[Seret]

Don't use them for braking, unless you calculated it beforehand.

TBH I have used them for braking, my station around Kerbin has several seperatrons on each escape pod to put them on an entry trajectory. Tbh I could probably have done the same for fewer parts using a tiny liquid fuel engine, but solid rockets just seemed right for an escape system.

[Kegereneku]

A 3 man capsule can return from the mun with one keypress using the RT10 SRB. (Of course, this only works from the Kerbin-facing side in a somewhat retrograde angle.)

This makes a very simple and reliable escape pod to bring everybody home ASAP.

@Kegereneku

While that may be true, the SRB's also have a very low dry weight. If your payload is small and requires a high TWR, it is actually one of the most efficient engines.

Take it as the following : A longer burn with a lighter LiquidFuel+Engine is better than a short burn with the heavier solidfuel-booster.

...as long as the added TWR is higher than the body you are ascending from of course. But concerning Duna it mean roughly everything even if it add only 0.1m/s² more, as long as you don't loose more than what you lost moving around the mass of the solid booster.

[Sharpened Alliance]

SRBs are only good for take offs. If you were landing on Duna or Laythe then you could have used parachutes and assisted them with some liquid fuel. If you are really crazy you try landing with jets on Laythe, the problem with that is they have a thrust cut off (they stop working if you turn them below about 1/5 thrust). But in the end, trying to land with SRBs is like trying to land without touching the [shift] key or the [Ctrl] key. After wasting tons of fuel on carrying some 1 meter cylinders of fancy explosive rocks with you.

[DrMonte]

I challenge that statement!

http://imgur.com/a/cbbjG#61

Yep, I must fix myself:

When you'll understand how these charts were generated, only then you'll know where it is reasonable to use SRBs.

TWR >= 4 OR mass < 1t. Are kidding me?

[numerobis]

Oooh, there's another time the RT-10 is most efficient, and this one is actually marginally useful: on Eve, on a payload of slightly less than 8t, put on one RT-10; it will provide about 750 m/s, for a Kerbin-relative TWR of 2.2. By the time it burns out you'll have gone up a ways and you'll be going about terminal velocity. But if you change mass about 100 kg or you feel like staging a bit earlier or later, the RT-10 will no longer win. In 0.21 the RT-10 had a lot more usefulness; buffing the 48-7S makes most other engines pointless. Note also that tavert's tables don't take account of drag -- so actually I bet the RT-10 is never the most efficient.

To the OP: If what you're worried about is fuel, how does the type of fuel change your worries? Just pack more fuel for the return trip.

[codepoet]

I think that this is definitely an interesting possibility! I myself have used two seperatrons on a Minmus lander to do a suicide burn right before landing. It seems that you would need Kerbal Engineer or similar to calculate the TWR, in conjunction with precisely planned numbers for velocity. In short, it would take a LOT of precise planning, and it may not be worth it in the long run. I can see this being useful on atmospheric bodies if you don't want to tote around a lot of parachutes, especially for Duna or Laythe, which have thinner atmospheres.

I have been experimenting with landing base components on laythe using just a small parachute, and then a couple of sepratrons for a final braking burn prior to touchdown. It is hard and does takea lot of planning, but the advantage is that you can fly all the way to laythe using electric engines and to do not need to pack a heavy chemical rocket at all which dramatically reduces the payload to orbit and hencevthe size fo the rocket. Ouf course it also means lots of long difficult burns that also require lots of planning - but then this is rocket science, it is not supposed to be easy.

[ammonia_ocean]

Thanks for all the help, guys! Sorry, I may have been a bit unclear in my post – I didn't use the SRBs for braking burns, just for takeoffs. (Although I have been thinking of trying to use them to land on the mün, just as a challenge). After looking at some of the stats I think I agree with you – they're cool-looking, but generally inefficient. Next multi-stage lander I make, I'll probably use liquid-fuel boosters.

[6677]

I tried landing on kerbin using SRB's to slow the decent as a laugh. "Strategic lithobraking" occured (although the pod *did* survive). Think it was all the way back in the demo days.

[Psycix]

Yep, I must fix myself:

When you'll understand how these charts were generated, only then you'll know where it is reasonable to use SRBs.

TWR >= 4 OR mass < 1t. Are kidding me?

Aside from various challenges and random abuse of physics I can provide you with one valid scenario: Launch escape systems!

But yeah, I think that SRB usage will skyrocket as soon as cost is implemented. They are indeed a bit pointless right now.

[Spyritdragon]

In general this might be a nice idea, but they are very very expensive delta-V wise to get to Duna or Laythe - they weigh very much, so taking them to space is usually something mqny people avoid. But purely from a takeoff point of view it could work, yeah :-).