designing a LEO rocket for a payload

[Skript]

I'm working on designing various rockets, for research purposes.

I want to get a feel for the actual math and engineering that goes into designing these vehicles from the ground up.

So far I've been able to mathematically design a suborbital rocket that should in theory reach the edge of space. 

However I kind of came up with the design process in a way that seems entirely backwards.

I started with an engine design, since the most detailed documentation I could find was about engines and thrust generation. I was able to design a liquid bi propellant 250lbf engine(most documentation is old nasa work which was done in lbs) then I built the rocket up around that, knowing that I wanted an initial TWR of 1.7 I knew the rocket could weigh around 147lbs when full, and since I wanted a Delta V of 1.5km/s and the engine had an ISP of 300 that the empty rocket had to weigh 88lbs. 

This method is perfectly fine for a high power rocket experiment or demonstration. But it seems backwards for practical use.

rockets usually carry a payload, I'm wondering what a better process would be if I started knowing the payload mass. And worked down the rocket similar to the way rockets are built in KSP.

Lets say I start knowing my payload mass and where I wanted it in space. For instance 5kg to LEO 

LEO delta V is about 10km/s by my understanding. And for simplicity the rocket will be two stages.

So how would I work from the payload mass down the rocket instead of from the engine up the rocket?

[Elthy]

Afaik its "best" for launchers to LEO to have the delta-v split evenly between the stages. This way you would design two 5km/s rockets, one being the payload of the other.

[Skript]

3 hours ago, Elthy said:

Afaik its "best" for launchers to LEO to have the delta-v split evenly between the stages. This way you would design two 5km/s rockets, one being the payload of the other.

I see. 

[UmbralRaptor]

To minimize overall craft mass, you'll tend to want delta-v to be proportional to stage Isp. In practice it gets a bit more complicated as lower Isp stages often use rather denser propellants, and sometimes the "best" split gives insufficient TWR in the upper stage.

 

While it's really aimed at KSP, I do have some single and multistage analytic models lying around.

Edited September 16, 2016 by UmbralRaptor

[Vanamonde]

Since you are talking about non-Kerbal rocket science, the thread has been moved to Science and Spaceflight. 

[magnemoe]

Note that in KSP you need 3 km/s not 9 to reach space, atmosphere is also lower so 70-100 km is low orbit. 
This has some effects, first is that difference between suborbital and orbital is less, you will also need higher TWR on upper stage as you have shorter time to do the burn.
 

[Canis Dirus Leidy]

On 14.09.2016 at 8:24 PM, Skript said:

So how would I work from the payload mass down the rocket instead of from the engine up the rocket?

Simplest variant is shown on Atomic Rockets.

[A Fuzzy Velociraptor]

On 9/14/2016 at 4:32 PM, Elthy said:

Afaik its "best" for launchers to LEO to have the delta-v split evenly between the stages. This way you would design two 5km/s rockets, one being the payload of the other.

Many textbooks on the subject will bring up optimization of rocket stages. For optimization it is generally desired for the non-propellant mass ratios to be the same for both craft. In the case that the effective velocity of both stages is the same the required delta V will be the same. This is generally not the case outside of textbooks or craft which operate entirely within one regime. Also there are many situations when designing to maintain similar mass ratios between the two stages may not be possible due to the non-linear weight-volume scaling relationships. However except in extreme circumstances (i.e. hydrolox or electric upper stage or solid lower stages) splitting the delta-V between the stages is normally a good first order approximation.

[Sereneti]

The mass of the first stage are.... nearly .... not important.

you can use cheap SRB ore cheap hybrid-rockets.


the second stage has to be as light as possible.


Buuut:

you have to ignite the second stage, after the staging. ...


you cann use a bigger "second stage" ignite it on earth, and add "some" booster....
 

[Jovus]

Are we talking about paper rockets, rockets for a 'real' launch company, or rockets for your backyard?

It makes a difference. For example, if we're talking about you personally funding a rocket to go to LEO, cost scales are...quite different. (Also, good luck.)

[Kryten]

It's rare in practice for rocket designs to be completely clean-sheet, especially in terms of engines. Take the Delta II, one of the most flown rockets in history; the first stage motor is an RS-27A, which was developed from the Saturn I's H-1 engine, which was developed from the S-3D for the Jupiter and Thor programmes, which was developed from the engine for the Navajo booster... and so on, right back to the V-2 engine. The upper stage motor is an AJ-10, which has a similarly convoluted heritage leading to the mid-40s Aerobee sounding rocket. Same for most engines today.

[wumpus]

I'm failing with google, but I'm pretty sure there is a "list of rules for rockets" that includes such things as "any program that includes a launch vehicle becomes a launch vehicle program".  The idea being that designing a rocket capable of launching a spacecraft (into orbit and beyond) is so difficult and expensive that it overwhelms the entire program.  Rocket programs typically find the most existing reliable rocket with the least cost (in that order) to provide the delta-v for the mass they need.

I'd have to say the simple answer is to play KSP and figure it out.  Note that you need not buy a copy to [mostly] answer the question, the game demo works reasonably well to learn the difficulties of getting to orbit (assuming that someone else supplies all the parts).  If you did buy a copy, then installing the Realism Overhaul mods should allow getting the realism details much closer than stock KSP (hopefully you also downloaded KSP 1.1, getting Realism Overhaul to *all* work at 1.2 might take a while).

 

[magnemoe]

10 hours ago, Sereneti said:

The mass of the first stage are.... nearly .... not important.

you can use cheap SRB ore cheap hybrid-rockets.


the second stage has to be as light as possible.


Buuut:

you have to ignite the second stage, after the staging. ...


you cann use a bigger "second stage" ignite it on earth, and add "some" booster....
 

Yes, this is why it works to do an burn back to pad and land the first stage, also the problem with the space shuttle. 
its cheap to add more dry weight to first stage or use less efficient solid rockets. 
Upper stage you want to keep weight down, 3rd stage, put it inside the fairing even if this increase weight 


 

[wumpus]

On 9/19/2016 at 10:37 AM, Sereneti said:

The mass of the first stage are.... nearly .... not important.
you can use cheap SRB ore cheap hybrid-rockets.
the second stage has to be as light as possible.
Buuut:
you have to ignite the second stage, after the staging. ...
you cann use a bigger "second stage" ignite it on earth, and add "some" booster....

Oddly enough, had I worked on the Falcon Heavy, I would think long and hard about adding some COTS (commercial off the shelf) SRBs (solid rocket boosters) to the Falcon's upper (non-recoverable) stage.  While reducing first stage mass is relatively unimportant, *recoverable* mass is even more of an issue.  While sticking the SRBs on the upper stage might not help delta-vs as much (to be honest, they might help even if they slightly reduce delta-v), the point is that to reduce the delta-v coming from the middle booster.  Since that booster needs to be recovered, keeping the delta-v low (and thus the boost-back low) is critical.  Since neither the upper stage (nor the SRB boosters) would ever be recovered, they can have all the delta-v you need.

Most of this can be found by either playing KSP or studying the rocket equation.  Since the effective "dry mass" of the lower stages includes the entire upper stage, the Isp of the lower stages becomes far less critical, while the Isp of the upper stages becomes important (because the "dry mass" is only the dry mass of the upper stage + cargo).  If you want higher mass fractions (and bigger cargoes) even the Isp becomes less important and you look to things like cheap power (which in KSP means kicker SRBs, but this isn't necessarily true about real life).

Examples of getting big payloads into LKO (low kerbal orbit) on the cheap...

[Steel]

14 minutes ago, wumpus said:

Oddly enough, had I worked on the Falcon Heavy, I would think long and hard about adding some COTS (commercial off the shelf) SRBs (solid rocket boosters) to the Falcon's upper (non-recoverable) stage.  While reducing first stage mass is relatively unimportant, *recoverable* mass is even more of an issue.  While sticking the SRBs on the upper stage might not help delta-vs as much (to be honest, they might help even if they slightly reduce delta-v), the point is that to reduce the delta-v coming from the middle booster.  Since that booster needs to be recovered, keeping the delta-v low (and thus the boost-back low) is critical.  Since neither the upper stage (nor the SRB boosters) would ever be recovered, they can have all the delta-v you need.

Most of this can be found by either playing KSP or studying the rocket equation.  Since the effective "dry mass" of the lower stages includes the entire upper stage, the Isp of the lower stages becomes far less critical, while the Isp of the upper stages becomes important (because the "dry mass" is only the dry mass of the upper stage + cargo).  If you want higher mass fractions (and bigger cargoes) even the Isp becomes less important and you look to things like cheap power (which in KSP means kicker SRBs, but this isn't necessarily true about real life).

Examples of getting big payloads into LKO (low kerbal orbit) on the cheap...

I'm not sure I understand your argument for using SRBs on the second stage, what are you trying to achieve here?

[wumpus]

1 minute ago, Steel said:

I'm not sure I understand your argument for using SRBs on the second stage, what are you trying to achieve here?

Reduced back burn on the recoverable stage.  Boosters have so far been recovered after getting to 1-2km/s.  A Falcon Heavy middle booster carrying a cargo beyond what a falcon 9 (unrecoverable, something like 28tons) is likely to require >6km/s.  Coming down from those speeds have been iffy (although this middle booster has been specifically "stretched" for such a flight).  Dropping the middle stage earlier and applying delta-v directly to the upper stage would be the whole point.

I wouldn't be surprised if the economics dictate that spacex will simply throw away the center booster for heavier flights, and still be more economic than multiple flights with full recovery.

[Steel]

Ah ok, I see.

I would imagine that the reason is that the FH wasn't designed from the ground up to be recoverable. Most likely it was more of a "Hey, our heavy lifter design could also use some of the reusability that we've managed to get to work on the F9!". With this generation of it's rockets i feel that SpaceX want to get the basics right (i.e. being a successful lifter) and then extend that program to begin to field test recovery and reusability elements. Then maybe in the next generation the whole lift system can be designed with reusability in mind from the ground up.

[Kryten]

They could just stretch the second stage to get a similar effect more efficiently. Besides, there's pretty much no current demand for payloads heavy enough that 3-core RTLS isn't an option. FH manifest now, apart from Red Dragon, is all ~6 ton GSO comsats; they're only just too heavy for F9.

[Sereneti]

4 hours ago, wumpus said:

Oddly enough, had I worked on the Falcon Heavy, I would think long and hard about adding some COTS (commercial off the shelf) SRBs (solid rocket boosters) to the Falcon's upper (non-recoverable) stage.  While reducing first stage mass is relatively unimportant, *recoverable* mass is even more of an issue.  While sticking the SRBs on the upper stage might not help delta-vs as much (to be honest, they might help even if they slightly reduce delta-v), the point is that to reduce the delta-v coming from the middle booster.  Since that booster needs to be recovered, keeping the delta-v low (and thus the boost-back low) is critical.  Since neither the upper stage (nor the SRB boosters) would ever be recovered, they can have all the delta-v you need.

Most of this can be found by either playing KSP or studying the rocket equation.  Since the effective "dry mass" of the lower stages includes the entire upper stage, the Isp of the lower stages becomes far less critical, while the Isp of the upper stages becomes important (because the "dry mass" is only the dry mass of the upper stage + cargo).  If you want higher mass fractions (and bigger cargoes) even the Isp becomes less important and you look to things like cheap power (which in KSP means kicker SRBs, but this isn't necessarily true about real life).

Examples of getting big payloads into LKO (low kerbal orbit) on the cheap...

only on my curiosity:

Why not use SRBs on the first stage?

They are droped with parachutes and are reusable.

Cheap dv/TWR...

moar Payload with a little moar of money....

Edited September 20, 2016 by Sereneti

[wumpus]

21 hours ago, Sereneti said:

Why not use SRBs on the first stage?

Because while I might be able to get them back, they are basically just shells of steel.  What I want back are the liquid-fuel 1[.5] stages.

Work back from orbit: A Falcon 9 booster has to provide roughly 7-8km/s delta-v for a lightweight rocket (when the booster lands) or a 4-5km/s delta-v for a heavy one (where it doesn't).  The upper stage of a Falcon Heavy will be able to provide even less delta-v (unless I missed something, it is the same as the Falcon 9) as it is designed for an even heavier load.  This puts the middle stage as providing at least 6km/s of delta-v (and thus hurtling towards space at 6km/s, and have to boost back nearly the entire velocity).

If you want to avoid that tremendous boostback, you need to add delta-v to the upper stage.  Thus the SRBs on the upper stage.

If you aren't worried about the boostback (and not only is Spacex apparently less worried about it, Blue Origins appears blase about it to with New Glenn), then don't bother (or put the SRBs on the first stage).

[Kryten]

SRBs would require you to redesign the upper stage anyway, so why not just stretch it? Or replace it with a bigger one with a raptor engine, like USAF are already paying them to do?

[wumpus]

3 minutes ago, Kryten said:

SRBs would require you to redesign the upper stage anyway, so why not just stretch it? Or replace it with a bigger one with a raptor engine, like USAF are already paying them to do?

I suspect they can do the backburn well enough or they would have stretched it during 1.1 (or possibly added room for SRBs).  While I like the raptor engine even more, I suspect that is due to my lack of RO play (where I often run into needing moar thrust on my second stage.  That doesn't seem to be a real issue in real life).

[Sereneti]

On 21.9.2016 at 7:28 PM, wumpus said:

Because while I might be able to get them back, they are basically just shells of steel.  What I want back are the liquid-fuel 1[.5] stages.

Work back from orbit: A Falcon 9 booster has to provide roughly 7-8km/s delta-v for a lightweight rocket (when the booster lands) or a 4-5km/s delta-v for a heavy one (where it doesn't).  The upper stage of a Falcon Heavy will be able to provide even less delta-v (unless I missed something, it is the same as the Falcon 9) as it is designed for an even heavier load.  This puts the middle stage as providing at least 6km/s of delta-v (and thus hurtling towards space at 6km/s, and have to boost back nearly the entire velocity).

If you want to avoid that tremendous boostback, you need to add delta-v to the upper stage.  Thus the SRBs on the upper stage.

If you aren't worried about the boostback (and not only is Spacex apparently less worried about it, Blue Origins appears blase about it to with New Glenn), then don't bother (or put the SRBs on the first stage).

Yes.

but:
The falcon first stage can land.
In KSP, if i want to increase the payload a little bit , and dont want to re-desing the whole rocket, i put some Bosters on the rocket.
And then i have the same flight-path then before.

(like : get the boosters back (on parachutes) -  and then : get the [first-stage] engine back.)