Rules of Thumb for Building Cheap and Cheerful Rockets

[GoSlash27]

Norcalplanner,

I would imagine if you're using stage recovery it'd be in your best interest to make more of the total DV come from the booster stage and less from the insertion stage.

Also, I've got that stack up to 38t payload now.

Best,

-Slashy

[GoSlash27]

As long as you're playing around with stage recovery, this one should be your best bet:

Cheep 20 craft file

This one does 20t for 20 grand. Just boost vertically until 60m/sec then hold D until SRB burnout.

Best,

-Slashy

[Meithan]

And this one does 18t to 80 km orbit for 24k funds, without stage recovery:

First stage is three Kickbacks, second (serial) stage is one Skipper and an orange tank. Control authority during first stage burn provided by reaction wheel hidden inside fairing (below payload decoupler), with a probe core and two batteries to deorbit the spent upper stage:

I'm not claiming any record here (we need a challenge for that! -- Norcalplanner: I'd be willing to review entries or even host the challenge, but I've never done so and I'd need help with the rules), that was just the end result of a couple hours spent redesigning my launchers to be Cheap and Cheerful.

I also tried two liquid stages with SRBs and one big central liquid stage with SRBs, but they were more expensive.

Edited October 25, 2015 by Meithan

[Alshain]

Norcalplanner,

I would imagine if you're using stage recovery it'd be in your best interest to make more of the total DV come from the booster stage and less from the insertion stage.

Also, I've got that stack up to 38t payload now.

Best,

-Slashy

Not Necessarily. As I said earlier, the further the stage lands from KSC, the more it cuts into your refund. A longer burning booster stage gets just that much further before discarding. The engine is the most expensive part, adding a little more fuel to the booster stage might get you refunded the cost of a tank but it will severely cut the overall refund for the engine.

- - - Updated - - -

So here's what I did that caused me to change my position on recovering Kickbacks using StageRecovery.

I took Slashy's rocket, the cheep31, and modified it a bit. The cheep32 simply removed the Rockomax adapter, to reduce cost a little bit. That rocket cost 46,956 funds, and didn't have any appreciable change in performance from the original version.

A few iterations later, the cheep34 had eight radial parachutes on it, which added 3200 funds in cost, driving the price up to 50,156 funds.

Recovery of the bottom stage (7 Kickbacks, Rockomax decoupler, 2 Delta fins, and the 8 parachutes) was 115.44 km from KSC. Terminal velocity from the 8 parachutes was 7.95 m/s. At that distance and speed using stock StageRecovery settings, the recovery percentage is 82.8%, or 10,710 funds out of the total value of 12,930. Net cost of the rocket after the recovery was 39,446 funds.

That's 7,510 funds cheaper than the version without the parachutes.

Clearly more research is warranted and photos need to be put up, but it's late and I need to get up early in the morning. More information (including photos) should be posted tomorrow.

But did you factor in the mass of those parachutes. I don't think it is so black and white. In some cases this might work but in other cases, like that 120ton lifter I posted earlier, it may push you over your TWR limit and then you have to add another Kickback... and another parachute, and another decoupler. The lifter itself matters too. When you are talking about a lifter that costs 145,000 funds, 7,000 seems like pocket change.

Edited October 25, 2015 by Alshain

[Norcalplanner]

Not Necessarily. As I said earlier, the further the stage lands from KSC, the more it cuts into your refund. A longer burning booster stage gets just that much further before discarding. The engine is the most expensive part, adding a little more fuel to the booster stage might get you refunded the cost of a tank but it will severely cut the overall refund for the engine.

- - - Updated - - -

But did you factor in the mass of those parachutes. I don't think it is so black and white. In some cases this might work but in other cases, like that 120ton lifter I posted earlier, it may push you over your TWR limit and then you have to add another Kickback... and another parachute, and another decoupler. The lifter itself matters too. When you are talking about a lifter that costs 145,000 funds, 7,000 seems like pocket change.

OK, finally back at the computer.

It was actually odd with the modified version of Slashy's rocket - whether I had no chutes or 8 chutes, I always seemed to get into an 80 km orbit with around 180 m/s left in the tank. I was flying everything manually, so maybe I was just getting better with that rocket - still, it seemed to be pretty insensitive to the additional 0.8 tons and additional drag in the first stage.

Now the plan is to play around with both StageRecovery and the stock recovery mechanic, and try to get a better handle on when it makes sense and when it doesn't.

[Norcalplanner]

More StageRecovery Information and Examples

I've assembled an imgur album showing a few different scenarios. Essentially, if you decide to use StageRecovery, recover everything you can. It's a net improvement of 250 to 300 funds per Kickback to place a Mk16-XL on top of each one instead of leaving them bare. Also recover your LFO tanks and engines if they'll be staged while the rocket is going less than 2,000 m/s in atmosphere.

I created a rocket with a 167+ ton payload, similar to Alshain's, to use as a testbed. Without StageRecovery and without any parachutes, the lifter portion of the rocket costs 137,600 funds, for a rate of 820.3 funds per ton of payload to an 80 km orbit. For the StageRecovery rates, take a look at the album.

Javascript is disabled. View full album

Takeaway - StageRecovery is a gamechanger which will drastically reduce the cost to orbit if you choose to use it, provided you design and fly your rockets in a way that is compatible with the recovery mechanic.

Edited October 26, 2015 by Norcalplanner
clarifying a few things

[Norcalplanner]

And this one does 18t to 80 km orbit for 24k funds, without stage recovery:

http://i.imgur.com/3hezQ7Ll.jpg

First stage is three Kickbacks, second (serial) stage is one Skipper and an orange tank. Control authority during first stage burn provided by reaction wheel hidden inside fairing (below payload decoupler), with a probe core and two batteries to deorbit the spent upper stage:

http://i.imgur.com/7bq5V0Wm.jpg

I'm not claiming any record here (we need a challenge for that! -- Norcalplanner: I'd be willing to review entries or even host the challenge, but I've never done so and I'd need help with the rules), that was just the end result of a couple hours spent redesigning my launchers to be Cheap and Cheerful.

I also tried two liquid stages with SRBs and one big central liquid stage with SRBs, but they were more expensive.

Meithan,

Thanks for showing your ships and for the offer on the challenge. I'll try to carve out some free time to come up with a ruleset which will work. Then we can all start comparing apples to apples with our Cheap and Cheerful designs. :-)

- - - Updated - - -

Norcalplanner,

I would imagine if you're using stage recovery it'd be in your best interest to make more of the total DV come from the booster stage and less from the insertion stage.

Also, I've got that stack up to 38t payload now.

Best,

-Slashy

Slashy,

That's pretty much what I ended up doing - see the last part of the imgur album I posted. The last craft uses a 2.5 stage design where the last stage only has 600 m/s of delta V. Everything else is recovered with StageRecovery.

[celem]

Thanks for crunching the numbers on those Norcal, nice clear examples.

I will also go ahead and say that I've taken a look at the current incarnation of SR and some of my topstage concerns have been somewhat alleviated. The author has done some solid work since I used it, with allowances for heatshields and powered landings.

It would be possible with this to recover upper stages too, even those moving over 2kms, but the truth is that your gonna get awful distance %'s. I might take a look at heat-shielding some stuff..but those are both heavy and yet more money, and then more chutes..and so on.

[Earthslug]

Hey man, great tutorial.

Playing for over a year vanilla and still learning each day. Today I finally installed KER and discovered situations where I was using engines way wrong.

Like you said, terrier or spark engines can outperform poodle on rockomax tanks by 100s dV. I was blinded by Isp and engine size on its respective fuel tanks for over a year. Sure TWR is less but who cares, I want to build more efficient rockets and if those smaller ones get the job done....

I saw some other nice tips and tricks in your guide. I'll try some more of them.

Tnx man, great contribution to the community, and not only the newer players!

[Norcalplanner]

On 1/3/2016 at 3:36 PM, Earthslug said:

Hey man, great tutorial.

Playing for over a year vanilla and still learning each day. Today I finally installed KER and discovered situations where I was using engines way wrong.

Like you said, terrier or spark engines can outperform poodle on rockomax tanks by 100s dV. I was blinded by Isp and engine size on its respective fuel tanks for over a year. Sure TWR is less but who cares, I want to build more efficient rockets and if those smaller ones get the job done....

I saw some other nice tips and tricks in your guide. I'll try some more of them.

Tnx man, great contribution to the community, and not only the newer players!

Earthslug,

I'm glad you found the guide to be helpful. If you have any questions as you move forward, feel free to ask.

[wumpus]

Some notes:

I'm pretty sure you should never lower the thrust in your SRBs (unless it's over 4 or something silly).  I'll have to get back to this and make a thread on it.  It seems to be based on "leftover wisdom" from the old aero model (where 2.1 [fixed] TWR was the ideal) and assumptions about performing a gravity turn.

After making an "expensive and cheerful" (i.e. relatively cheap for 7000 m/s delta-v) out of [kicker] SRBs and a nuclear engine I noticed the following:

TWR of .4 (or less) isn't going to cut it.  It takes longer than 2 minutes of flight between SRB cutoff to apogee to get to orbital velocity.  If your C&C rocket is going to get into space with .3-.4TWR, it will do it by burning enough fuel in the second stage to get a TWR high enough.  My solution was pretty weird: use 4 kickers attached laterally and make a second stage of a vertically mounted kicker.  This added enough delta-v and time to apogee that the .4TWR could get up to speed in the two minutes it had to apogee.  I'm pretty sure that by list-price this was cheaper, but serious fans of recovery funds could build a second stage that got to orbit easier, and then could drop near KSC.

 

[Norcalplanner]

9 hours ago, wumpus said:

Some notes:

I'm pretty sure you should never lower the thrust in your SRBs (unless it's over 4 or something silly).  I'll have to get back to this and make a thread on it.  It seems to be based on "leftover wisdom" from the old aero model (where 2.1 [fixed] TWR was the ideal) and assumptions about performing a gravity turn.

After making an "expensive and cheerful" (i.e. relatively cheap for 7000 m/s delta-v) out of [kicker] SRBs and a nuclear engine I noticed the following:

TWR of .4 (or less) isn't going to cut it.  It takes longer than 2 minutes of flight between SRB cutoff to apogee to get to orbital velocity.  If your C&C rocket is going to get into space with .3-.4TWR, it will do it by burning enough fuel in the second stage to get a TWR high enough.  My solution was pretty weird: use 4 kickers attached laterally and make a second stage of a vertically mounted kicker.  This added enough delta-v and time to apogee that the .4TWR could get up to speed in the two minutes it had to apogee.  I'm pretty sure that by list-price this was cheaper, but serious fans of recovery funds could build a second stage that got to orbit easier, and then could drop near KSC.

 

Wumpus,

Thanks for the post.  While I agree that having a higher initial TWR off the pad is a good way to minimize gravity losses, it's not necessarily the best way to go when minimizing cost per ton to orbit.  It can also introduce aero losses which begin to offset the gravity losses if you go too fast when still deep in the atmosphere.  Especially when it comes to Kickbacks, thrust reducing the SRBs also has the benefit of making them run longer before burnout.  I'll frequently find that the initial burn with thrust-limited Kickbacks lasts for the first 90 or so seconds of the launch, with a steadily increasing TWR that whole time. 

Nearly all of this tutorial was written based on 1.0.4 game physics, so it's definitely not a case of old aero.

In terms of having a low TWR for your orbital insertion stage, that's primarily for serially staged LFO rockets where the low TWR stage only has to do 500 to 700 m/s of burn to achieve orbit.  I've put the imgur album below which is the single time I had an insertion TWR of 0.42, which was for a payload mass fraction challenge that used cross-feeding.  The orbital insertion stage only has to do the last 700 m/s to get to orbit, and the vehicle was already at 44 km with a speed of 1,700 m/s when that stage started to fire. I also note that 0.5 or 0.6 is likely more suitable for an orbital insertion stage, but also only for limited delta V requirements.  If a craft needs 1,200 m/s for orbital insertion, then the TWR is going to need to be up around 0.8 or higher, and you'll need a more vertical gravity turn. 

 

TWR of 0.3 was intended to be the minimum for a transfer stage for a craft which is already in orbit (such as a Jool mothership) - I apologize if that wasn't clear.  In running tests, I found that having a TWR of 0.3 was the minimum reasonably required to reach common destinations with a single transfer burn while still retaining a fair bit of accuracy from a 250 km parking orbit.  Steering losses really start to mount when the TWR is down at 0.2 when you're going to Jool unless the ship's is in a ridiculously high parking orbit, which gives up a lot of Oberth effect.

Your Kickback rocket design sounds interesting - any chance you could post a pic of it?  Sounds like it may be similar in concept to Slashy's, where he had different levels of thrust limiting on the initial clusters of SRBs, but without any staging to minimize the expense.