Solid Boosters vs Hydrogen Boosters

[dwightlooi]

It is well known that solid boosters have inferior performance to liquid boosters, and nothing can be further apart than a Hydrogen Booster and a Solid Booster. Is that really true? Well, if you measure performance in terms of payload mass as a fraction of lift off mass, it is definitely true. But is that important?<br /><br />To illustrate the point let us consider two boosters -- the all hydrogen Delta IV (medium 4,0) and a hypothetical Affordable Solid Launch Vehicle (ASLV). Both are 2-stage vehicles. <b>The Delta IV lifts off at 260 tons and puts 8.6 tons into LEO. </b>That is a payload fraction of 3.3%. <b>The ASLV takes off at 1185 tons and it puts 9 tons into LEO.</b> That is a pathetic payload fraction of 0.76%. This is really bad even for all-solid boosters -- for reference, the Taurus 4-stage all solid LV gets a payload fraction of 1.87%. But we are trying to build a simple vehicle whose only purpose is to meet or exceed the Delta IV's payload capacity – let efficiency be damned -- so we are going to make it an inefficient 2-stage design. So, yes, the Delta IV is 4.34 times more efficient. <b>But guess what? The two rockets ARE the same size! </b>You see... Solid boosters are about 1150 kg per cubic meter, whereas hydrogen boosters are around 280 kg per cubic meter. So in the end, you have two boosters which are practically the same size; same diameter and height. <b>And, they have comparable payload capacity. So one has to ask... does mass efficiency matter? Should it be the leading factor to consider when designing a launch vehicle?</b><br /><br />Surely the solid booster is simpler. Instead of two sets of engines with turbopumps, combustors and complex plumbing, you have nothing. No moving parts. Instead of huge tanks full of highly cryogenic liquids, and the need for insulation and ground support facilities to deal with them, you have nothing. No liquids, no fueling apparatus, no insulation, no fear of low temperature cracking, no need to fuel or defuel th

 

[propforce]

What are your assumptions in the hypothetical ASLV configuration ? <div class="Discussion_UserSignature"> </div>

 

[dwightlooi]

The size of the booster was intentionally selected to be the same as the Delta IV. That is the booster body -- 1st + 2nd stage -- is defined as 5 m x 53 m. I approximated the ASLV stages' masses by assuming that the booster's density (mass/volume ratio) is roughly the same as that of the MX missile's 1st stage. The structural mass to propellant mass fraction is assumed to be the same as the shuttle SRB. The thrust is selected for a comfortable thrust to weight ratio. The IpSec is what is typical of decent SRBs. And the burn time was figured out by dividing the propellant mass by the thrust and multiplying the resulting fraction by the IpSec. I try to err on the conservative side when it comes to payload ratings.

 

[gunsandrockets]

I can see all kinds of problems with the monster 5 meter diameter solid rocket. Just building it could be trouble. That's a 5 meter diameter thrust chamber, and a 5 meter diameter fuel grain. I don't think something that size has ever been tried.<br /><br />Then there's the ground transportation hassle. The completed weight is almost half that of a Saturn V rocket. Once all the pieces are in place it won't go anywhere very fast.<br /><br />And comparing an all solid rocket to the Delta IV is stacking the deck for comparison purposes. Most rockets will only use LH2 in an upper stage. That's why the single core Delta IV can't lift as much to LEO as the smaller Atlas V can. <br /><br />

 

[gunsandrockets]

"The IpSec is what is typical of decent SRBs."<br /><br />Which SRB has a vacuum ISP of 290?

 

[najab]

><i>Most rockets will only use LH2 in an upper stage. That's why the single core Delta IV can't lift as much to LEO as the smaller Atlas V can.</i><p>First off, I'd argue that they don't use <b>only</b> LH2 in the upper stage, and secondly: what propellants does Delta's upper stage use?</p>

 

[gunsandrockets]

najaB<br /><br />Ahem. Most rockets only use LH2 fuel in an upper stage, not in all stages as does the Delta IV.<br /><br />Is that clearer?

 

[nacnud]

I think he wanted a reference to LOX.

 

[drwayne]

"It is well known that solid boosters have inferior performance to liquid boosters"<br /><br />Well known? <br /><br />I think many here understand full well that Isp is not the whole story.<br /><br /><img src="/images/icons/smile.gif" /><br /><br />Wayne <div class="Discussion_UserSignature"> <p>"1) Give no quarter; 2) Take no prisoners; 3) Sink everything."  Admiral Jackie Fisher</p> </div>

 

[najab]

Aha, I was mis-reading your post. My apologies.

 

[gunsandrockets]

"I think he wanted a reference to LOX. "<br /><br />Why should I bother stating what is obvious?

 

[nacnud]

Never mind you stated that you mean LH2 fuel, therefore LOX was obvious. There is a long running habit (that I approve of) to correct people confusing fuel with propellant. After all the devil is in the detail. <img src="/images/icons/smile.gif" />

 

[drwayne]

"There is a long running habit (that I approve of) to correct people confusing fuel with propellant. "<br /><br />Sometimes you just have to say - "I don't give a FLOX!"<br /><br /><img src="/images/icons/wink.gif" /><br /><br />Wayne <div class="Discussion_UserSignature"> <p>"1) Give no quarter; 2) Take no prisoners; 3) Sink everything."  Admiral Jackie Fisher</p> </div>

 

[nacnud]

Time to don a flame retardent suit? <img src="/images/icons/laugh.gif" />

 

[drwayne]

I think FLOX is hypergolic with those too. I know Flourine is hypergolic with asbestas<br /><br /><img src="/images/icons/wink.gif" /><br /><br />Wayne <div class="Discussion_UserSignature"> <p>"1) Give no quarter; 2) Take no prisoners; 3) Sink everything."  Admiral Jackie Fisher</p> </div>

 

[no_way]

When building a new rocket, your last concern should be performance. Why ? Because the problem with current launchers isnt performance, its high cost.<br /><br />And high performance is basically the enemy of low cost. From operational aspect, solids have many features that make them preferable to liquids ( the ones you stated, like simplicity of the thing itself, simplified launch operations, highly storable ) but also some drawbacks like already mentioned transportation hassles.<br /><br />A good middle ground is LOX/hydrocarbons. You avoid hassles with LH2 ( LOX is much easier to deal with ), making your rocket more storable. There's a reason why russian boosters are relatively cheap, even though their technology heritage is still in sixties without major new designs : they are mostly LOX/Kerosene, making their launch operations relatively simple and requiring small crew.<br /><br />But you still have the relative complexity of rocket engine where most of your complexity and high development and manufacturing costs is in turbomachinery. Note that your assertion that Solids have no moving parts is wrong: you still have thrust vectoring.<br />Turbomachinery could possibly be avoided with lower-performing piston- or "high pressure pumps" or entirely pressure-fed design. <br />( John Carmack has a good chapter written on their reasoning on selecting a pump over pressure-fed ) <br /><br />Finally, if you want reusability, the holy grail of rocketry at present, solids will get very difficult. From operational standpoint, pure liquids are pretty much a given, or easily exchangeable hybrid cores ( which will be very problematic when you get up to orbital performance and sizes )

 

[dwightlooi]

<i><br />"The IpSec is what is typical of decent SRBs." <br /><br />Which SRB has a vacuum ISP of 290?</i><br /><br />A lot of them.... for example all the Pegasus upper stage solids and all the PAM kickers have IpSecs in the 290 to 293 sec range. Like liquids, solids perform better with a low pressure surrounding too. At sea level, solids tend to fall in the 230-240 range.<br /><br /><i> And comparing an all solid rocket to the Delta IV is stacking the deck for comparison purposes. Most rockets will only use LH2 in an upper stage. That's why the single core Delta IV can't lift as much to LEO as the smaller Atlas V can.</i><br /><br />I know most rockets use LH2 only in the upper stages. UDMH, MMH, Aerozine with N2O4, Kerosene and solids are popular as 1st stages. In fact just about everything – except the Delta IV medium (4,0) that does light a LH2 burner at liftoff also use solids to shove the rocket off the pad. Just to name a few, the Shuttle and the Arianne V both do this.<br /><br />But I was trying to make a point by comparing a high impulse LH2 solution with the low impulse solid solution. I think it is very interesting that they turn out to be pretty much the same size.<br /><br />Normally, yes, if you want to lift the most payload with the most compact airframe, you will use either a hydrocarbon or solid 1st stage and a LH2 upper stage. The lower stage makes up most of the rocket, so using a high density solution keeps size down and improves aerodynamics among other things. The upper stage can be a bulky LH2 design because upper stages are small no matter how bulky they get.<br /><br />For large solids, I think transportation is the biggest challenge. There reason the Shuttle SRB contract was given to Thiokol years ago wasn’t they their design was the highest performing or the cheapest to build – it fact the reverse is true – it was that their segmented design was the only one which was transportable by rail. You cannot put a single 600 ton stick on a rail car, you cann

 

[najab]

It's my opinion that Thiokol got the contract largely for political reasons. Someone (I can't remember who at the moment, Aerojet?) demonstrated that they had the technology to cast monolithic boosters onsite at KSC, but they weren't awarded the contract. The rub is that the monolithic boosters would not only have been safer - no joints to leak - but would also have been cheaper in the long run!

 

[propforce]

<blockquote><font class="small">In reply to:</font><hr /><p> I approximated the ASLV stages' masses by assuming that the booster's density (mass/volume ratio) is roughly the same as that of the MX missile's 1st stage. The structural mass to propellant mass fraction is assumed to be the same as the shuttle SRB. The thrust is selected for a comfortable thrust to weight ratio. The IpSec is what is typical of decent SRBs....<p><hr /></p></p></blockquote><br /><br />You'll need a much bigger rocket on the ASLV to lift that payload !! <img src="/images/icons/laugh.gif" /><br /><br />Right now, based on my calculation, your ASLV can't even get to orbit. In order to launch the same payload as DIV-M, you'll need <font color="yellow">6 SRBs</font>on your 1st stage! <br /><br />Check your numbers. <br /><br /> <div class="Discussion_UserSignature"> </div>

 

[gunsandrockets]

"A lot of them.... for example all the Pegasus upper stage solids and all the PAM kickers have IpSecs in the 290 to 293 sec range."<br /><br />After some googling I see that you are correct. The solid IUS even has an ISP as high as 304 seconds. That's very surprising to me.

 

[chris_in_space]

I agree with propforce, check your numbers dwightlooi since your ASLV wouldn't make it to LEO. From my calculations the delta velocity it can get is around 6500 m/s so even without taking into account gravity and drag losses (around 1500-2000 m/s) it isn't enough to get it to LEO velocity (~7800m/s).

 

[dwightlooi]

Do you guys have a reliable computerized solution? I am doing it all on paper with a calculator. It is a hassle not to mention it creates all kinds of potential for stupid mistakes.<br /><br />I re-did the math... you guys are right. In fact I got a LOWER number of 6047 m/s. The problem with the original calculation lay in the empty mass fraction of the 2nd stage being incorrect. I use a constant Cd of 0.1 for the rocket for the first stage burn and zero drag for the second stage.

 

[propforce]

You'll need more stages for solid rockets. It is more efficient to shed mass as you go up, esp. due to the low Isp nature of solids.<br /><br />Check out the Minotaur , it's a 4 stage using surplus Minuteman I & II stages plus Pegasus upper stages. You can reduce your GLOW significantly if you use a 4-stage approach rather than a 2-stage. <div class="Discussion_UserSignature"> </div>